LIFE HISTORY CHARACTERISTICS AND HABITAT QUALITY OF ... · quality in a Colorado population of Flammulated Owls ( Otus flammeolus ) in a 19-yr study. The owl is a small, monogamous

LIFE HISTORY CHARACTERISTICS AND HABITAT QUALITY OF

FLAMMULATED OWLS (OTUS FLAMMEOLUS) IN COLORADO

By

BRIAN DWIGHT LINKHART

B.S., Colorado State University, 1981

M.S., Colorado State University, 1984

A thesis submitted to the

Faculty of the Graduate School of the

University of Colorado in partial fulfillment

of the requirement for the degree of

Doctor of Philosophy

Department of Environmental, Population, and Organismic Biology

2001

ii

This thesis entitled:

Life History Characteristics and Habitat Quality of Flammulated Owls (Otus flammeolus) in Colorado

written by Brian Dwight Linkhart

has been approved for the Department of Environmental, Population, and Organismic Biology

________________________________________ Dr. Carl Bock, committee chair

_________________________________________ Dr. Dave Chiszar

_________________________________________ Dr. Sharon Collinge

_________________________________________

Dr. Alexander Cruz

_________________________________________ Dr. Yan Linhart

Date ____________________________

The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly

work in the above mentioned discipline.

iii

Linkhart, Brian Dwight (Ph.D., Environmental, Population, and Organismic Biology) Life history characteristics and habitat quality of Flammulated Owls (Otus

flammeolus) in Colorado

Thesis directed by Dr. Carl E. Bock

Abstract. – I determined the life history characteristics and the components of habitat

quality in a Colorado population of Flammulated Owls (Otus flammeolus) in a 19-yr

study. The owl is a small, monogamous Neotropical migrant that nests in mature

conifer forests in western North America, and is considered sensitive by the USDA

Forest Service. Conservation planning is limited by lack of data regarding the owl’s

population dynamics and habitat requirements. I assessed population dynamics based

on density, territory fidelity, dispersal, survival, and reproduction of the owls. Most

owl territories were constant in time and space despite turnover of individuals.

Density of breeding pairs showed little annual variation. Up to 70% of territories

were occupied annually by bachelor males, suggesting that females have lower

survival. Compared to other North American owls, Flammulated Owls have a small

and unvarying clutch size, high nesting success, and a long breeding lifespan,

indicating they have a life history similar to larger owls.

Territory fidelity was male-biased, as it is with most birds, and pairs rarely

divorced. Most breeding dispersals were by females that moved one or two territories

away from their original territories. However, females whose nests failed the previous

year had lower return rates to the study area than females whose previous nests were

iv

successful. Dispersal distance may be bimodal with females dispersing longer distances

after nesting failure and shorter distances after successful nests. Females dispersed to

territories where total productivity during the study was higher than on original territories,

suggesting they assessed territory quality before dispersing.

Characteristics of high-quality breeding habitats were determined by

correlating long-term demographic parameters of owls with habitat characteristics on

their territories. Territories differed significantly in total years they were occupied by

breeding pairs and in total productivity. Availability of cavity-trees for nesting

determined where owls established territories, while forest type and structure

determined whether a territory was more often occupied by breeding pairs or bachelor

males. High-quality breeding habitat for Flammulated Owls was characterized as

mature, open stands of ponderosa pine (Pinus ponderosa) mixed with Douglas-fir

(Pseudotsuga menziesii) with sufficient cavity-trees for nesting.

v

Acknowledgements

I am grateful for all of the people who provided support for this project. First,

I thank my major advisor, Dr. Carl Bock, for his guidance, advice, and support

throughout this project. I also extend special thanks to my committee members, Dr.

Dave Chiszar, Dr. Sharon Collinge, Dr. Alexander Cruz, Dr. Yan Linhart, and Dr.

Carol Wessman.

Had it not been for the U.S. Forest Service, this project could never have

started or continued over the past 19 years. The Rocky Mountain Research Station

always has been the primary source of funding and logistical support. The Manitou

Experimental Forest proved to be a perfect location for this long-term study, offering

as natural and unmodified montane forest as might be found anywhere in the region.

My discoveries and experiences on this forest always will be part of my memories.

Several employees of the U.S. Forest Service have had vital roles in this

study. I am particularly grateful to Dr. Richard T. Reynolds, who has had a marked

effect on my career. Dr. Reynolds first offered me a job as a summer technician

assisting his raptor research projects throughout Colorado when I was an

undergraduate at Colorado State University, and he sparked my interests in research

and taught me how to become a good field biologist. Over the years his supervision,

support, encouragement, and friendship have been essential to my success. I am

indebted to his efforts.

I also thank Carl Edminster and Dr. Wayne Shepperd, who have administered

the Manitou Experimental Forest over the years, and William Knott, Ross Watkins,

vi

and Steve Tapia, who as forest caretakers always were eager to help. I especially

thank Steve Tapia, who always has gone out of his way to assist with arranging and

procuring necessary equipment and materials, helping in the field, and locating

potential volunteers. He also has been a good friend. I thank Mike Morrison and

Frank Romero, who were instrumental in providing digitized data for use in

Geographic Information Systems (GIS), and Mark Roper, who assisted with GPS

equipment. Without the friendship and advice of Suzanne Joy, who dispensed

invaluable advice on GIS matters day or night, I might still be hopelessly punching

the keyboard. Finally, Rudy King was especially helpful in assisting me with

statistical analyses and his advice was greatly appreciated.

I am very grateful to several people in the Geography Department at the

University of Colorado who provided technical skill, advice, and computer equipment

for my GIS analyses including Dr. Barbara Buttenfield, Sara Farenkopf, Jim Robb,

Chris Hanson, and Tom Dickinson. I am particularly grateful to Dr. Robin Reich,

Department of Forest Sciences at Colorado State University, for providing several

months of assistance and instruction with vegetation modeling.

A project of this duration necessitates a large number of field workers and I

am very grateful to the many individuals who offered their assistance in data

collection. Especially notable were several individuals who each volunteered over

four hundred hours, including Buzz Morrison, Scott Hiller, Patrick Leavey, and

Nicholas Tucey. The commitment of these individuals, and the extent to which they

benefited my study, cannot be understated. Also assisting with field work were Josie

Bamford, Holly Barnard, Christina Bauman, Cloud Ridge Naturalists, Sam Berry,

vii

Rob Bringuel, Michelle and Tim Connelly, Peg Coontz, Dave and Laura (Scott)

Denton, Peter Gaede, Jennifer Gass, Matt Harper, Emily Holt, Stephanie Howard,

Sam Johnson, Littleton High School students, Steve Mata, Terry and Amelia

McGlynn, William Merkle, Monica Mohr, Monica Molachy, Mark and John

Morgenstern, Nickolas Neitzel, Charlie Nemkevich, Andrew Orahoske, Summer

Orniz, Amy Ottaway, Amy Pabski, Mark Platten, Jenna Sanchez, David Stahl, Sue

Stern, Greg Styduhar, and Steve Tapia.

In addition to the Rocky Mountain Research Station, financial support was

provided by Littleton Public Schools, the University of Colorado, the Department of

Environmental, Population, and Organismic Biology, the Colorado Mountain Club

Foundation, Colorado Natural History Small Grants, and the Cooper Ornithological

Society.

The support I have received from my friends and family have been profoundly

important. I thank Audrey and Jim Benedict, and Tim and Michelle Connelly, whose

friendship and encouragement have been sources of strength for over two decades. I

particularly thank my mom and dad, whose love of nature was instilled in me at a

very young age. It is to them that I owe my curiosity of life, and for that I will always

be deeply appreciative. Finally, I am especially grateful for my wife, Marlene, who

provided tremendous love and support through all years of this project when I was

spending innumerable hours at night running over mountains in search of singing

owls and hooting at the moon. She truly has been the wind in my sails and I owe the

completion of this effort to her.

viii

Table of Contents

CHAPTER

I. BACKGROUND AND OUTLINE OF DISSERTATION TOPICS

Introduction………………………………………………………… 1

Outline of dissertation topics………………………………………. 4

Literature Cited ……………………………………………………. 7

II. DEMOGRAPHY OF FLAMMULATED OWLS

Abstract …………………………………………………………… 11

Introduction ………………………………………………………. 13

Methods …………………………………………………………… 14

Results ……………………………………………………………. 21

Discussion ………………………………………………………… 33

Literature Cited …………………………………………………… 38

III. LIFETIME REPRODUCTION OF FLAMMULATED OWLS

Abstract …………………………………………………………… 43

Introduction ……………………………………………………….. 44

Methods …………………………………………………………… 46

Results …………………………………………………………….. 49

Discussion …………………………………………………………. 61


ix

IV. MATE AND SITE FIDELITY AND DISPERSAL IN

FLAMMULATED OWLS

Abstract …………………………………………………………… 70

Introduction ……………………………………………………….. 72

Methods …………………………………………………………… 74

Results …………………………………………………………….. 81

Discussion …………………………………………………………. 89

Literature Cited …………………………………………………… 98

V. DETERMINING HABITAT QUALITY FROM LONGTERM

DEMOGRAPHICS IN FLAMMULATED OWLS

Abstract …………………………………………………………… 105

Introduction ……………………………………………………….. 108

Methods …………………………………………………………… 112

Results …………………………………………………………….. 128

Discussion ………………………………………………………… 159


VI. CONCLUSIONS ………………………………………………….. 180


VII. LITERATURE CITED ……………………………………………. 189

x

List of Tables

Table 1 Mate improvement hypothesis; comparison of lifetime

production of owlets by original males on territories from

which females dispersed to lifetime production of owlets

by new males on territories where females dispersed.

………. 88

Table 2 Territory improvement hypothesis; comparison of total

owlets produced over 19 yr on territories where females

nested before dispersal (“original territory”) to total

owlets produced on territories where females dispersed

(“new territory”).

………. 90

Table 3 Disperser-enhancement hypothesis; comparison of mean

brood size for females before they dispersed to mean

brood size on territories after they dispersed.

………. 91

Table 4 Forest overstory and understory variables used in

correlations with demographic variables.

………. 120

Table 5 Demography on owl territories from 1981-1999. ………. 131

Table 6 Comparisons of owlets over 19 yr on territories where ……….. 138

xi

females nested before dispersal (“original territory”) to

owlets produced on territories to which those same

females dispersed (“new territory”).

Table 7 Area and percentage of forest types in owl territories,

non-territory habitat, and over the entire study area.

………. 139

Table 8 Comparison of forest structure variables among forest

types.

………. 141

Table 9 Correlations among demographic variables and forest

types within owl territories.

………. 147

Table 10 Correlations among demographic and forest structure

variables within owl territories.

………. 148

Table 11 Comparison of forest structure variables in territory vs

non-territory habitat.

………. 151

Table 12 Comparison of forest structure variables among non-

territory and three classes of territory habitat.

………. 153

Table 13 Density of cavity trees among owl territories and in non- ………. 156

xii

territory habitat.

Table 14 Comparison of relative arthropod abundance between a

high-productivity territory (A4) and a low-productivity

territory (A18) during 1998-1999.

………. 158

xiii

List of Figures

Figure 1 Location of owl territories on the Manitou

Experimental Forest study area, 1981-1999.

………. 16

Figure 2 Number of territories occupied annually by bachelor

(unpaired) males and breeding pairs, 1981-1999.

………. 23

Figure 3 Number of nesting attempts and successful nests per

yr on 14 territories, 1981-1999.

………. 25

Figure 4 Annual mean number of eggs per clutch, number of

owlets per brood, and number of owlets per successful

brood, 1981-1999.

……….. 26

Figure 5 Total eggs and fledglings produced in all territories

annually from 1981-1999.

………. 28

Figure 6 Total years of return to the study area by male and

female owls.

………. 31

Figure 7 Estimated survival curves for male and female owls. ………. 32

xiv

Figure 8 Lifetime number of nesting attempts by female and

male owls.

………. 51

Figure 9 Lifetime number of successful nests by female and

male owls.

………. 52

Figure 10 Lifetime percentage of successful nests by female and

male owls.

………. 53

Figure 11 Lifetime production of eggs by female and male owls. ………. 55

Figure 12 Lifetime production of fledglings by female and male

owls.

………. 56

Figure 13 Relationship between total breeding years and total

fledglings for female and male owls.

………. 58

Figure 14 Relationship between total eggs and fledglings for

female and male owls.

………. 59

Figure 15 Percent of total fledglings produced by varying

percentages of female and male owls.

………. 60

xv

Figure 16 Lifetime number of new mates for female and male

owls.

………. 62



………. 82



………. 115

Figure 19 Contribution of individual territories to total owlets

produced by combined territories from 1981-1999.

………. 136

Figure 20 Distribution of owl territories, cavity trees, and forest

types on the Manitou Experimental Forest study area.

………. 140

Figure 21 Correlations between demographic variables and

forest types across owl territories.

………. 146

CHAPTER I

BACKGROUND AND OUTLINE OF DISSERTATION TOPICS

Life histories of avian species have evolved to maximize lifetime reproductive

output in particular environments (Stearns 1992). Life histories, which consist of

coevolved characteristics including reproductive rate, natal and breeding dispersal,

and breeding life span, vary markedly across avian species (Moreau 1944, Lack 1954,

Ricklefs 2000). Despite variance in life histories, studies of demography have

revealed several internally-consistent patterns such as correlation between body mass,

fecundity and longevity. Large species, including many raptors, are generally long-

lived and have low fecundity while small species are typically short-lived and have

high fecundity (Newton 1979, Johnsgard 1988, Gill 1995). Large, long-lived species

also generally have low rates of annual turnover with more overlap among

generations compared to small, short-lived species (Newton 1998, Ricklefs 2000).

Investigations of life histories are important in population ecology for several

reasons. First, life history characteristics such as breeding density, reproduction, and

survival determine population dynamics. Studies of these characteristics reveal the

relative stability of populations in time and space (Grant 1986, Woolfendon and

Fitzpatrick 1984), and provide insight in determining how populations are affected by

ecological factors such as habitat quality (Newton and Marquiss 1982, Forsman et al.

1996), prey abundance (Nol and Smith 1987, Korpimaki 1988), nest predation

2

(Martin 1988, Bosque and Bosque 1995), and extreme weather (Grant 1986, Owen

and Black 1989). Life history characteristics are also important in developing avian

conservation strategies because species may respond differently to environmental

perturbations based on patterns in their reproduction and survival (Newton 1995,

Forsman et al. 1996).

Second, life history investigations that focus on lifetime reproductive success

(LRS), the total offspring raised by individuals over their lifetimes, are useful for

determining reproductive strategies of species and variance in productivity among

individuals. For example, LRS data show that Kingfishers (Alcedo atthis) exhibit a

limited but highly productive breeding life, breeding annually for no more than 4 yr,

and producing more than 20 young per yr (Bunzel and Druke 1989). In contrast, LRS

data show that Barnacle Geese (Branta leucopsis) have a long but relatively

unproductive breeding life. These waterfowl initiate breeding at nearly 7 yr, produce

fewer than 5 young per yr, and total number of offspring rarely exceeds 10 in a

lifetime that may be greater than 20 yr (Owen and Black 1989). In both species, less

than one-third of the breeding population produces 50% of all offspring (Bunzel and

Druke 1989, Owen and Black 1989). LRS also provides one of the best estimates of

individual fitness, and may allow identification of individual attributes and

environmental correlates that contribute most importantly to fitness (Newton 1989).

Third, studies of mate and site fidelity (e.g., Greenwood 1980, Gavin and

Bollinger 1988), and natal and adult dispersal (e.g., Greenwood and Harvey 1982,

Forero et al. 1999), are important because movement behaviors strongly influence

population structure and gene flow (Johnson and Gaines 1990). Moreover,

3

understanding the ecological correlates of dispersal is useful because factors such as

nesting success (Haas 1998), mate quality (Goodburn 1991), and territory quality

(Korpimaki 1988) influence dispersal rate and distance, which are vital considerations

in the development of avian conservation plans (Forsman et al. 1996).

Finally, investigations of life history characteristics figure prominently in the

reliable determination of habitat quality. Most studies have inferred the quality of

avian breeding habitats based solely on relative abundance or density (e.g., Whitmore

1977, James and Warner 1982, Wenny et al. 1993), but this approach is not always

reliable and may misrepresent suitability of habitats for breeding (Van Horne 1983,

Maurer 1986, Martin 1992). While theoretical literature strongly advocates use of

direct measures of fitness (i.e., reproduction and survival) in identifying important

breeding habitats (e.g., Van Horne 1983, Martin 1992), relatively few studies of

habitat quality have been based on demography because these data are time and

energy-intensive.

Discerning patterns in life-history characteristics is best determined from

longitudinal studies, where data are collected over entire lifetimes of marked

individuals, rather than from cross-sectional studies, where data are collected at

specific points in time and from different and unknown individuals (Newton 1989).

Longtitudinal data are necessary to distinguish effects caused by individual attributes

such as age and condition (Coulson 1966, Nol and Smith 1987, Ens et al. 1992), and

stochastic environmental variation such as extreme weather phenomena (Grant 1986,

Van Horne et al. 1997).

The overall objectives of my long-term (1981-1999) investigation on

4

Flammulated Owls (Otus flammeolus) were to determine the life history

characteristics and the components of habitat quality for a population in central

Colorado. The owl breeds in montane forests from the Rocky Mountains to the

Pacific Coast, and from southern British Columbia to Vera cruz, Mexico (McCallum

1994, Linkhart et al. 1998). Determination of the life history of Flammulated Owls is

interesting because, while the owl’s life history might be expected to be similar to

that of other raptors (e.g., low fecundity and high survival), it is one of the smallest

North American owls and annually migrates the greatest distance between summer

and winter grounds (Johnsgard 1988). The owl’s population dynamics are poorly

known anywhere in their range. These data are sorely needed because the owls are

listed as sensitive and vulnerable by the United States (USDA Forest Service; Verner

1994) and Canada (van Woudenberg 1992), primarily because they are obligate

cavity nesters and because densities have declined following timber harvests

(Marshall 1957, 1988, Phillips et al. 1964, Franzreb and Ohmart 1978). While the

owl’s habitat has been described in mostly anecdotal reports (e.g., Phillips et al. 1964,

Bull and Anderson 1978; but see Linkhart and Reynolds 1997, Linkhart et al. 1998), a

clear understanding of its habitat requirements necessitates more intensive study.

In Chapter 2, my objective was to evaluate the demography of Flammulated

Owls over the 19 yr study. Specifically, I determined the density of breeding and

bachelor (unpaired) males, reproductive parameters such as productivity and nesting

success, longevity, and survival of owls associated with 14 owl territories on 511 ha.

On the basis of these data, I also assessed the owl’s life history strategy. Compared to

5

other, larger raptors, I predicted that Flammulated Owls would show more annual

variation in density, higher annual recruitment, and shorter longevity.

In Chapter 3, my objective was to determine LRS of male and female adult

owls over 19 yr. LRS is little studied in long-lived birds including raptors, and sexual

differences in LRS have only been assessed for three species. Specifically, I

compared LRS within and between sexes, investigated life-history attributes that may

have important influences on LRS, and compared the reproductive strategy of

Flammulated Owls to other raptors. Compared to other, larger raptors, I predicted

that Flammulated Owls would have a shorter reproductive lifespan, and produce more

offspring over their lifetimes.

In Chapter 4, my objective was to quantify territory and mate fidelity and

dispersal of owls. Despite their importance in affecting population dynamics and in

developing conservation plans, these life history characteristics are poorly understood

in birds because study generally requires long time periods and large areas of

investigation (Paradis et al. 1998, Forero et al. 1999). Specifically, I describe: (1)

sex differences in return rates to the study area; (2) sex differences in territory

fidelity, and I evaluate effects of previous nest failure, breeding status (paired vs

unpaired), and return of previous mate on fidelity; (3) patterns in mate fidelity and

apparent benefits of maintaining pair bonds; and (4) sex differences in breeding

dispersal. I also tested six possible correlates of breeding dispersal.

Finally, in Chapter 5 my objective was to identify characteristics of high-

quality breeding habitats, by correlating habitat characteristics on territories with

long-term demographic parameters. Specifically, I: (1) describe demographic

6

performance (territory occupancy, reproductive success, territory tenure, pair

duration, and breeding dispersal) on territories, and identify variables that

distinguished among territories; and (2) identify components of habitat quality by

correlating habitat variables with demographic performance on territories. I

evaluated habitat quality based on two questions: (a) Across territories, was

demographic performance associated with forest type and structure (e.g., tree density,

basal area, and crown volume)? (b) Does forest structure differ among territories and

between territory and non-territory (i.e., unoccupied) habitat? I predicted that

reproductive success could be positively associated with area in ponderosa

pine/Douglas-fir, a forest type and structure with which Flammulated Owls have been

associated in other studies (McCallum 1994). I also evaluated three possible limiting

factors associated with the owl’s habitat relationships, and predicted that highest-

quality territories were characterized by highest densities of cavity trees, lowest rates

of nest predation, and greatest prey abundance.

7

LITERATURE CITED

Bosque, C., and M. T. Bosque. 1995. Nest predation as a selective factor in the evolution of developmental rates in altricial birds. Amer. Natural. 145:234-260.

Bull, E. L., and E. G. Anderson. 1978. Notes on Flammulated Owls in northeastern

Oregon. Murrelet 59:26-27. Bunzel, M. and J. Druke. 1989. Kingfisher. Pages 107-117 in Lifetime reproduction

in birds (I. Newton, Ed.). Academic Press, San Diego, California. Coulson, J. C. 1966. The influence of the pair-bond and age on the breeding biology

of the kittiwake gull Rissa tridactyla. J. Anim. Ecol. 35:269-279. Ens, B. J., M Kersten, A Brenninkmeijer, and J. B. Hulscher. 1992. Territory

quality, parental effort and reproductive success of Oystercatchers (Haimatopus ostralegus). J. Anim. Ecol. 61:703-715.

Forero, M. G., J. A. Donozar, J. Blas, and F. Hiraldo. 1999. Causes and

consequences of territory change and breeding dispersal distance in the Black Kite. Ecol. 80: 1298-1310.

Forsman, E.D., S. Destefano, M. G. Raphael, and R. J. Gutierrez. 1996.

Demography of the Northern Spotted Owl. Studies in Avian Biol. No. 17, Cooper Ornithol. Soc.

Franzreb, K. E., and R. D. Ohmart. 1978. The effects of timber harvesting on

breeding birds in a mixed-coniferous forest. Condor 80:431-441. Gavin, T. A., and E. K. Bollinger. 1988. Reproductive correlates of breeding-site

fidelity in Bobolinks (Dolichonyx oryzivorus). Ecol. 69:96-103. Gill, F. B. 1995. Ornithology. W. H. Freeman and Co., New York. 2nd Ed. 766 pp. Goodburn, S. F. 1991. Territory quality or bird quality? Factors determining

breeding success in the Magpie Pica pica. Ibis 133:85-90. Grant, P. R. 1986. Ecology and evolution of Darwin’s Finches. Princeton Univ.

Press, Princeton, New Jersey. Greenwood, P. J. 1980. Mating systems, philopatry, and dispersal in birds and

mammals. Anim. Behav. 28:1140-1162. Greenwood, P. J., and P. H. Harvey. 1982. The natal and breeding dispersal of birds.

Ann. Rev. of Ecol. and Syst. 13:1-21.

8

Haas, C. 1998. Effects of prior nesting success on site fidelity and breeding

dispersal: an experimental approach. Auk 115:929-936. James, F. C., and N. O. Warner. 1982. Relationships between temperate forest bird

communities and vegetation structure. Ecol. 63:159-171. Johnson, M. L., and M. S. Gaines. 1990. Evolution of dispersal: theoretical models

and empirical test using birds and mammals. Ann. Rev. Ecol. Syst. 21:449-480.

Johnsgard, P. A. 1988. North American owls: Biology and natural history.

Smithsonian Institution Press, Washington, D.C. Korpimaki, E. 1988. Effects of territory quality on occupancy, breeding performance

and breding dispersal in Tengmalm’s Owl. J. Anim. Ecol. 57:97-108. Lack, D. 1954. The natural regulation of animal numbers. Oxford, University Press. Linkhart, B. D. and R. T. Reynolds. 1997. Territories of Flammulated Owls: Is

occupancy a measure of habitat quality? Pp. 250-254 in Biology and conservation of owls of the northern hemisphere (J. R. Duncan, D. H. Johnson, and T. H. Nichols, Eds.). USDA Forest Serv. Gen Tech. Rep. NC-190.

Linkhart, B. D., R. T. Reynolds, and R. A. Ryder. 1998. Home range and habitat of

breeding Flammulated Owls in Colorado. Wilson Bull.. 110:342-351. Marshall, J. T., Jr. 1957. Birds of pine-oak woodland in southern Arizona and

adjacent Mexico. Pacif. Coast Avif. 32:1-125. Marshall, J. T., Jr. 1988. Birds lost from a giant sequoia forest during fifty years.

Condor 90:359-372. Martin, T. E. 1988. Processes organizing open-nesting bird assemblages:

competition or nest predation. Evol. Ecol. 2:37-50. Martin, T. E. 1992. Breeding productivity considerations: what are the appropriate

habitat features for management? Pages 455-473 in Ecology and conservation of Neoptropical migrant landbirds (J. M. Hagan III and D. W. Johnston, eds.). Smithsonian Institution Press, Washington, D.C.

Maurer, B. A. 1986. Predicting habitat quality for grassland birds using density-

habitat correlation. J. Wildl. Manage. 50:556-566.

9

McCallum, A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North America, No. 93 (A. Poole and F. Gill, eds.). Academy of Natural Sciences of Philadelphia; American Ornithologists’ Union, Washington, D.C.

Moreau, R. E. 1944. Clutch size: a comparative study, with references to African

birds. Ibis 86:286-347. Newton, I. 1979. Population ecology of raptors. T & A D Poyser Ltd., London. 399

pp. Newton, I. 1989. Lifetime reproduction in birds. Academic Press, San Diego,

California. Newton, I. 1995. The contribution of recent research on birds to ecological

understanding. J. Anim. Ecol. 64:675-696. Newton, I. 1998. Population limitation in birds. Academic Press, N.Y. 597 pp. Newton, I., and M. Marquiss. 1982. Fidelity to breeding area and mate in

Sparrowhawks Accipiter nisus. J. Anim. Ecol. 51:327-341. Nol, E., and J. N. M. Smith. 1987. Effects of age and breeding experience on

seasonal reproductive success in the Song Sparrow. J. Anim. Ecol. 56:301-313.

Owen, M., and J. M. Black. 1989. Barnacle Goose. Pages 349-362 in Lifetime

reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, CA. Paradis, E., S. R. Baillie, W. J. Sutherland, and R. D. Gregory. 1998. Patterns of

natal and breeding dispersal in birds. J. Anim. Ecol. 67:518-536. Phillips, A. R., J. T. Marshall, and G. Monson. 1964. The birds of Arizona. Univ.

Ariz. Press, Tucson. 212 pp. Ricklefs, R. E. 2000. Density dependence, evolutionary optimization, and

diversification of avian life histories. Condor 102:9-22. Stearns, S. C. 1992. The evolution of life histories. Oxford Univ. Press, New York.

249 pp. Van Horne, B. 1983. Density as a misleading indicator of habitat quality. J. Wildl.

Manage. 47:893-901. Van Horne, B., G. S. Olson, R. L. Schooley, J. G. Corn, and K. P. Burnham. 1997.

Effects of drought and prolonged winter on Townsend’s Ground Squirrel demography in shrubsteppe habitats. Ecol. Monogr. 67:295-315.

10

Verner, J. 1994. Current management situation: Flammulated Owls. Pages 10-13 in

Flammulated, Boreal, and Great Gray Owls in the United States: a technical conservation assessment (G. D. Hayward and J. Verner, eds.). USDA Forest Serv. Gen. Tech. Rep. RM-253.

Wenny, D. G., R. L. Claswon, J. Faaborg, and S. L. Sheriff. 1993. Population

density, habitat selection, and minimum area requirements of three forest-interior Warblers in central Missouri. Condor 95:968-979.

Whitmore, R. C. 1977. Habitat partitioning in a community of passerine birds.

Wilson Bull. 89:253-265. Woolfenden, G. E., and J. W. Fitzpatrick. 1984. The Florida Scrub Jay:

demography of a cooperative-breeding bird. Monogr. Pop. Biol. 20, Princeton Univ. Press, Princeton, NJ.

van Woudenberg, A. M. 1992. Integrated management of Flammulated Owl

breeding habitat and timber harvest in British Columbia. Masters thesis, Univ. British Columbia, Vancouver.

11

CHAPTER II

DEMOGRAPHY OF FLAMMULATED OWLS IN COLORADO

Abstract. - I investigated the demography of Flammulated Owls (Otus flammeolus) in

Colorado from 1981-1999. Fourteen territories occurred on the 511 ha study area

during the 19 yr study, most of which were constant in time and space despite

periodic turnover of individuals on territories. Mean (+ SE) annual density was 0.9 +

0.1 breeding pairs 100 ha-1 and 0.7 + 0.1 bachelor (unpaired) males 100 ha-1.

Numbers of territories occupied by breeding pairs annually was relatively constant,

totaling 4 – 5 pairs in 79% of yr. Nesting success was high; 82% of 79 nests

successfully fledged > 1 owlet. Most (13 of 14) nesting failures occurred during

incubation due to predation, and failures appeared to be caused by pine squirrel

(Tamiasciurus hudsonicus) predation. Productivity of pairs varied little during the

study. Mean clutch size was 2.5 + 0.1 eggs (range = 1-3, n = 29) and mean brood size

of successful nests was 2.4 + 0.1 owlets (range = 1-4, n = 68). Total annual

production of all pairs on the study area was 9.6 + 0.5 eggs and 8.2 + 0.5 fledglings.

Reproductive success was not correlated with breeding experience; dates of initiation

of incubation and brood size were not different for either males or females breeding

for the first time on the study area vs those that bred > 2 yr. Total number of yr

banded males returned to the study area was significantly greater than for females

(3.2 + 0.6 yr vs 2.0 + 0.3 yr). Annual return rate was greater for males (0.59; 17 of

12

29) than for females (0.37; 14 of 38); either adult males had higher survival rates than

females or females more often dispersed from the study area. Reproduction and

survival indicate that Flammulated Owls have a life history strategy more typical of

larger birds, and relatively constant densities of breeding pairs suggest the owls bred

in a stable environment.

13

INTRODUCTION

Birds have diverse life histories, resulting from the cumulative evolutionary

effect of many abiotic and biotic factors whose relative importance is still debated

(Moreau 1944, Lack 1954, Ricklefs 2000). Despite wide variance among species,

studies of demography have revealed several patterns in life histories. Large species,

including many raptors, are generally long-lived and have low fecundity while small

species are typically short-lived and have high fecundity (Newton 1979, Johnsgard

1988, Ricklefs 2000). In addition, mortality rates generally decline with age, with

adults surviving at higher rates than juveniles (e.g., Perrins and Geer 1980,

Woolfenden and Fitzpatrick 1984), while nesting success and annual number of

young produced generally increases with age (e.g., Curio 1983, Nol and Smith 1987).

Life histories have strong influence on avian population dynamics. Large,

long-lived species, such as Northern Goshawks (Accipiter gentilis; Reynolds et al., in

prep.) and Northern Spotted Owls (Strix occidentalis; Forsman et al. 1996), tend to

have relatively small fluctuations in annual population densities in response to short-

term environmental oscillations because overlapping generations result in low

turnover rates by individuals on territories. Populations of these species also contain

a relatively large non-breeding segment, typically consisting of immature individuals,

which buffer the population against environmental fluctuations (Forsman et al. 1996).

In contrast, small, short-lived species, such as the Blue Tit (Parus caeruleus; Dhondt

1989), show high turnover rates of individuals on territories due to little overlap in

generations, resulting in large annual fluctuations in population densities. However,

because of high fecundity, populations of small, short-lived species can rebound more

14

quickly than large, long-lived species (Newton 1998).

I studied the demography of a Flammulated Owl population (Otus

flammeolus) in Colorado from 1981-1999 using capture-recapture methods. This

insectivorous owl is a Neotropical migrant that breeds in montane forests of western

North America as far north as southern British Columbia and winters as far south as

El Salvador (McCallum 1994, Linkhart et al. 1998). The owl’s population dynamics

are poorly known anywhere in its range. These data are sorely needed because the

owl is listed as sensitive and vulnerable by the United States (USDA Forest Service;

Verner 1994) and Canada (van Woudenberg 1992), primarily because they are

obligate cavity nesters and because densities have declined following timber harvests

(Marshall 1957, 1988, Phillips et al. 1964, Franzreb and Ohmart 1978).

Here I report data on the density, reproduction, longevity, and survival of

adult Flammulated Owls. I predicted that Flammulated Owls, which are the second-

smallest North American Owl (Johnsgard 1988), should have higher annual variance

in density, higher annual recruitment, and shorter longevity than other, larger raptors.

Elsewhere I report on other life history characteristics, including lifetime

reproduction (Chapter 3), territory and mate fidelity and dispersal (Chapter 4), and the

determination of habitat quality based on long-term demography (Chapter 5).

METHODS Study Area

The study was conducted on the Manitou Experimental Forest in Teller Co.,

Colorado. I established boundaries of the 511 ha study area after initial surveys

(1980) for territorial Flammulated Owls. After I confirmed the presence of owls,

boundaries were drawn around a sufficient area to contain approximately 20

15

territorial males, based on an estimate of territory size for this species (274 m in

diameter; Marshall 1939). Forests within the study area consist of (1) ponderosa pine

(Pinus ponderosa) mixed with Douglas-fir (Pseudotsuga menziesii), generally on

ridgetops and south- and west-facing slopes, (2) quaking aspen (Populus tremuloides)

stands on lower slopes and bottoms of moist drainages, (3) quaking aspen stands

mixed with blue spruce (Picea pungens) in bottoms, lower slopes, and benches in

mesic areas, and (4) Douglas-fir mixed with blue spruce on higher slopes and on

north-facing slopes. The area has been characterized by an absence of management

activities since the 1880’s, when there was a single-tree selection cut for railroad ties

(Reynolds et al. 1985). Snags and trees with cavities are relatively abundant

throughout the study area (Reynolds et al. 1985). The forest understory, consisting of

over 100 species of grasses, forbs, and shrubs, is poorly developed in all but the moist

creek bottoms (Reynolds et al. 1985). Terrain is moderately steep (20-80% slope)

and elevations ranged from 2,550-2,855 m.

Territory Occupancy and Population Density

Each spring and summer from 1981-1999, I systematically searched the entire

study area for territorial males (Reynolds and Linkhart 1984). I identified territory

boundaries by marking territorial song-trees of males (Reynolds and Linkhart 1984)

and by monitoring movements with radio-telemetry (1982-1983) which assisted in

determining range in territory sizes and identifying topographic characteristics of

boundaries (Linkhart et al. 1998). Boundaries of territories changed little from year

to year (Fig. 1), despite turnover of males on territories (Chapter 5). Replacement

males often sang from the same tree or groups of trees throughout their territories and

16

Figure 1. Location of owl territories (black polygons) on the Manitou Experimental

Forest study area (white boundary), 1981-1999. Heavy white polygons

represent territories (A15 and A24) not occupied after 1984 and light white

polygons represent territories (A4, A8, A11, and A29) prior to boundary shifts.

17

along boundaries that adjoined neighboring territories (Linkhart 1984, Linkhart et al.

1998). I may have missed some annual fluctuation of territory boundaries, especially

where there were no adjacent neighbors or when adjacent territories were not

occupied. However, lack of change in most boundaries indicated that territories were

relatively constant in time and space.

I mapped all suitable nest cavities (tree cavities with entrance diameters > 4

cm) within territories and checked each for nesting owls each year (Reynolds and

Linkhart 1984). Singing by nesting males dramatically declined after egg-hatch,

whereas bachelor (unpaired) males typically sang throughout a breeding season

(Reynolds and Linkhart 1987). Due to seasonal nest searching efforts, I was

confident that all nests were located each year. I determined annual density of

breeding pairs and bachelor males each yr by converting each to the frequency per

100 ha.

Nest Success and Reproduction

I found most nests during incubation (late May and early June) and I checked

the status of nests at least weekly (often two or three times per week) until the young

fledged (mid July). Breeding adults were captured at nests (occasionally on day

roosts) and banded with U. S. Fish and Wildlife Service leg bands (Reynolds and

Linkhart 1984). Sex of adults was determined by behavior at nests and weight (see

Reynolds and Linkhart 1984). I banded owlets at 14-24 days after hatching (fledging

occurs at 22-24 days; Reynolds and Linkhart 1987). A total of 215 Flammulated

Owls were banded on the study area from 1981-1999: 146 owlets (sex not

18

determined) and 69 adults (29 males and 40 females). Because bachelor males were

difficult to capture, all banded males but one were owls that bred at least once.

I defined a nesting attempt as a nest in which > 1 egg was laid, and a

successful nest as a nest that fledged > 1 owlet. I determined annual nesting success

by dividing total number of successful nests in a year by total nesting attempts in that

year. Total nesting success was the total number of successful nests over all years

divided by total nest attempts over all years. Number of eggs per clutch was

determined during egg-laying or incubation, and number of owlets per brood was

determined at the time owlets were banded. To determine total annual production of

eggs and owlets, each year I summed eggs and owlets from all clutches and broods.

For calculating nesting success, mean clutch and brood sizes, and mean annual

production of eggs and owlets, I excluded 6 nests whose outcome was uncertain (two

in 1981 and one in 1990) or failed due to human disturbance (one each in 1984, 1991,

and 1994).

To determine if breeding experience influenced the timing of breeding or

number of young produced, I compared mean dates of initiation of incubation and

mean owlets per brood for males and females breeding for the first time on the study

area vs males and females that bred two or more years on the study area. I could not

determine if males or females bred previously before their tenure on the study area.

Initiation of incubation was determined by observations of females incubating eggs,

and by back-dating from hatching (mean duration of incubation was 22 d; Reynolds

and Linkhart 1987) for nests discovered after females initiated incubation.

Return Rate, Longevity, and Survival

19

Annual return rate to the study area, longevity, and survival were determined

from capture-recapture of adults banded from 1981-1999. I determined sex-specific

return rates to the study area by dividing the number of banded males and females

that were recaptured for > 2 consecutive yr by total number of banded owls

(excluding one male and two females banded in 1999). Longevity was conservatively

estimated by summing total yr of return to the study area. I assumed that individuals

died if they failed to return to the study area. Some individuals, especially females,

may have dispersed from the study area rather than died, since females had higher

rates of breeding dispersal (0.22) on the study area than males (0.02; Chapter 4).

To estimate adult survival, I divided the number of adults captured or

recaptured in yr X that returned to breed in yr X + 1 by the total number of adults

captured or recaptured in yr X. I also plotted an approximate survival curve for each

sex following Murphy (1996), in which I treated the year an owl was banded as yr 0

and converted the number of adults banded each yr to an initial population of 1000. I

then averaged the number of owls in each cohort (1981-1998) that returned in the first

yr after banding (= yr 1) to determine survival over the first yr of banding. To

determine survival from yr 1 to yr 2, I excluded those owls banded in 1998 because

they did not yet have a second yr of potential survival, and averaged the number of

owls alive for each cohort banded in 1981-1997. For each subsequent yr, I followed

the same procedure for estimating survival. This procedure potentially

underestimates survival because it does not account for recapture efficiency, the

probability of recapturing individuals (Martin et al. 1995). Because my sample sizes

were insufficient for calculating recapture probability, I could not use capture-

20

recapture models of survival such as those used in program MARK (Lebreton et al.

1992).

When calculating return rate, longevity, and survival, I assumed that a male

banded on a territory in yr X was present on the same territory in yr X + 1, despite

being uncaptured in yr X + 1, if: (1) a male was heard singing on the territory in yr

X + 1, and (2) the same male was recaptured in a subsequent yr on the same territory.

This assumption was supported by the fact that I noted no instances of males leaving

a territory for > 1 yr and returning in a subsequent yr. No unpaired females were

detected on territories in any yr, and I detected no instances in which a female failed

to nest in one yr but nested on the same territory in the previous yr and subsequent yr.

Therefore, I presumed a banded female that nested in yr X on a territory had not

returned to the same territory in yr X + 1 if no nest was found. Because I could not

distinguish between mortality and dispersal away from the study area, estimates of

longevity and survivorship are conservative.

Estimates of longevity and survival for adults banded in 1981 with unknown

histories and adults banded and/or recaptured in 1999 with unknown futures (‘fringe’

adults) did not significantly differ from adults whose breeding lifespans began after

1981 and terminated before 1999 (‘inclusive’ adults). Mean (+ SE) total yr of return

for ‘fringe’ males (n = 4 in 1981 and 5 in 1999) was 3.9 + 1.3 yr, and mean total yr of

return for ‘inclusive’ males was 2.6 + 0.5 yr (n = 22; t = -0.94, df = 11, P = 0.37, two-

tailed). Likewise, mean total yr of return for ‘fringe’ females (n = 5 in 1981 and 5 in

1999) was 2.4 + 0.6 yr and mean total yr of return for ‘inclusive’ females was 1.7 +

0.3 yr (n = 31; t = -1.03, df = 14, P = 0.32, two-sided). Consequently, to maximize

21

sample sizes I included the 9 adults banded in 1981 and the 10 adults banded and/or

recaptured in 1999 for calculating estimates of longevity and survival.

Statistical analyses were performed using Statistical Analysis System (SAS

Institute 1995). I used unpaired t-tests (two-tailed) assuming unequal variances

(PROC TTEST) to determine effect of breeding experience on mean date of initiation

of incubation and mean brood size. I used Wilcoxon’s test (PROC NPAR1WAY) to

evaluate sex differences in longevity, and Fisher’s exact test (PROC FREQ) to

evaluate sex differences in annual return rates to the study area. Throughout the

paper I report means + SE. Analyses were considered significant if P < 0.05.

RESULTS

Territory Occupancy

Fourteen owl territories were located on the 511 ha study area between 1981

and 1999 (Fig. 1). With a few exceptions, territories were generally fixed in time and

space despite individual turnover on each territory. First, territory A24 was only

occupied from 1981-1983. In 1984, a new male in an adjacent territory (A29)

expanded his territory to include the western portion of territory A24. The new

boundaries of A29 did not change over the remainder of the study (Fig. 1). Second,

the male in territory A4 expanded his territory in 1983 to include much of the eastern

portion of territory A15, which was only occupied in 1981-1982 and 1984 (Linkhart

et al. 1998; Fig. 1). I did not determine the boundaries of either A4 or A15 in 1984,

when only A15 territory was occupied by a breeding pair, but for the remainder of the

study (1984-1999), territory A4 contained the eastern half of territory A15. Finally,

in 1988 the male in territory A8 expanded his territory north into the southern portion

22

of A11 territory when the male in territory A11 did not return from migration, and

northeast into the northern portion of territory A15 (Fig. 1). Beginning in 1989,

territory A11 contained only the northern portion of the original territory A11 (Fig.

1). In each of the above instances, shifts in territory boundaries occurred when males

in adjacent territories did not return from spring migration. Boundaries of all other

territories remained unchanged over the study. Because males had high territory

fidelity (98%; Chapter 4), newly arriving males in the spring typically filled

geographic voids left by predecessors.

Population Density

Of the total 14 owl territories on the study area, males annually occupied a

mean 8.1 + 0.5 (58 + 4%) territories, ranging from 4 territories (29%) in 1986 to 13

territories (93%) in 1995. Mean annual density of territorial males was 1.7 + 0.1 100

ha-1. However, annual density of breeding pairs was 0.9 + 0.1 100 ha-1, reflecting the

fact that breeding pairs only occupied a mean 4.5 + 0.2 territories annually (32 + 3%;

Fig. 2). Nonetheless, frequency of territories occupied by breeding pairs each yr

remained relatively constant, between 3 and 6 territories, and between 4 and 5

territories in 79% of all yr (15 of 19; Fig. 2). Annual density of bachelor males was

0.7 + 0.1 100 ha-1, reflecting the fact that bachelor males occupied a mean 3.6 + 0.5

territories annually (26 + 3%; Fig. 2). While mean density of bachelor males was

similar to mean density of breeding pairs, frequency of territories annually occupied

by bachelor males was more variable, ranging from one in 1986 to eight in 1995 (Fig.

2).

23

Figure 2. Number of territories occupied annually by bachelor (unpaired) males

(light bars) and breeding pairs (dark bars), 1981-1999.

0

1

2

3

4

5

6

7

8

9

81 83 85 87 89 91 93 95 97 99

Year

No.

terr

itorie

s ye

ar-1

24

Nest Success and Reproduction

Annual Nesting success.—I recorded 85 total nesting attempts. There were no second

nesting attempts, even when nests failed in early incubation. Thus, mean total nesting

attempts per yr (4.5 + 0.2, range 3 - 6) was identical to mean total breeding pairs per

yr. Mean total successful nests was 3.4 + 0.2 annually (n = 79), ranging from 2 in

1993 and 1994 to 5 in 1982 (Fig. 3). Nesting success was high; 82% (65 of 79; 6

nesting attempts were excluded because I was uncertain whether young fledged [3],

or they failed due to human disturbance [3]) of all nests successfully fledged young.

Nesting success was 100% in 8 yr, 75-99% in 8 yr, 50-74% in 2 yr, and < 50% in 1

yr. A total of 14 nesting failures occurred in 9 of the 14 territories. One failure was

caused by predation of the female during late incubation. The remaining failures

were caused by predation on eggs or owlets during incubation or early nestling

periods. Although predators responsible for nesting failures were unknown, pine

squirrels (Tamiasciurus hudsonicus) were the most likely predator as few other

predaceous mammals (excepting one bushytail woodrat, Neotoma cinerea; also see

Linkhart and Reynolds 1994), and no predaceous birds, were ever observed in tree

cavities.

Reproduction.—Mean number of eggs per clutch and owlets per brood varied little

over the study. Mean clutch size over all yr was 2.5 + 0.1 eggs (n = 29; Fig. 4) and

all clutches contained two or three eggs. Combining successful and unsuccessful

nests, mean brood size was 2.0 + 0.6 owlets (n = 79), ranging from 0.6 owlets in 1993

to 3.0 owlets in 1990, while only successful nests over all yr contained 2.4 + 0.1

owlets (n = 65), ranging from 1.5 owlets in 1990 to 3.0 owlets in 1990 (Fig. 4). Other

25

Figure 3. Number of nesting attempts (dark bars) and successful nests (light bars) per

yr on 14 territories, 1981-1999 (n = 79; these data exclude 3 nests whose

outcome was uncertain [two in 1981 and one in 1990] and 3 nests that failed

due to human disturbance [one each in 1984, 1991, and 1994]).

0

1

2

3

4

5

6

81 83 85 87 89 91 93 95 97 99

Year

Tot

al n

estin

g at

tem

pts

or s

ucce

ssfu

l nes

ts

26

Figure 4. Annual mean number of eggs per clutch (dark bars; n = 29), number of

owlets per brood (white bars; n = 79), and number of owlets per successful

brood (stippled bars; n = 65), 1981-1999. These data exclude 3 nests whose

outcome was uncertain (two in 1981 and one in 1990) and 3 nests that failed

due to human disturbance (one each in 1984, 1991, and 1994).

Year

Mea

n cl

utch

or

bro

od s

ize

0

0.5

1

1.5

2

2.5

3

3.5

4

81 83 85 87 89 91 93 95 97 99

27

than one brood with one owlet and one brood with four owlets, all broods contained

two or three owlets. Similarity between mean clutch size and mean brood size

reflected high survival of eggs; 79% of all eggs (57 of 72; n = 29 clutches) over the

study survived to hatching, a value which was similar to nesting success because

predation, when it occurred, always resulted in the loss of entire clutches. Survival of

owlets was even higher; 95% of owlets (144 of 151; n = 61 broods) survived from

egg-hatching to fledging. Because no nests failed after the midpoint (day 12) of the

nestling period (22-24 d; Reynolds et al. 1987), mean number of fledglings per brood

was identical to mean number of banding-age owlets per brood. Survival of

fledglings prior to fall migration was unknown, but was probably low based on the

fact that 4 of 8 (50%) fledglings in four radio-tagged broods were killed by predators

in the first six weeks after they fledged in 1982-83 (Linkhart 1984).

Total eggs and fledglings produced annually by all breeding pairs were nearly

equivalent; owls annually produced a mean total 9.6 + 0.5 eggs and 8.2 + 0.5

fledglings (Fig. 5). Range in total eggs produced on the study area annually was

attributable to variance in number of breeding pairs since range in clutch sizes was

small; the fewest total eggs (5) were produced in 1986 when there were 3 breeding

pairs and the most total eggs (13) were produced in 1999 when there were 5 breeding

pairs. Difference between total eggs and total fledglings produced annually was

attributable to nest predation; the fewest fledglings (3) were produced in 1993 when 3

of 5 nests were lost to predators and the most fledglings (11) were produced in 1982

and 1985 when all nests were successful.

Effect of Breeding Experience on Reproductive Success—In many birds, increased

28

Figure 5. Total eggs (dark bars) and fledglings (light bars) produced in all territories

annually from 1981-1999. These data exclude 3 nests whose outcome was

uncertain (two in 1981 and one in 1990) and 3 nests that failed due to human

disturbance (one each in 1984, 1991, and 1994).

Year

Tot

al e

ggs

or fl

edg

ed o

wle

ts

0

2

4

6

8

10

12

14

81 83 85 87 89 91 93 95 97 99

29

breeding experience of adults is associated with earlier egg-laying and increased sizes

of clutches and broods (Pietiainen 1988). I assessed effect of breeding experience of

male and female Flammulated Owls on date of initiation of incubation and brood size.

For males breeding for the first time on the study area, incubation began 1 d later than

males with > 2 yr breeding experience (5 June + 2 d [n = 14, range 26 May-16 June],

vs 4 June + 1 d [n = 28, range 19 May –29 June]), but this difference was not

significant (t = 0.36, df = 24, P = 0.72). Similarly, females breeding for the first time

initiated incubation 2 d later than females with > 2 yr breeding experience (5 June + 2

d [n = 21, range 27 May-29 June], vs 3 June + 2 d [n = 21, range 19 May-24 June]),

but the difference also was not significant (t = 0.94, df = 40, P = 0.35). Breeding

experience was not associated with productivity for either sex. Males breeding for

the first time on the study area had a mean brood size similar to males with > 2 yr

breeding experience (2.3 + 0.2 owlets [n = 16, range 0-3], vs 2.2 + 0.2 owlets [n = 35,

range 0-4]; t = 0.40, df = 49, P = 0.69). Likewise, females breeding for the first time

had mean brood size similar to females with > 2 yr breeding experience (2.1 + 0.2

owlets [n = 23, range 0-3], vs 2.3 + 0.2 owlets [n = 28, range 0-4]; t = -0.72, df = 49,

P = 0.47). I could not determine if male or female owls bred previously before their

tenure on the study area.

Return Rate, Longevity, and Survival

For sexes combined, rate of return to the study area for > 2 consecutive yr was

0.46 (31 of 67). Return rate was greater for males (0.59; 17 of 29) than for females

(0.37; 14 of 38), but not significantly so (Fisher’s exact test, P = 0.09). Only two

30

owls (both females) returned to breed on the study area after each was undetected for

one breeding season (one in 1998 and 1999). Longevity, estimated by total yr of

return to the study area, was 2.5 + 0.3 yr for all adult owls (includes all breeders and

one bachelor male). However, mean total yr of return was greater for males (3.2 +

0.6 yr, n = 30) than for females (2.0 + 0.3 yr, n = 41; z = 2.0, P = 0.04; Fig. 6). Forty

percent of males returned for > 3 yr to the study area compared to just 18% of

females. Maximum yr of return was 12 yr for males and 8 yr for females. Because

age could not be determined when adults were first banded, total yr of return

represent minimum estimates of longevity. I documented only one instance of natal

dispersal; a male hatched in 1981 was found breeding on another territory 1.4 km

distant from 1987-1989 (Chapter 4). Age of this male was 8 yr, 1 mo in 1989.

Annual frequency of return likely provides a good basis for estimating

survival rates for male Flammulated Owls, given their high territory fidelity (98%;

Chapter 4). At 3 yr after banding, 32% of males (n = 30) were estimated to be alive,

and at 6 yr after banding, 15% of males were estimated to be alive (Fig. 7). At 10 yr

after banding, 11% of males were estimated to be alive, although this estimate was

based on just 2 males (Fig. 7). Annual frequency of return may underestimate

survival of females compared to males, due to their higher rates of breeding dispersal

(0.22 vs 0.02 for males; Chapter 4). Compared to males, survival estimates for

females (n = 41) were lower for all yr after banding; just 12% of females were

estimated to be alive 3 yr after banding (Fig. 7).

31

Figure 6. Total years of return to the study area by male (light bars, n = 30) and

female (dark bars, n = 41) owls. All owls were banded as adults at unknown

ages.

0

20

40

60

80

100

1 2 3 4 5 6 7 8 9 10 11 12

Years

Per

cent

of t

otal

mal

es o

r fe

mal

es

32

Figure 7. Estimated survival curves for male (solid line, n = 30) and female (dashed

line, n = 41) owls. All owls (except for one male )were banded as adults at

unknown ages.

Years since first banded

No.

indi

vidu

als

surv

ivin

g

0

200

400

600

800

1000

0 1 2 3 4 5 6 7 8 9 10 11 12

33

DISCUSSION

Life History

Several demographic characteristics indicate that Flammulated Owls, which

are the second-smallest North American strigiform (Johnsgard 1988), have a life

history more typical of larger birds, which are long-lived and have low fecundity

(Newton 1998). First, annual reproduction of Flammulated Owls is among the lowest

and least variable of North American and European strigiforms. In this study, mean

clutch size was 2.5 + 0.1 eggs and mean brood size (successful and unsuccessful) was

2.0 + 0.1 owlets. Size of clutches and broods was typically 2 or 3; I only observed

one brood of 1 owlet and one brood of 4 owlets. Only two other owls, the Great

Horned Owl (Bubo virginianus; 1300-1700 g) and Barred Owl (Strix varia; 600-800

g), much larger than Flammulated Owls (50-66 g; Reynolds and Linkhart 1987), have

a smaller clutch size (-x = 2.4 eggs/clutch for both species; Murray 1976, Johnson

1978). Second, at least 75% of nests were successful in 16 of 19 years for an overall

nesting success rate of 82% over the study. Among North American strigiforms, only

Spotted Owls (S. occidentalis) have a higher nesting success (85%; Forsman et al.

1984, Johnsgard 1988). Third, I never observed replacement clutches or multiple

broods in Flammulated Owls even when nests failed early in the breeding season.

Although multiple broods are relatively uncommon among raptors (but see Marti

1997), several small and some medium-sized species in temperate regions may lay

replacement clutches if their first clutch is lost in early incubation (Newton 1979,

Johnsgard 1988). Finally, while longevity is not known for most North American

34

strigiforms, male Flammulated Owls have greater maximum longevity (12+ yr) than

other small (< 150 g) owls and a longevity comparable to many larger owls (Glutz

and Bauer 1980, Clapp et al. 1983, Klimkiewicz and Futcher 1989, Klimkiewicz,

pers. comm). Coupled with low turnover rates on territories (Chapter 5), these data

suggest that male Flammulated Owls have relatively high annual survival, despite

their being the most migratory of North American strigiforms (Johnsgard 1988).

Breeding Densities

Density of territories occupied by breeding pairs varied little over the 19 yr

study; breeding pairs occupied either 4 or 5 territories in 79% (15 of 19) of yr. The

relatively constant annual breeding density suggests that the owls in the study area

occupied a relatively stable environment (sensu Pianka 1970). Other long-term

studies of raptors including strigiforms showed stable breeding populations over time

(e.g., Gargett 1977, Korpimaki 1988).

Underlying the relative constancy in annual density of breeding-pairs was the

fact that many of the same territories were reoccupied nearly every year by breeding

pairs. As a consequence of the continuous occupancy, these territories accounted for

the majority of total productivity over the study. Elsewhere I reported that 5 of 12

territories were occupied by breeding pairs > 8 yr (not necessarily consecutive) and

accounted for nearly 70% of total owlets produced on the study area from 1981-1999,

and 3 territories, occupied by breeding pairs for > 11 yr, accounted for 50% of total

owlets produced (Chapter 5). Productivity on territories was higher on territories

with mature, open forests of ponderosa pine/Douglas-fir forests (Chapter 5),

suggesting that availability of this forest type and structure were factors associated

35

with the relative constancy in density and reproduction of Flammulated Owls on the

study area. Other studies inferred that demographic parameters were most constant in

high-quality breeding habitats (e.g., Probst and Hayes 1987, Korpimaki 1988,

Steenhof et al. 1999).

Patterns in demographic characteristics in birds are often closely related to

food abundance. Several studies have found that annual densities of raptor

populations were strongly correlated with prey density (e.g., Craighead and

Craighead 1956,White and Cade 1971, Korpimaki 1988). Reproductive parameters

such as nesting success and clutch size are closely tied to food abundance in several

species of strigiforms (Johnsgard 1988) and other raptors (Steenhof et al. 1999). The

constancy of Flammulated Owl demographic characteristics during the study suggests

that prey generally was a reliable resource. Indeed, long-term owl productivity was

not associated with prey abundance sampled over 2 yr (Chapter 5), suggesting that

prey abundance was not limiting and was not associated with habitat quality.

Sex Differences In Longevity

Greater estimated longevity for male Flammulated Owls suggests a sex bias in

survival, emigration, or both. Mean total yr of return was significantly greater for

males (3.2 + 0.6 yr) than for females (2.0 + 0.3 yr), and 40% of males returned for > 3

yr to the study area compared to just 18% of females. Annual frequency of return

may give a biased estimate of survival because (1) females had higher rates of

detected breeding dispersal within the study area than males, and (2) females had

lower return rates following nest failure than males (Chapter 4). These data suggest

that some of the females that had not returned may have dispersed from the study area

36

(Chapter 4). Nonetheless, the fact that unpaired males annually occupied 10-70% of

the 14 territories strongly suggests a shortage of females. If differences in return rates

for males and females are related to survival, the underlying reasons for these

differences are uncertain. A possible explanation is that the cost of egg production in

female Flammulated Owls, whose clutches represent more of their mass (55-60%;

approximately 30 g) than most other strigiforms (Johnsgard 1988), coupled with

energetic costs of long-distance migration prior to egg-laying, may result in higher

adult female mortality. Other studies of monogamous birds have reported or

suggested a male-biased sex ratio in adults (e.g., Breitwisch 1989 and sources therein,

Burke and Nol 1998, Gibbs and Faaborg 1990, Payne and Payne 1990).

Factors Affecting Reproductive Success

Nest predation is a primary cause of nesting mortality for many bird species

(e.g., Skutch 1949, Ricklefs 1969), and is believed to be an important factor in the

evolution of life-histories (Slagsvold 1982, Sonerud 1985, Martin 1988, Bosque and

Bosque 1995). While nest predation was responsible for reducing nesting success in

certain years (e.g., 1993 and 1999), the high rate of nesting success (82%) over 19 yr

suggests that nest predation may not be a major factor influencing reproductive

success. Elsewhere I reported that, among territories differing in long-term

productivity, predation rates at artificial nests were not significantly different,

indicating that nest predation was not associated with territory quality (Chapter 5).

Contrary to studies that found that reproductive success was correlated with

breeding experience (e.g., Nol and Smith 1987, Pietiainen 1988), male and female

Flammulated Owls with > 2 yr breeding experience did not initiate incubation earlier

37

or have larger broods than males and females breeding for the first time on the study

area. However, if owls bred elsewhere prior to their first breeding on the study area,

then any differences between inexperienced (first-time) or experienced breeders

would have been diluted. Moreover, since adults at first capture could not be aged, I

could not separate the effects of breeding age from the effects of breeding experience.

Age, rather than experience per se, was known to affect reproductive success in

several birds (e.g., Harvey et al. 1979, Curio 1983, Nol and Smith 1987).

Conservation Implications

High-quality breeding habitat for Flammulated Owls in central Colorado was

characterized as mature, open forests of ponderosa pine/Douglas-fir and owls

preferentially foraged in this forest type (Chapter 5; Linkhart et al. 1998). Elsewhere

in its range, the owl has been generally associated with mature conifer forests (see

McCallum 1994), and these forests have been subjected to extensive tree harvesting

over the past several decades. In fact, tree harvesting caused population declines in

Flammulated Owls in some areas (Marshall 1957, 1988, Phillips et al. 1964, Franzreb

and Ohmart 1978). Based on the fact that the Flammulated Owl appears to have a K-

selected life-history strategy, characterized by low rates of reproduction and high

survival (Pianka 1970), these data suggest that the owl may be vulnerable to long-

term habitat changes. K-selected species typically respond slowly to environmental

perturbations because of their low fecundity and low density (Newton 1998). In order

to understand effects of habitat changes on dynamics and long-term viability of owl

populations, researchers need to undertake comparative demographic studies of owls

across multiple forest management regimes.

38

LITERATURE CITED

Bosque, C., and M. T. Bosque. 1995. Nest predation as a selective factor in the evolution of developmental rates in altricial birds. Amer. Natural. 145:234-260.

Breitwisch, R. 1989. Mortality patterns, sex ratios, and parental investment in

monogamous birds. Current Ornithol. 6:1-50. Burke, D. M., and E. Nol. 1998. Influence of food abundance, nesti-site habitat, and

forest fragmentation on breeding Ovenbirds. Auk 115:96-104. Clapp, R. B., M. K. Klimkiewicz, and A. G. Futcher. 1983. Longevity records of

North American birds: columbidae through paridae. J. Field Ornithol. 54: 123-137.

Coulson, J. C. 1966. The influence of the pair-bond and age on the breeding biology

of the kittiwake gull Rissa tridactyla. J. Anim. Ecol. 35:269-279. Craighead, J. J., and F. C. Craighead. 1956. Hawks, owls, and wildlife. Stackpole

Co., Pennsylvania. Curio, E. 1983. Why do young birds reproduce less well? Ibis 400-404. Dhondt, A. A. 1989. Blue Tit. Pages 15-33 in Lifetime reproduction in birds (I.

Newton, Ed.). Academic Press, San Diego, California. Ens, B. J., M Kersten, A Brenninkmeijer, and J. B. Hulscher. 1992. Territory


Forsman, E. D., E. C. Meslow, and H. M. Wight. 1984. Distribution and biology of

the Spotted Owl in Oregon. Wildl. Monogr. 87:1-64. Forsman, E.D., S. Destefano, M. G. Raphael, and R. J. Gutierrez. 1996.



breeding birds in a mixed-coniferous forest. Condor 80:431-441. Gargett, V. 1977. A 13-year population study of the Black Eagles in the Matopos,

Rhodesia, 1964-1976. Ostrich 48: 17-27.

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Gibbs, J. P., and J. Faaborg. 1990. Estimating the viability of Ovenbird and Kentucky Warbler populations in forest fragments. Conserv. Biol. 4:193-196.

Grant, P. R. 1986. Ecology and evolution of Darwin’s Finches. Princeton Univ.

Press, Princeton, New Jersey. Glutz Von Blotzheim, U. N., and K. M. Bauer. 1980. Handbuch der Vogel

Mitteleuropas, Vol. 9. Akademische Verlagsgesellschaft, Wiesbaden. Harvey, P. H., P. J. Greenwood, C. M. Perrings, and A. R. Martin. 1979. Breeding

success of great tits Parus major in relation to age of male and female parent. Ibis 121:186-200.

Johnsgard, P. A. 1988. North American owls: Biology and natural history.

Smithsonian Institution Press, Washington, D.C. Klimkiewicz, M. K., and A. G. Futcher. 1989. Longevity records of North American

birds. Supplement I. J. Field Ornithol. 60:469-494. Korpimaki, E. 1988. Effects of territory quality on occupancy, breeding performance

and breding dispersal in Tengmalm’s Owl. J. Anim. Ecol. 57:97-108. Lack, D. 1954. The natural regulation of animal numbers. Oxford, University Press. Lebreton, J. D., K. P. Burnham, J. Clobert, and D. R. Andersen. 1992. Modeling

survival and testing biological hypotheses using marked animals: a unified approach with case studies. Ecol. Monogr. 62:67-118.


breeding Flammulated Owls in Colorado. Wilson Bull.. 110:342-351. Linkhart, B. D., and R. T. Reynolds. 1994. Peromyscus carcass in the nest of a

Flammulated Owl. J. Raptor Res. 28:43-44. Linkhart, B. D. 1984. Range, activity, and habitat use by nesting Flammulated Owls

in a Colorado ponderosa pine forest. M. S. Thesis, Colorado State Univ., Fort Collins. 45 pp.

Marshall, J. T., Jr. 1939. Territorial behavior of the Flammulated Screech Owl.

Condor 41:71-78. Marshall, J. T., Jr. 1957. Birds of pine-oak woodland in southern Arizona and


Condor 90:359-372.

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Marti, C. D. 1997. Lifetime reproductive success in Barn Owls near the limit of the

species range. Auk 114:581-592. Martin, T. E. 1988. Processes organizing open-nesting bird assemblages:

competition or nest predation. Evol. Ecol. 2:37-50. Martin, T. E., J. Clobert, and D. R. Anderson. 1995. Return rates in studies of life

history evolution: are biases large? J. Appl. Stat. 22:863-875. McCallum, A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North

America, No. 93 (A. Poole and F. Gill, eds.). Academy of Natural Sciences of Philadelphia; American Ornithologists’ Union, Washington, D.C.

Moreau, R. E. 1944. Clutch size: a comparative study, with references to African

birds. Ibis 86:286-347. Murphy, M. T. 1996. Survivorship, breeding dispersal and mate fidelity in Eastern

Kingbirds. Condor 98:82-92. Murray, G. A. 1976. Geographic variation in the clutch sizes of seven owl species.

Auk 93:602-613. Newton, I. 1979. Population ecology of raptors. T & A D Poyser Ltd., London. 399

pp. Newton, I. 1998. Population limitation in birds. Academic Press, New York. 597

pp. Nol, E., and J. N. M. Smith. 1987. Effects of age and breeding experience on


Payne, R. B., and L. L. Payne. 1990. Survival estimates of Indigo Buntings:

comparisons of banding recoveries and local observations. Condor 92:938:946.

Perrins, C. M., and T. A. Geer. 1980. The effect of Sparrowhawks on Tit

Populations. Ardea 68:133-142. Phillips, A. R., J. T. Marshall, and G. Monson. 1964. The birds of Arizona. Univ.

Ariz. Press, Tucson. 212 pp. Pianka, E. R. 1970. On r- and K-selection. Amer. Natural. 104:592-597.

41

Pietiainen, H. 1988. Breeding season quality, age, and the effect of experience on the reproductive success of the Ural Owl (Strix uralensis). Auk 105:316-324.

Probst, J. R., and J. P. Hayes. 1987. Pairing success of Kirtland’s Warblers in

marginal vs. suitable habitat. Auk 104:234-241. Reynolds, R. T., and B. D. Linkhart. 1984. Methods and materials for capturing and

monitoring Flammulated Owls. Great Basin Natural. 44:49-51. Reynolds, R. T., B. D. Linkhart, and J. Jeanson. 1985. Characteristics of snags and

trees containing cavities in a Colorado conifer forest. USDA Forest Serv. Res. Note RM-455. 6 pp.

Reynolds, R. T., and B. D. Linkhart. 1987. The nesting biology of Flammulated

Owls in Colorado. Pages 239-248 in Biology and conservation of northern forest owls: symposium proceedings (R. W. Nero, R. J. Clark, R. J. Knapton, R. H. Hamre, eds.). USDA Forest Serv. Gen. Tech.. Rep. RM-142.

Ricklefs, R. E. 1969. An analysis of nesting mortality in birds. Smithson. Contrib.

Zool. 9:1-48. Ricklefs, R. E. 2000. Density dependence, evolutionary optimization, and

diversification of avian life histories. Condor 102:9-22. SAS Institute. 1996. SAS user’s guide: Statistics, version 6.12. SAS Institute, Inc.,

Cary, North Carolina. Skutch, A. F. 1949. Do tropical birds rear as many young as they can nourish? Ibis

91:430-455. Slagsvold, T. 1982. Clutch size variation in passerine birds: the nest predation

hypothesis. Oecologia 54:159-169. Sonerud, G. A. 1985. Risk of nest predation in three species of hole nesting owls:

influence on choice of nesting habitat and incubation behaviour. Ornis Scand. 16:261-269.

Steenhof, K., M. N. Kochert, L. B. Carpenter, and R. N. Lehman. 1999. Long-term

Prairie Falcon population changes in relation to prey abundance, weather, land uses, and habitat conditions. Condor 101:28-41.

Van Horne, B., G. S. Olson, R. L. Schooley, J. G. Corn, and K. P. Burnham. 1997.


42

van Woudenberg, A. M. 1992. Integrated management of Flammulated Owl breeding habitat and timber harvest in British Columbia. Masters thesis, Univ. British Columbia, Vancouver.



White, C. M., and T. J. Cade. 1971. Cliff-nesting raptors and ravens along the

Colville River in arctic Alaska. Living Bird 10: 107-150. Wiens, J. A. 1986. Spatial scale and temporal variation in studies of shrubsteppe

birds. Pages 154-172 in Community ecology (J. Diamond and T Case, Eds.). Harper and Row Publishers, New York.

Woolfenden, G. E., and J. W. Fitzpatrick. 1984. The Florida Scrub Jay:




43

CHAPTER III

LIFETIME REPRODUCTION OF FLAMMULATED OWLS

Abstract. - I investigated lifetime reproduction of 22 male and 37 female adult

Flammulated Owls (Otus flammeolus) on 511 ha in Colorado from 1981-1999. Mean

(+ SE) clutch size over the study was 2.5 + 0.1 eggs and mean brood size for

successful nests was 2.4 + 0.1 owlets. Total years that individual owls bred

successfully (i.e., produced at least one owlet/yr) was 1.7 + 0.3 yr (range 1 to 7 yr, n

= 36) for females and 2.5 + 0.4 yr (range 1 to 9 yr, n = 22) for males. Individual

females produced a total 4.9 + 0.8 eggs (range 1 to 19 eggs) and 4.1 + 0.7 owlets

(range 0 to 18 owlets), and males produced 6.8 + 1.1 eggs (range 2 to 25 eggs) and

6.1 + 1.0 owlets (range 0 to 22 owlets), in their lifetimes. Relatively few individuals

accounted for a large proportion of the total production of offspring. Seventeen

percent of 37 females produced 50% of total owlets, while 27% of 22 males produced

50% of total owlets. Adults with longer breeding lifespans produced more owlets in

their lifetimes, and differences between sexes in lifetime reproduction and lifetime

number of mates were associated with greater longevity on the study area by males.

Flammulated Owls have breeding lifespans comparable to other, larger strigiforms,

and coupled with the fact that I never observed renesting or multiple broods, these

data support reports that the owl generally has a K-selected life-history strategy.

44

INTRODUCTION

Avian reproductive strategies have been shaped by a variety of environmental

factors including climate, habitat, food resources, and predators (Stearns 1976,

Southwood 1977). These factors have diversified reproductive strategies among

species by influencing parameters such as age of first breeding (e.g., Pietiainen 1988),

sex- and age- dependent reproduction (e.g., Pianka 1970, Pianka and Parker 1975,

Orians and Beletsky 1989, Fitzpatrick and Woolfenden 1989), and breeding lifespan

(e.g., Owen and Black 1989). Studying reproductive strategies is important because

they offer insight into evolutionary factors influencing reproductive rate (e.g., Lack

1947, Skutch 1949), and because this information helps determine how species may

respond to environmental changes (Newton 1998).

Reproductive strategies of species are perhaps best determined by

measurements of lifetime reproductive success (LRS), the total offspring raised by

individuals over their lifetimes. LRS, which is based on longitudinal studies,

provides more accurate estimates of reproductive contributions by individuals and of

variation in reproductive output among individuals than annual measures from cross-

sectional studies (Newton 1989a, Clutton-Brock 1988). LRS may also provide the

best estimate of biological fitness for individuals because these data elicit information

on the relative contribution of individual genotypes, and allow identification of traits

that may most contribute to fitness (Williams 1966, Newton 1989a).

LRS studies have revealed patterns in reproductive strategies and variance in

productivity among birds. Small species generally have shorter but more productive

45

breeding lives than large species. For example, the relatively small Kingfisher

(Alcedo atthis) breeds at less than one yr, seldom lives more than 4 yr, and may

produce more than 20 young per yr (Bunzel and Druke 1989). In contrast, the much

larger Barnacle Geese (Branta leucopsis) generally initiates breeding at 7 yr, and

produces no more than 10 offspring in a lifespan that may exceed 20 yr (Owen and

Black 1989). Despite ecological differences among birds, most studies of LRS have

shown that a few individuals produce a disproportionately large percentage of the

next generation (Clutton-Brock 1988, Newton 1989a). In both the Kingfisher and

Barnacle Geese, for example, fewer than one-third of the breeding population

produces 50% of all offspring (Bunzel and Druke 1989, Owen and Black 1989).

LRS is little studied among raptors because most of these species are

relatively long-lived. LRS has been estimated for Eurasian Sparrowhawk (Accipiter

nisus; Newton 1989b), Merlin (Falco columbarius; Wiklund 1995), Eastern Screech

Owl (Otus asio; Gehlbach 1989), Ural Owl (S. uralensis; Saurola 1989), Osprey

(Pandion heliaetus; Postupalsky 1989), Tengmalm’s Owl (Aegolius funereus;

Korpimaki 1992), and Barn Owl (Tyto alba; Marti 1997).

I present LRS data for a population of Flammulated Owls (O. flammeolus) in

Colorado studied for 19 years (1981-1999). The Flammulated Owl is a cavity-nester

that breeds in montane forests of western North America (McCallum 1994, Linkhart

et al. 1998). These owls are entirely insectivorous, feeding mostly on small moths

(Reynolds and Linkhart 1987), and are among the most migratory of all North

American strigiforms (Johnsgard 1988), breeding as far north as southern British

Columbia and wintering as far south as El Salvador (McCallum 1994).

46

Determination of LRS in Flammulated Owls is interesting because, while the owl’s

lifetime reproduction might be expected to be similar to that of other raptors (e.g.,

long breeding lifespan), it is one of the smallest North American owls and annually

migrates the greatest distance (Johnsgard 1988). Specifically, my objectives were to:

(1) describe individual variation in LRS among both sexes; (2) compare LRS

between sexes; (3) identify life-history attributes that have important influences on

LRS; and (4) compare the reproductive strategy of Flammulated Owls to other

raptors. Compared to other, larger raptors, I predicted that Flammulated Owls would

have shorter reproductive lifespans, and produce more offspring over their lifetimes.

METHODS

Study Area

The study area was a 511 ha tract on the Manitou Experimental Forest in

Teller Co., Colorado. Forests within the study area consisted of (1) ponderosa pine

(Pinus ponderosa) mixed with Douglas-fir (Pseudotsuga menziesii), generally on

ridgetops and south- and west-facing slopes, (2) quaking aspen (Populus tremuloides)

stands on lower slopes and bottoms of moist drainages, (3) quaking aspen stands

mixed with blue spruce (Picea pungens) in bottoms, lower slopes, and benches in

mesic areas, and (4) Douglas-fir mixed with blue spruce on higher slopes in drainages

and on north-facing slopes. Tree cutting on the study area has not occurred since the

1880s, when a light harvest for railroad ties occurred (Reynolds et al. 1985). Snags

and trees with cavities were relatively abundant throughout the study area (Reynolds

et al. 1985). Elevations ranged from 2,550-2,855 m.

Data Collection and Analysis

47

From 1981-1999, I collected data on the demographic performance of

Flammulated Owls on the study area. Each spring and summer, I searched the entire

study area for territorial males (Marshall 1939). Territories were identified by

marking territorial song-trees of males (Reynolds and Linkhart 1984) and using radio-

telemetry in 1982-1983 (Linkhart et al. 1998). Once territory boundaries were

delineated, I located all suitable nesting cavities (tree cavities with entrance diameters

> 4 cm) within territories and checked each for nesting owls (Reynolds and Linkhart

1984). Unpaired males typically sang throughout a breeding season, whereas singing

in nesting males dramatically declined after egg-hatch (Reynolds and Linkhart 1987).

Because I spent considerable time during each breeding season monitoring singing

males and searching for nests in their territories, I was confident that all nests were

located. Most nests were found during incubation and nests were checked at least

weekly (often two or three times per week) until the young fledged. Breeding adults

were captured at nests (occasionally on perches or day roosts) and banded with U. S.

Fish and Wildlife Service leg bands (Reynolds and Linkhart 1984). I banded owlets

when 2-3 weeks old (fledging occurs at 22-24 days; Reynolds and Linkhart 1987).

Because no nests failed beyond the midpoint of the nestling period (duration of

nestling period was 22-24 d; Reynolds et al. 1987), mean number of fledglings per

brood was identical to mean number of banding-age owlets per brood.

I determined lifetime reproduction for all males and females captured and

recaptured between 1981 and 1999 according to the following criteria. First, I

included only individuals that bred at least once during the study (territorial, non-

breeding males were rarely captured). Second, individuals whose breeding lifespans

48

included 1981 or 1999 (the first and last years of study) were included only if their

total annual breeding attempts were greater than the mean for all individuals whose

known breeding lifespans began after 1981 and terminated before 1998 (henceforth

“inclusive owls”). Mean (+ SE) total breeding attempts for inclusive adults was 2.5 +

0.5 (n = 18) for males and 1.7 + 0.3 (n = 31) for females, so this criterion excluded

seven adults—four males (two males with two breeding attempts and two with one

breeding attempt) and three females (each with one breeding attempt) from LRS

calculations. Third, breeding individuals were excluded from calculations of certain

parameters of lifetime reproduction if their identity was unknown (i.e., they were not

recaptured) in any given year or if their full breeding histories were unknown

(excepting criterion #2 above). Age of first-time breeders could not be determined.

Data on reproductive parameters are therefore conservative if owls nested outside, but

immigrated onto, the study area prior to being banded. This bias may be small given

that I documented just nine cases of breeding dispersal between 1981-1999, with

dispersers usually moving to adjacent territories (Linkhart and Reynolds 1998).

For all individual owls meeting the above criteria, I determined the following

parameters for both sexes: (1) lifetime breeding attempts, defined as the combined

total of successful (i.e., fledged at least one owlet) and failed breeding attempts; (2)

lifetime successful breeding attempts; (3) lifetime production of eggs and fledglings;

(4) relationship between total breeding years and production of fledglings; (5)

relationship between total fledglings and total eggs, (6) contribution to total offspring

by individual adults; and (7) lifetime number of new mates.

Statistical analyses were performed using SAS (SAS Institute 1995). I used

49

Wilcoxon’s test (PROC NPAR1WAY) to evaluate sex differences in reproductive

parameters and effect of breeding experience on brood sizes, and simple linear

regression (PROC GLM) to examine relationships between variables. Means are

presented + standard error (SE), and analyses were considered significant if P < 0.05.

RESULTS

Over the 19 yr study, I recorded 82 breeding attempts (i.e., at least one egg

laid) by 65 adults. I documented the reproductive lives of 59 of these adults: 22

males and 37 females. Except for four males and three females that nested in 1999,

no individuals who nested in prior years on the study area were known to be alive in

1999. Only two individuals (females) returned to breed on the study area (one in

1998 and 1999) after being absent for a breeding season; these females returned to

breed one and two territories distant from original territories (Chapter 4). Unless

otherwise noted, the following are based on these 59 adults.

Lifetime Breeding Attempts

Lifetime reproduction of individuals is the product of mean clutch or brood

size and lifetime breeding attempts. Elsewhere I reported that, over the 19 yr study

for this population, mean clutch size was 2.5 + 0.1 eggs (n = 29, range = 2–4) and

mean brood size for successful nests was 2.4 + 0.1 (n = 67, range = 1-4; Chapter 2).

Total breeding attempts is dependent upon the age of first breeding, number of

breeding attempts per year, and breeding lifespan (i.e., duration of breeding life). I

was unable to determine age of first breeding because adults could not be aged. Most

raptors produce only one brood per year, although several small species of hawk and

falcon and some medium to large owl species renest after initial breeding attempts

50

fail (Newton 1989a, Johnsgard 1988). I never documented any instances of renesting

or multiple broods in Flammulated Owls during the study, even for pairs whose nests

failed early in the breeding cycle. Since this species has a maximum of one breeding

attempt per year, lifetime breeding attempts was equivalent to breeding lifespan.

Most owls in this study bred more than once in their lifetimes, and the mean

total breeding attempts was 2.4 + 0.3 (range = 1 to 10). However, males had

significantly more mean total breeding attempts in their lifetimes (3.0 + 0.5) than

females (2.0 + 0.3; z = 2.38, P = 0.02; Fig. 8). Just 20% of females had three or more

breeding attempts compared to nearly 50% of males (Fig. 8).

Because unsuccessful breeding attempts do not contribute to an individual’s

LRS, I determined total years that owls bred successfully (i.e., produced at least one

owlet). Most owls in the population bred successfully more than once in their

lifetimes, and mean total successful nest attempts for all owls was 2.0 + 0.2 (range =

0 to 9). However, on average males bred successfully more yr (2.5 + 0.4 yr) than

females (1.7 + 0.3 yr; z = 2.46, P = 0.01; Fig. 9). Just 20% of females bred

successfully for > 3 yr compared to nearly 50% of males. These data are similar to

lifetime breeding attempts for both sexes because only 14 nests failed during the

study—82% of all breeding attempts were successful (Chapter 2). The percentage of

individuals that had greater than 80% lifetime nesting success was 73% among males

and 75% among females (Fig. 10).

Lifetime Reproduction

Most owls produced or tended nests that produced > 5 eggs in their lifetimes;

mean total production was 5.7 + 0.7 eggs. However, males tended nests that

51

Figure 8. Lifetime number of nesting attempts by female (light bars) and male (dark

bars) owls.

Per

cent

of i

ndiv

idua

ls

Years

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9 10

52

Figure 9. Lifetime number of successful nests by female (light bars) and male (dark

bars) owls.

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6 7 8 9

Total successful nests

Per

cent

of i

ndiv

idua

ls

53

Figure 10. Lifetime percentage of successful nests by female (light bars) and male

(dark bars) owls.

Percent of nests successful

Per

cent

of i

ndiv

idua

ls

0

20

40

60

80

100

0-20 21-40 41-60 61-80 81-100

54

produced significantly more mean eggs in their lifetimes (-x = 6.8 + 1.1) than females

produced (-x = 4.9 + 0.8; z = 2.29, P = 0.02; Fig. 11). Less than 30% of females

produced > 6 eggs while nearly 50% of males produced > 6 eggs. Yr of breeding

experience were not associated with larger clutch sizes in either sex; females breeding

for the first time on the study area had mean clutch sizes of 2.5 + 0.1 eggs (n = 29)

and females breeding for > 2 yr had clutches of 2.6 + 0.2 eggs (n = 8; z = 2.16, P =

0.03). Mean clutch size of males breeding for the first time and males breeding for >

2 yr were nearly identical (both produced clutches of 2.6 + 0.2 eggs, n = 14 and 8,

respectively; z = -0.35, P = 0.73).

Owls produced or tended nests that produced a mean 4.8 + 0.6 fledglings over

their lifetimes. As with lifetime production of eggs, however, over their lifetimes

males tended nests that produced more mean fledglings (-x = 6.1 + 1.0) than females

produced (-x = 4.1 + 0.7; z = 2.48, P = 0.01; Fig. 12). Fourteen percent of females

and 5% of males did not produce any fledglings (their nests failed), while 25% of

females and 47% of males produced > 6 fledglings in their lifetimes. Since my data

on lifetime production of eggs and fledglings do not include territorial but non-

nesting males—who comprised approximately half of all territorial males annually

and who may not produce any offspring (Chapter 4)—estimates of LRS parameters

for males may be inflated relative to females.

Previous breeding experience did not affect brood sizes of either sex. Females

breeding for the first time on the study area had mean brood size of 2.1 + 0.2 owlets

(n = 21) while females breeding for > 2 yr had broods of 2.0 + 0.3 owlets (n = 13; z =

0.39, P = 0.69). Males breeding for the first time on the study area had mean brood

55

Figure 11. Lifetime production of eggs by female (light bars) and male (dark bars)

owls.

0

5

10

15

20

25

30

35

40

45

1 3 5 7 9 11 13 15 17 19 21 23 25

Total eggs

Per

cent

of i

ndiv

idua

ls

56

Figure 12. Lifetime production of fledglings by female (light bars) and male (dark

bars) owls.

Total fledglings

Per

cent

of i

ndiv

idua

ls

0

5

10

15

20

25

30

35

1 3 5 7 9 11 13 15 17 19 21 23

57

size of 2.3 + 0.2 owlets (n = 14) while males breeding for > 2 yr years had broods of

2.2 + 0.2 owlets (n = 14; z = 0.73, P = 0.46).

Breeding lifespan (i.e., total years in which owls bred) appeared to be an

important determinant of lifetime reproduction. Breeding lifespan was positively

correlated with total production of fledglings, although the relationship was more

strongly linear for females (r = 0.96, F = 389.4, P < 0.001; Fig. 13A) than for males

(r = 0.87, F = 63.0 P = 0.001; Fig. 13B). Females produced a mean 4.1 fledglings in

2.0 breeding years, and males produced a mean 6.1 fledglings in 3.0 breeding years.

Thus, the more years that Flammulated Owls bred, the more fledglings they

produced. Total owlet production was positively correlated with total egg production

for both sexes (r = 0.97 for females; r = 0.95 for males; Fig. 14A and 14B), and the

strength of the correlation reflected the fact that relatively few nests failed.

The contribution of total offspring by individuals varied greatly for adults of

both sexes. The most productive female produced 18 fledglings over a breeding

lifespan of 8 yr, accounting for 12% of total fledglings produced during the study.

Overall, 17% of females produced 50% of total fledglings (Fig. 15A). The most

productive male tended nests that produced 22 fledglings over a breeding lifespan of

10 yr, accounting for 16% of all fledglings produced during the study. Overall, 27%

of males produced 50% of total fledglings (Fig. 15B).

Lifetime Number of New Mates

Lifetime number of new mates is defined as the total number of unique

partners with which an individual bred over his/her lifetime. Most owls had one or

two new mates in their lifetimes; number of mates for sexes combined was 1.6 + 0.1

58

Fig. 13. Relationship between total breeding years and total fledglings by (A) female

and (B) male owls. Duplicate symbols are omitted from the graph.

B

y = 1.6x + 1.1

R2 = 0.76

0

5

10

15

20

25

0 2 4 6 8 10

y = 2.3x - 0.5

R2 = 0.93

0

5

10

15

20

25

0 2 4 6 8 10

Total breeding years

Total breeding years

Tot

al fl

edgl

ings

T

otal

fled

glin

gs

A

59

Fig. 14. Relationship between total eggs and fledglings for female (A) and male (B)

owls. Duplicate symbols are omitted from the graph.

y = 0.9x + 0.3

R2 = 0.95

0

5

10

15

20

25

0 5 10 15 20 25

B

A

Tot

al fl

edgl

ings

T

otal

fled

glin

gs

Total eggs

Total eggs

y = 0.9x - 0.1

R2 = 0.97

0

5

10

15

20

0 5 10 15 20

A

60

Fig. 15. Percent of total fledglings produced by varying percentages of female (A)

and male (B) owls. Axes showing percent of males and females were based

on ordering individuals from highest to lowest productivity.

0

20

40

60

80

100

0 20 40 60 80 100

0

20

40

60

80

100

0 20 40 60 80 100

Per

cent

of t

otal

fled

glin

gs

(n =

138

) P

erce

nt o

f tot

al fl

edgl

ing

s (n

= 1

34)

Percent of females (n = 35)

Percent of males (n = 22)

B

A

61

(range = 1-5 mates). However, on average males had more mates in their lifetimes

(2.0 + 0.3) than females (1.3 + 0.1; z = -2.28, P = 0.02; Fig. 16). Just 22% of females

had > 2 breeding partners in their lifetimes compared with 50% of males. These

differences are likely associated with greater male longevity on the study area

compared to females (Chapter 2).

DISCUSSION

Reproductive Strategy

Flammulated Owls, which are the second-smallest (mass 50-60 g) North

American strigiform, have a reproductive strategy that resembles larger strigiforms

for which comparable data exist. Male Flammulated Owls have a longer mean

breeding lifespan (3.0 yr) than Tengmalm’s Owls (1.5 yr; Korpimaki 1992) and Barn

Owl (1.2 yr; Marti 1997), despite the fact that Tengmalm’s Owls and Barn Owls have

greater mass by about 200% and 800%, respectively. Mean lifetime production of

fledglings by Flammulated Owls (6.1 fledglings) was somewhat more than

Tengmalm’s Owls (5.4 fledglings; Korpimaki 1992), and Barn Owls (4.7 fledglings;

Marti 1997). For females, Flammulated Owls have a longer mean breeding lifespan

(2.0 yr) than Barn Owls (1.4 yr; Marti 1997), but shorter than the much larger Ural

Owls (4.9 yr; Saurola 1989), whose mass is about 1500% that of Flammulated Owls.

Mean lifetime production of fledglings by females is less for Flammulated Owls (4.1

fledglings) than Barn Owls (6.0 fledglings; Marti 1997) and Ural Owls (8.2

fledglings; Saurola 1989), but more than Eastern Screech Owls (2.9 fledglings;

Gehlbach 1989), whose mass is about 300% of Flammulated Owls.

62

Fig. 16. Lifetime number of new mates for female (light bars) and male (dark bars)

owls.

0

20

40

60

80

100

1 2 3 4 5

Per

cent

age

of in

divi

dual

s

Total new mates

63

Elsewhere (Chapter 2) I reported data that showed, compared to other North

American strigiforms, Flammulated Owls had (1) one of the smallest and least

variable clutch sizes, (2) one of the highest rates of nesting success, and (3)

relatively long lifespan. Coupled with the fact that lifetime production of fledglings

and breeding lifespan is similar to that of several larger strigiforms, and that I never

observed renesting or multiple broods, these data indicate that Flammulated Owls

have a life-history strategy resembling other, larger strigiforms (e.g., Tawny Owls,

Southern 1970; Ural Owls, Saurola 1989; Spotted Owls, S. occidentalis, Forsman et

al. 1984). This strategy contrasts with that of Barn Owls, whose large clutch size,

propensity for multiple clutches annually by some individuals, and short breeding

lifespans resemble many passerine species (Marti 1997).

Individual Variation in LRS

Studies of LRS have revealed three patterns regarding the extent to which

individuals contribute offspring to future generations. First, some individuals that

attempt to breed fail to produce any offspring during their lifetimes (Clutton-Brock

1988, Newton 1989a). Among all birds, the proportion of individuals that attempted

to breed but produced no offspring ranges from 5% (Blue Tits, Parus caeruleus;

Dhondt 1989) to 49% (Barnacle Geese, Owen and Black 1989). In Flammulated

Owls, 14% of females and 5% of males that attempted to breed produced no

fledglings. However, this latter percentage is underestimated, because most unpaired

males, which occupied 30-70% of territories annually (Chapter 4) could not be

captured. The proportion of unpaired males that eventually bred was unknown

(Chapter 4). Second, a small percentage of breeding adults account for the majority

64

of the total fledglings produced in the population (Clutton-Brock 1988, Newton

1989a). Among all birds, the percentage of females that accounted for 50% of total

fledglings ranges from 15% (Red-billed Gulls; Mills 1989) to 31% (Kingfishers;

Bunzel and Druke 1989), and the percentage of males ranges from 14% (Indigo

Buntings; Payne 1989) to 30% (Kingfishers; Bunzel and Druke 1989). In

Flammulated Owls, 17% of females and 27% of males accounted for 50% of total

fledglings. As noted above, the percentage of fledglings accounted for by males is

perhaps inflated. Third, a high proportion of fledglings die before ever attempting to

breed (Clutton-Brock 1988, Newton 1989a). Among all birds, the percentage of

fledglings that die before attempting to breed ranges from 42% (Barnacle Geese;

Owen and Black 1989) to 86% (Blue Tits; Dhondt 1989). I could not determine the

proportion of Flammulated Owls that died before breeding because I could not

distinguish between death and dispersal for fledglings and adults.

Sexual Variation in LRS

In many bird species the extent of sexual differences in variance of LRS is

correlated with the degree of sexual dimorphism (Newton 1989a). Species showing

the least sexual dimorphism exhibit the least sexual variance in LRS, such as in

Florida Scrub Jays (Aphelocoma c. caerulescens; Fitzpatrick and Woolfenden 1989),

while species showing the most sexual dimorphism exhibit greatest differences

between the sexes, with the most dimorphic sex exhibiting the greatest variance in

LRS such as in Red-winged Blackbirds (Agelaius phoeniceus; Orians and Beletsky

1989). Inter-sexual variation in LRS of raptors, which exhibit varying degrees of

reversed sexual size dimorphism (Snyder and Wiley 1976, Mueller 1986), has been

65

studied in only three species: Osprey, Barn Owls, and Flammulated Owls (this

study). Extent of dimorphism is similar among these species (females have 20-30%

more mass than males), but inter-sexual differences in LRS exist only within the two

owl species. Male and female Barn Owls produced a similar mean number of total

eggs and fledglings, but females had longer breeding lifespans than males (Marti

1997). In contrast, mean lifetime production of eggs and fledglings were similar

between male and female Flammulated Owls. However, males had longer breeding

lifespans and more mates over their lifetimes than females. These differences may

result from males having greater longevity than females, which is suggested by an

apparent surplus of males—only about 50% of males breed annually (Chapter 5).

The underlying reasons for differences in longevity are uncertain, but females may

suffer higher mortality than males due to the high cost of egg production immediately

following spring migration (Chapter 2). Alternatively, higher rates of breeding

dispersal by females (Chapter 4) may mean that some females may breed either prior

to, or subsequent to, their arrival on my study area.

Demographic and Ecological Correlates of LRS

My data showed that total breeding years were strongly correlated with

lifetime productivity for female (r = 0.96) and male (r = 0.87) Flammulated Owls,

because clutch sizes varied little and nesting success was high. In fact, breeding

lifespan has emerged as the major demographic determinant of LRS (Newton 1989a).

Among all birds, regression analyses have shown that variance in fledgling

production that was accounted for by breeding lifespan ranged from 29% (Barnacle

Geese; Owen and Black 1989) to 86% (Blue Tits; Dhondt 1989). For some species,

66

other important factors contributing to differences in LRS among individuals were

offspring survival between the egg and fledgling stages and lifetime fecundity

(Newton 1989a).

Few studies have examined the ecological factors associated with individual

variation in LRS. Newton (1989b) reported that LRS of European Sparrowhawks

was greatly influenced by territory quality, where better territories were characterized

as those most frequently occupied annually in a previous study. In my study,

variance in LRS also may have been associated with habitat quality. Males and

females producing the most eggs and fledglings over their lifetimes did so on four

territories (A4, A8, A11, and A29) that were, over the 19 yr study, the most

productive of all territories (Chapter 5). Productivity over all territories was

positively correlated with mature, open stands of ponderosa pine/Douglas-fir and

negatively correlated with Douglas-fir forests, which were denser and consisted of

smaller trees (Chapter 5). Variance in LRS was associated with variable food

abundance in Tengmalm’s Owl (Korpimaki 1988, 1992), nest predation in Merlins

(Falco columbarius; Wiklund 1995), and extreme weather in Great Tits (Parus

major; McCleery and Perrins 1989) and Barnacle Geese (Owen and Black 1989).

Annett and Pierotti (1999) found that long-term reproductive output in Western Gulls

(Larus occidentalis) was strongly influenced by choice of diet, with increasing

amounts of fish in the diet associated with greater survival and reproduction.

67

Literature Cited Annett, C. A. and R. Pierotti. Long-term reproductive output in Western Gulls:

consequences of alternate tactics in diet choice. Ecol. 80:288-297. Bunzel, M. and J. Druke. 1989. Kingfisher. Pages 107-117 in Lifetime reproduction

in birds (I. Newton, Ed.). Academic Press, San Diego, California. Clutton-Brock, T. H. 1988. Reproductive success. University of Chicago Press,

Chicago. Dhondt, A. A. 1989. Blue Tit. Pages 15-33 in Lifetime reproduction in birds (I.

Newton, Ed.). Academic Press, San Diego, California. Fitzpatrick, J. W. and G. E. Woolfenden. 1989. Florida Scrub Jay. Pages 201-218 in

Lifetime reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.


the Spotted Owl in Oregon. Wildl. Monogr. 87:1-64. Gehlbach, F. R. 1989. Screech-owl. Pages 315-326 in Lifetime reproduction in

birds (I. Newton, Ed.). Academic Press, San Diego, California. Johsnsgard, P. A. 1988. North American owls: Biology and natural history.

Smithsonian Institution Press, Washington, D.C. Korpimaki, E. 1988. Costs of reproduction and success of manipulated broods under

varying food conditions in Tengmalm’s Owls. J. Anim. Ecol. 57:97-108. Korpimaki, E. 1992. Fluctuating food abundance determines the lifetime

reproductive success of male Tengmalm’s Owls. J. Anim. Ecol. 61:103-111. Linkhart, B. D. and R. T. Reynolds. 1997. Territories of Flammulated Owls: Is



breeding Flammulated Owls in Colorado. Wilson Bull. 110:342-351. Marshall, J. T., Jr. 1939. Territorial behavior of the Flammulated Screech Owl.

Condor 41:71-78.

68

Marti, C. D. 1997. Lifetime reproductive success in Barn Owls near the limit of the

species range. Auk 114:581-592. McCallum, D. A. 1994. Flammulated Owl (Otus flammeolus). In The birds of North

America, No. 93 (A. Poole and F. Gill, Eds.). Academy of Natural Sciences, Philadelphia; and American Ornithologists’ Union, Washington, D.C.

McCleery, R. H., and C M. Perrins. 1989. Great Tit. Pages 35-53 in Lifetime

reproduction in birds (I. Newton, Ed.). Academic Press, San Diego, California.

Mills, J. A. 1989. Red-billed Gull. Pages 387-404 in Lifetime reproduction in birds

(I. Newton, Ed.). Academic Press, San Diego, California. Mueller, H. C. 1986. The evolution of reversed sexual dimorphism in owls: an

empirical analysis of possible selective factors. Wilson Bull. 98:387-406. Newton, I. 1989a. Lifetime reproduction in birds. Academic Press, San Diego,

California. Newton, I. 1989b. Sparrowhawk. Pages 279-296 in Lifetime reproduction in birds

(I. Newton, Ed.). Academic Press, San Diego, California. Newton, I. 1998. Population limitation in birds. Academic Press, New York. 597

pp. Orians, G. H. and L. D. Beletsky. 1989. Red-winged Blackbird. Pages 183-197 in


Owen, M., and J. M. Black. 1989. Barnacle Goose. Pages 349-362 in Lifetime


Payne, R. B. 1989. Indigo Bunting. Pages 153-172 in Lifetime reproduction in birds

(I. Newton, Ed.). Academic Press, San Diego, California. Pianka, E. R. and W. S. Parker. 1975. Age-specific reproductive tactics. Amer.

Natural. 109:453-464. Pietiainen, H. 1970. Breeding season, age, and the effect of experience on the

reproductive success of the Ural Owl (Strix uralensis). Auk 105:316-324. Postupalsky, S. 1989. Osprey. Pages 297-313 in Lifetime reproduction in birds (I.

Newton, Ed.). Academic Press, San Diego, California.

69

Reynolds, R. T. and B. D. Linkhart. 1984. Methods and materials for capturing and

monitoring Flammulated Owls. Great Basin Nat. 44:49-51. Reynolds, R. T., B. D. Linkhart, and J. Jeanson. 1985. Characteristics of snags and


Reynolds, R. T. and B. D. Linkhart. 1987. The nesting biology of Flammulated

Owls in Colorado. Pp. 239-248 in Biology and conservation of northern forest owls (R. W. Nero, R. J. Clark, R. J. Knapton, and R. H. Hamre, Eds.). USDA Forest Serv. Gen. Tech. Rep. RM-142.

SAS Institute, Inc. 1995. SAS user’s guide, SAS Institute, Inc. Cary, North

Carolina. Saurola, P. 1989. Ural Owl. Pages 327-345 in Lifetime reproduction in birds (I.

Newton, Ed.). Academic Press, San Diego, California. Skutch, A. F. 1949. Do tropical birds rear as many young as they can nourish? Ibis

91:430-455. Snyder, N. F. and J. W. Wiley. 1976. Sexual size dimorphism in hawks and owls of

North America. Ornith. Monogr. 20:1-96. Southwood, T. R. E. 1977. Habitat, the templet for ecological strategies? J. Anim.

Ecol. 46:337-365. Stearns, S. C. 1976. Life history tactics, a review of the ideas. Quart. Rev. of Biol.

51:3-47. Southern, H. N. 1970. The natural control of a population of Tawny Owls (Strix

aluco). J. Zool. (London) 162:197-285. Wiklund, C. G. 1995. Nest predation and life-span: components of variance in LRS

among Merlin females. Ecol. 76:1994-1996. Williams, G. C. 1966. Natural selection, the costs of reproduction, and a refinement

of Lack’s Principle. Amer. Natural. 100:687-690.

70

CHAPTER IV

MATE AND SITE FIDELITY AND BREEDING DISPERSAL IN

FLAMMULATED OWLS

Abstract. - I investigated territory and mate fidelity and dispersal in a migratory

population of Flammulated Owls (Otus flammeolus) on 511 ha in central Colorado. Over

the 19 yr (1981-1999) study, a mean 8.0 + 0.5 (SE) territories were occupied annually by

breeding pairs or unpaired males. Rate of return to the study area was higher for males

(59%) than for females (37%), and mean tenure on territories was nearly twice as long for

males (3.0 + 0.5 yr) as for females (1.6 + 0.2 yr). Males also showed greater annual

territory fidelity than females; 98% of returning males stayed on original territories

compared to 78% of females. Failure of previous year’s nest and breeding status (paired

vs unpaired) were associated with reduced territory fidelity in females but not males,

while return of previous mate did not affect territory fidelity for either sex. Mean pair

duration was 1.4 + 0.1 yr and mate fidelity was high; 96% of pairs retained the same mate

when both mates returned from migration. Reproductive success was not correlated with

length of pair bond; annual initiation of egg-laying and productivity (mean

fledglings/brood) were similar for pairs that bred two or more years and pairs that bred for

the first time. I documented one instance of natal dispersal. Breeding dispersal was

female-biased; 8 of 9 dispersals were by females that moved one or two territories away

from their original territories. Females whose nests failed the previous year had lower

return rates to the study area than females whose previous nests were successful.

71

Consequently, dispersal distance may be bimodal with females dispersing longer distances

after nesting failure and shorter distances after successful nests. Dispersal was not

correlated with previous breeding experience, previous nest failure, mate quality, and

productivity, but females often dispersed when mates did not return and they dispersed to

territories on which total productivity over the study was higher than on original

territories.

72

INTRODUCTION

In stable environments many birds annually return to the same breeding sites

and retain the same breeding partners (Greenwood 1980, Black 1996). Particularly

for migratory species, site fidelity may confer benefits that include better knowledge

of locations for foraging, nesting, and escaping predators, and improved chance of

maintaining a breeding territory and mating (Hinde 1956, Greenwood 1980, Shields

1984, Part 1994, 1995). If neighbors return, site-faithful individuals also benefit by

avoiding initial cost of contesting territory boundaries with unfamiliar individuals

(Krebs 1982, Maynard Smith 1982). Mate fidelity is thought to improve coordination

and cooperation between mates, prolong biparental investment, and reduce costs

associated with mate sampling such as risk of predation or failing to find a suitable

mate in time to breed (Black 1996). Studies have shown that mates who bred

together previously had larger clutches and higher nest success than newly formed

pairs (Mills 1973, Coulson and Thomas 1983, Newton and Marquiss 1982,

Korpimaki 1988, Bradley et al. 1990, Orell et al. 1994, Murphy 1996).

In spite of benefits, mate and site fidelity involve some costs. Site faithfulness

increases the inbreeding probability, and may lower chances for reproductive success

due to decrease in habitat quality resulting from habitat changes (i.e., succession or

disturbance), predation, or competition (Greenwood et al. 1978, Oring and Lank

1982). For migratory species, costs of mate fidelity may include waiting for mates to

return or searching for new mates (Black 1996). When costs associated with mate or

site fidelity exceed benefits, individuals should disperse. In fact, several studies have

shown that fitness of individuals increased following breeding dispersal, when adults

73

moved to new breeding sites (Shields 1984, Payne and Payne 1993, Part 1995, Forero

et al. 1999).

Despite much study the past two decades, avian fidelity and dispersal are still

poorly understood primarily because investigations require gathering longitudinal

data on marked individuals and studying sufficiently large areas to document longer-

distance dispersal (Paradis et al. 1998, Forero et al. 1999). These problems are the

primary reasons that raptors, which are relatively long-lived and disperse over large

areas, have been little-studied. Diurnal species studied five or more years include

Merlins (Falco columbarius; Warkentin et al. 1991), European Sparrowhawks

(Accipiter nisus; Newton and Marquiss 1982, Newton 1993), and Black Kites (Milvus

migrans; Forero et al. 1999), and nocturnal species include Tengmalm’s Owl

(Aegolius funereus; Korpimaki 1988, 1993), Flammulated Owls (O. flammeolus;

Reynolds and Linkhart 1987a), Eastern Screech Owls (Otus asio; Gelhbach 1994),

and Barn Owls (Tyto alba; Marti 1999). Of these species, only Black Kites and

Flammulated Owls are migratory. Because dispersal influences population structure

and dynamics (e.g., Greenwood and Harvey 1982, Johnson and Gaines 1990),

understanding the ecological correlates of dispersal is important for avian

conservation.

Here I describe fidelity and dispersal for a Colorado population of

Flammulated Owls, which are among the most migratory of North American

strigiforms (Johnsgard 1988). I extend the study by Reynolds and Linkhart (1987a)

to include 19 yrs of data (1981-1999) on the same owl population. Specifically, I

describe: (1) sex differences in return rates to the study area; (2) sex differences in

74

territory fidelity, and I evaluate effects of previous nest failure, breeding status

(paired vs unpaired), and return of previous mate on fidelity; (3) patterns in mate

fidelity and I assess benefits of maintaining pair bonds; and (4) sex differences in

breeding dispersal. I also test possible correlates of breeding dispersal: (a) owls

dispersed due to failure of previous nest attempt (nest-failure hypothesis); (b) owls

dispersed because original mates did not return (non-return of mate hypothesis); (c)

owls that dispersed were inexperienced breeders (inexperienced-breeder hypothesis);

(d) owls were able to acquire higher quality mates (mate-improvement hypothesis);

(e) owls were able to acquire higher quality territories (territory-improvement

hypothesis); and (f) owls were able to increase their productivity (disperser-

enhancement hypothesis).

METHODS

Species

Flammulated Owls are insectivorous, feeding mostly on small lepidopterans

(Reynolds and Linkhart 1987b). The owls are obligate cavity-nesters that breed in

montane forests of western North America as far north as southern British Columbia

and winter as far south as El Salvador (McCallum 1994, Linkhart et al. 1998). They

are “sensitive species” in four of six regions of the USDA Forest Service (Verner

1994). Flammulated Owls are monogamous although extra-pair copulations are

known (Reynolds and Linkhart 1990); females are sole incubators of eggs while

males defend territories and provide food for their mates and fledglings (Reynolds

and Linkhart 1987b, Linkhart et al. 1998). Nest success by Flammulated Owls is

75

among the highest (82%) and clutches are among the smallest (2.5 + 0.5 eggs) of

North American strigiforms (Chapter 2; Johnsgard 1988).

Study Area


Colorado. I established boundaries of the 511 ha study area after territorial owls were

found to be present in initial surveys (1980). Boundaries were drawn around an area

large enough to contain approximately 20 territorial males, based on an estimate of

territory size (274 m in diameter; Marshall 1939). Forests within the study area

consist of (1) ponderosa pine (Pinus ponderosa) mixed with Douglas-fir

(Pseudotsuga menziesii), generally on ridgetops and south- and west-facing slopes,

(2) quaking aspen (Populus tremuloides) stands on lower slopes and bottoms of moist

drainages, (3) quaking aspen stands mixed with blue spruce (Picea pungens) in

bottoms, lower slopes, and benches in mesic areas, and (4) Douglas-fir mixed with

blue spruce on higher slopes in drainages and on north-facing slopes. Tree cutting on

the study area has not occurred since the 1880s, when a light single-tree selection cut

for railroad ties occurred (Reynolds et al. 1985). Snags and trees with cavities are

relatively abundant throughout the study area (Reynolds et al. 1985). The forest

understory, consisting of over 100 species of grasses, forbs, and shrubs, is poorly

developed in all but the moist creek bottoms (Reynolds et al. 1985). Terrain is

moderately steep (20-80% slope) and elevations ranged from 2,550-2,855 m. The

study area is surrounded by forests composed of a similar mix of forest types and

ages.

Locating and Capturing Owls

76

Each spring and summer from 1981-1999, I searched the entire study area for

territorial males (Reynolds and Linkhart 1984). I identified territory boundaries by


telemetry in 1982-1983 (Linkhart et al. 1998). I located all suitable nesting cavities

(tree cavities with entrance diameters > 4 cm) within territories and checked each for

nesting owls (Reynolds and Linkhart 1984). Unpaired males typically sang

throughout a breeding season, whereas singing by paired (nesting) males dramatically

declined after egg-hatch (Reynolds and Linkhart 1987b). Due to extensive nest

searching throughout each summer I was confident that all nests were located. Most

nests were found during incubation (late May and early June) and all nests were

checked weekly (most often two or three times per week) until the young fledged

(mid July). Breeding adults were captured at nests (occasionally on perches or day


Linkhart 1984). I determined sex of adults by behavioral and morphological

characteristics (see Reynolds and Linkhart 1984). I banded owlets when they were 2-

3 weeks old (fledging occurs at 22-24 days; Reynolds and Linkhart 1987b). A total

215 Flammulated Owls were banded on the study area from 1981-1999, including

146 owlets (sex unknown) and 69 adults (29 males and 40 females). Because

unpaired males were difficult to capture, all banded males but one bred at least once.

Unless otherwise noted, data on territory and mate fidelity and natal and breeding

dispersal were determined from annual capture-recapture data on all adults banded

from 1981 to 1999.

Return Rate, Turnover, and Fidelity to Territory and Mate

77

I determined sex-specific return rates to the study area by dividing the number

of banded males and females that were recaptured for two or more consecutive years

by total number of banded owls (excluding one male and two females banded in

1999). I defined territory tenure as total years an owl occupied the same territory. I

defined turnover as the replacement of a marked individual on a territory by another.

Turnover was calculated for each sex by dividing the number of occasions in which a

marked individual was known to have been replaced on a territory by the total

number of occasions (opportunities) in which identities of individuals on a territory

were known in consecutive yr. Territory fidelity, reoccupancy of the same territory

by the same owl for consecutive years, was calculated by dividing total years in

which banded owls returned to original territories by total owl-years (count of years

in which I could ascertain that males or females returned to original territories or

dispersed to new territories). I presumed a banded male on a territory in year X was

present on the same territory in year X + 1, despite not recapturing him in year X + 1,

if: (1) a male was heard singing on the territory in year X + 1; and (2) the original

male was recaptured in a subsequent year on the same territory. This assumption,

which accounted for 7 different males, was based on the fact that there were no

instances of a male leaving a territory and returning in a subsequent year to the same

territory. I presumed a banded female that nested in year X on a territory did not

return to the same territory in year X + 1 if she was not seen or if no nest was found

that year, since there were no instances of a female failing to nest in one year but

nesting on the same territory in the previous year and subsequent year.

To determine if previous nesting failure affected territory fidelity, I compared

78

return of owls to original territories in year X + 1 following nest success in year X to

return of owls to original territories following nesting failure in year X. To determine

if breeding status affected territory fidelity, I compared return of owls that were

paired (new or original mate) in year X to return of owls that were unpaired in year X.

Finally, to determine if return of previous mate affected territory fidelity, I compared

return of owls to original territories when original mates also returned to original

territories to return of owls to original territories when original mates did not return to

original territories. I omitted cases of known breeding dispersal when evaluating

effects of the above factors on territory fidelity. I could not distinguish between

mortality and dispersal beyond the study area for owls that did not return to the study

area.

I defined pair duration as total years in which the same male and female

remained paired, and mate fidelity as the same male and female remaining paired for

multiple years. Mate fidelity was calculated by dividing total years in which the same

male and female remained paired for subsequent years by total pair-years (total years

in which both pair members were known to be alive after migration). Divorce

occurred when both pair members from a particular year were known to be alive in a

subsequent year but one or both had different mates (sensu Rowley 1983). I

calculated divorce rate by dividing total divorces by total pair-years. I assessed affect

of pair duration on reproductive success by comparing initiation of incubation and

productivity (i.e., mean owlets per brood) for pairs breeding for the first time and

pairs that bred two or more years. Initiation of incubation was determined either by

observing nest contents and female behavior during egg-laying and incubation, or by

79

back-dating from hatching (mean duration of incubation was 22 days; Reynolds and

Linkhart 1987) if nests were discovered after females began incubating.

My estimates of territory tenure, territory fidelity, pair duration, and mate

fidelity included adults banded in 1981 (n = 9 unknown histories) and adults banded

and/or recaptured in 1999 (n = 10 unknown futures). In 1981 I banded 4 males (one

returned for 3 yr, one for 2 yr, and two for 1 yr) and 5 females (one returned for 7 yr,

one for 4 yr, one for 3 yr, and two for 1 yr), and in 1999 I banded or recaptured 5

males (one had returned for 12 yr, one for 8 yr, one for 5 yr, one for 2 yr, and one for

1 yr) and 5 females (three had returned for 2 yr and two for 1 yr). Mean total years of

return for “fringe” males whose histories included 1981 or 1999 (3.9 + 1.3 yr) was

actually greater than “inclusive” males whose known breeding lifespans began after

1981 and terminated before 1998 (2.6 + 0.5 yr, n = 22), although the difference was

not significant (t = -0.94, df = 11, P = 0.37, two-tailed). Likewise, mean total years

of return for fringe females (2.4 + 0.6 yr) was greater than those of inclusive females

(1.7 + 0.3 yr, n = 31), but the difference also was not significant (t = -1.03, df = 14, P

= 0.32, two-tailed). In 1981 I identified 5 pair bonds (pair duration for one was 3 yr,

one was 2 yr, and for three was 1 yr), and in 1999 I identified 5 pair bonds (pair

duration for two was 2 yr and for three was 1 yr). Mean duration of fringe pair bonds

(1.6 + 0.2 yr) was greater than inclusive pair bonds (1.4 + 0.1 yr, n = 44), but not

significantly (t = -0.72, df = 12, P = 0.49, two-tailed).

Natal and Breeding Dispersal

I defined natal dispersal as movement from birth territory to breeding

territory, and breeding dispersal as movement between territories by breeding owls

80

(sensu Greenwood and Harvey 1982). Because Flammulated Owls nested a

maximum of once per breeding season (Linkhart et al. 1998, Chapter 2), breeding

dispersal refers to owls changing territories between years. Breeding dispersers may

or may not have retained the same mate. Breeding dispersal distances were straight-

line distances between nest trees in successive territories, and were measured in

Arcview (ESRI 1995).

I evaluated six correlates of breeding dispersal, two of which were possible

causes of dispersal. To test a nest-failure hypothesis, I determined if breeding

dispersal in yr X + 1 was preceded by nest failure in yr X. To test a non-return of

mate hypothesis, I determined if dispersal occurred when original mates did not return

to original territories. To test an inexperienced-breeder hypothesis, I compared the

proportion of dispersers that had one yr of breeding experience to the proportion with

> 2 yr of breeding experience. I could not determine whether owls bred elsewhere

before breeding on the study area. To test a mate-improvement hypothesis, I defined

quality of mates by their lifetime reproductive success (LRS), a good estimate of

individual fitness (Williams 1966, Newton 1989), and compared LRS of new mates

of dispersers with LRS of original mates. To test a territory-improvement hypothesis,

I defined territory quality by total owlets produced on a territory over the 19 yr study.

Thus, I compared total owlets produced on original territories of dispersers to total

owlets produced on new territories. Finally, to test a disperser-enhancement

hypothesis, I compared mean brood size of dispersers on original territories to mean

brood size on new territories.

Statistical analyses were performed using Statistical Analysis System (SAS

81

Institute 1995). I used Wilcoxon’s test (PROC NPAR1WAY) to evaluate sex

differences in total yr of return, and Fisher’s exact test (PROC FREQ) and Chi-square

contingency table analyses to evaluate sex differences in return rates and territory

fidelity, and effect of factors on territory fidelity. I used Wilcoxon and unpaired t-

tests assuming unequal variances (PROC TTEST) to evaluate differences between

sexes for fidelity and dispersal parameters, and to evaluate dispersal hypotheses; all

tests were two-tailed. Analyses were considered significant if P < 0.05. Throughout

the paper I report means + SE.

RESULTS

Territory Occupancy and Return Rate

Over the 19 yrs I determined fidelity and dispersal parameters of owls on 14

territories (Fig. 17). Each year a mean 8.1 + 0.5 territories were occupied by owls;

breeding pairs occupied a mean 4.5 + 0.2 territories annually while non-breeding

males occupied 3.6 + 0.5 territories (Chapter 2). Most territories remained fixed over

years; boundaries of territories shifted little despite turnover of owls on territories

(Chapter 5).

Annual return of adult Flammulated Owls from spring migration appeared to

be sex-biased, as in many migratory birds (Greenwood 1980, Ens et al. 1996). Males

settled into territories between 1-15 May. Females settled in territories over a longer

time period; in some years females appeared on territories as early as the first week of

May or as late as mid-June, but most were detected on territories between 10-20 May.

For sexes combined, rate of return to the study area for two or more consecutive years

82


Forest study area (white boundary), 1981-1999.

83

was 0.46 (31 of 67); return rate for males was 0.59 (17 of 29) and return rate for

females was 0.37 (14 of 38; Fisher’s exact test, P = 0.09). Only two owls (both

females) returned to breed on the study area after each was undetected for one

breeding season (one in 1998 and 1999).

Territory Fidelity

Mean tenure on territories for all adults was 2.1 + 0.2 yr (n = 78) but varied by

sex. Mean tenure was greater for males (3.0 + 0.5 yr, n = 31) than for females (1.6 +

0.2 yr, n = 47; z = 2.47, P = 0.02). Thus, mean tenure of males on territories was

nearly twice mean tenure for females. Thirty-nine percent (12 of 31) of males

occupied the same territory for > 3 yr, compared to just 13% (6 of 47) of females.

As data on tenure indicate, turnover on territories was female biased. Annual

turnover rate for females was 37% (16 of 43 opportunities) while turnover rate for

males was just 10% (7 of 69 opportunities; Fisher’s exact test, P = 0.001). Each year

there were 1-6 opportunities to assess turnover of each sex on breeding territories; 1

or 2 females were replaced on breeding territories in 15 of 19 yr while < 1 male was

replaced in any year and no males were replaced during the last 9 yr of the study

(1991-1999).

Calculations of territory fidelity were based on 62 male-years and 36 female-

years. Territory fidelity by both sexes was 91%. However, males were significantly

more faithful to territories than females; fidelity by males was 98% (61 of 62) while

fidelity by females was 78% (28 of 36; χ2 = 11.6, df = 1, P = 0.001). Estimates of

territory fidelity for both sexes may be overestimated given the high proportion of

territories that occurred near study boundaries. Fifty percent (7 of 14) of territory

84

centers were within one mean territory-diameter (439 + 77 m; Chapter 5) of a study

boundary and 100% were within two mean territory-diameters of a study boundary

(Fig. 17), suggesting that some individuals may have dispersed undetected.

Failure of previous year’s nest did not affect territory fidelity in males; return

to original territories following success of previous year’s nest was 76% (38 of 50)

while return following failure of the previous year’s nest was 78% (7 of 9). However,

54% (26 of 48) of females returned to original territories following a successful nest

while only 9% (1 of 11) returned following failure of the previous year’s nest

(Fisher’s exact test, P = 0.008). Thus, while the latter sample size was small these

data suggest that effect of nest failure on territory fidelity was female-biased.

Breeding status (paired or unpaired) appeared to affect territory fidelity of

females but not males. Seven males continued to occupy their original territories

despite being unpaired for 4 consecutive yr (1 male), 3 consecutive yr (1 male), 2

consecutive yr (1 male), and 1 yr (4 males). In contrast, while detecting females was

more difficult than males because females did not defend territories, I documented no

instances in which females remained unpaired during a breeding season.

Return of previous mate did not affect territory fidelity in either sex. For

males, return to original territories when previous mates returned was 70% (23 of 33)

and return to original territories when previous mates did not return was 83% (19 of

23; χ2 = 1.21, df = 1, P = 0.27). For females, return to original territories when

previous mates also returned was 51% (23 of 45) and return to original territories

when previous mates did not return was 40% (4 of 10; χ2 = 0.4, df = 1, P = 0.53).

Mate Fidelity

85

I recorded 82 nesting attempts (i.e., at least one egg laid) by 53 unique pairs

involving 47 females and 31 males. Mean pair duration was 1.4 + 0.1 yr; 74% (39 of

53) of pair bonds endured one year, 15% (8 of 53) endured two years, 9% (5 of 53)

endured three years, and 2% (1 of 53) endured four years. However, faithfulness to

mates was high; 96% of pairs (22 of 23 pair-years) retained the same mate from one

nesting attempt to the next. Only one divorce occurred in 19 yr. The divorced female

nested successfully with her original mate in 1997 and was found nesting with a new

male two territories distant in 1999. Although I did not locate her breeding in 1998,

her original male nested in 1998 and 1999 with a new female. I was unable to

determine cause of the divorce.

I assessed effect of pair duration on two measures of reproductive success,

initiation of incubation and productivity (i.e., mean owlets per brood), which were

associated with pair duration in other birds (e.g., Newton and Marquiss 1982,

Korpimaki 1988). Pairs that bred for the first time initiated incubation only 3 days

later (5 June + 1 day, n = 26, range = 27 May-20 June) than pairs that bred two or

more years (2 June + 2 days, n = 16, range 20 May-13 June), and this difference was

not significant (t = 1.31, df = 34, P = 0.20). First-year pairs had only slightly fewer

mean owlets per brood (2.2 + 0.1 owlets, n = 29, range 0-3) than pairs that bred two

or more years (2.4 + 0.2 owlets, n = 20, range 0-4), and again this difference was not

significant (t = -0.40, df = 33, P = 0.69). Thus, mates that bred together two or more

years did not appear to have significant reproductive advantages over pairs breeding

for the first time.

86

Natal Dispersal

I documented one instance of natal dispersal; a male owlet banded on the

study area in 1981 was found breeding on another territory 1.4 km distant in 1987.

This male also bred in 1988 and 1989 with a different female in each year, and

produced five total owlets in three successful nest attempts (Chapter 3). Whether this

male bred in years prior to 1987 outside of the study area was unknown.

Breeding Dispersal

I documented 9 instances of breeding dispersal, including one pair, in 98 bird-

years for a dispersal rate of 0.092. Females dispersed more frequently than males;

females moved to new territories on 8 occasions (in 36 female-years) for a dispersal

rate of 0.22, while just 1 male (in 62 male-years) moved to a new territory for a

dispersal rate of 0.02. Mean dispersal distance for all owls (570 + 52 m, n = 9) was

approximately 1.3 times mean diameter of territories (439 + 77 m; Chapter 5); 78% (7

of 9 owls, including the pair) dispersed to adjacent territories while 22% (2 of 9)

dispersed two territories away from original territories. Mean dispersal distance for

females was 580 + 58 m; one female dispersed 375 m, four dispersed 400-599 m,

three dispersed 600-799 m, and one dispersed 845 m. Dispersal distance for the one

male was 480 m. Given the high proportion of territories that occurred near study

boundaries (see Territory Fidelity), estimates of breeding dispersal rate and distance

may be underestimated.

87

Correlates of Breeding Dispersal

Nest-failure Hypothesis.—Eighty-eight percent of females (7 of 8) and the one male

dispersed after nesting successfully the previous year. Thus, failure of previous nest

was not associated with breeding dispersal.

Non-return of Mate Hypothesis.—In all cases where identity of mates was known,

female dispersal occurred when original males did not return. Sixty-seven percent of

females (4 of 6, excluding the divorced female and the female that dispersed with her

original male) dispersed when their original males did not return to original

territories. In the two remaining instances of female dispersal, unidentified males that

were unpaired occupied original territories. However, females did not always

disperse when original males did not return, based on the fact that 4 of 6 (67%) other

females did not disperse when original males failed to return. Thus, these data

suggest that factor(s) other than non-return of mate were involved in dispersal.

Breeding-experience Hypothesis.—Dispersers generally had > 1 yr of breeding

experience; 63% (5 of 8) of females had > 2 yr of breeding experience on original

territories and the one male had 3 yr of breeding experience. Thus, most owls that

moved to new territories were not inexperienced breeders.

Mate Improvement Hypothesis.—LRS of new mates on new territories was higher

than original mates on original territories, although not significantly. LRS of original

mates was 5.0 + 1.0 owlets (range 2 to 8) while LRS of new mates after dispersal was

7.1 + 0.9 owlets (range 2 to 9; t = -1.58, df = 12, P = 0.14; Table 1). The pair that

dispersed together was excluded from this analysis.

Territory Improvement Hypothesis.—Females dispersed to territories of higher long-

88

Table 1. Mate improvement hypothesis; comparison of lifetime production of owlets

by original males on territories from which females dispersed to lifetime production

of owlets by new males on territories where females dispersed.

Original Male New Male

Territory

(yr)

Lifetime

Owlets

Territory

(yr)

Lifetime Owlets

A15 (1982) 3 A4 (1983) 2

A15 (1984) 2 A12 (1985) 7

A12 (1986) --1 A12 (1986) --1

A29 (1983) 8 A12 (1984) 9

A24 (1983) 4 A29 (1984) 9

A7 (1990) 3 A29 (1991) 9

A13 (1996) 7 A8 (1998) 7

A10 (1997) 8 A8 (1999) 7

MEAN 5.0 MEAN 7.1

SE 1.0 SE 0.9

1 Female was excluded because she had the same mate before and after dispersal.

89

term productivity; territories where females originally nested produced a mean of 9.0

+ 1.8 owlets over the study while territories to which females dispersed produced a

mean of 20.5 + 3.8 owlets (t = -2.53, df = 7, P = 0.04; Table 2). In the one case of

male dispersal, movement was to a territory whose long-term productivity (n = 35

owlets) over the study was greater than on the territory where he originally bred (n =

7 owlets).

Disperser-enhancement Hypothesis.—Overall, dispersers did not attain higher

productivity on territories following dispersal. For females, mean brood size on

territories before they dispersed was 2.3 + 0.2 owlets, and mean brood size on

territories after they dispersed was 2.3 + 0.2 owlets (t = 0.25, df = 10, P = 0.81; Table

3). Mean brood size for the one dispersing male was 2.3 + 0.4 owlets (n = 3 nests)

before dispersal and 2 owlets (n = 1 nest) after dispersal.

DISCUSSION

Territory Fidelity

Male Flammulated Owls had significantly longer tenure on territories than

females, which probably resulted from males having greater territory fidelity than

females (98% vs 78%), and an apparently longer male lifespan. Male-biased site

fidelity is widespread among birds (e.g., passerines: Shields 1984, Murphy 1996;

shorebirds: Mills et al. 1996, Sydeman et al. 1996; raptors: Newton and Marquiss

1982, Korpimaki 1988, Forero et al. 1999), perhaps because in resource-defense

mating systems (Emlen and Oring 1977) males have more to gain by being faithful to

breeding territories than females (Greenwood 1980). This may be particularly true in

90

Table 2. Territory improvement hypothesis; comparison of total owlets produced

over 19 yr on territories where females nested before dispersal (“original territory”) to

total owlets produced on territories where females dispersed (“new territory”).

Original Territory New Territory Net Change

in

Territory

(yr)

Total Owlets Territory (yr) Total Owlets Owlets (%)

A15 (1982) 7 A4 (1983) 35 400

A29 (1983) 17 A12 (1984) 7 -59

A24 (1983) 4 A29 (1984) 17 325

A15 (1984) 7 A12 (1985) 7 0

A12 (1986) 7 A4 (1987) 35 400

A7 (1990) 3 A29 (1991) 17 467

A13 (1996) 14 A8 (1998) 23 64

A10 (1997) 13 A8 (1999) 23 77

MEAN 9.0 MEAN 20.5 MEAN 209

SE 1.8 SE 3.8

91

Table 3. Disperser-enhancement hypothesis; comparison of mean brood size for

females on territories before they dispersed (“original territory”) to mean brood size

on territories after they dispersed (“new territory”).

Original Territory New Territory

Territory (yr) Mean Brood

Size (N)

Territory (yr) Mean Brood

Size (N)

A15 (1981-2) 2.5 + 0.5 (2) A4 (1983) 2 (1)

A15 (1984) 2 (1) A12 (1985-6) 2.5 + 0.5 (2)

A12 (1985-6) 2.5 + 0.5 (2) A4 (1987-90) 2.5 + 0.3 (4)

A29 (1981-3) 2.7 + 0.4 (3) A12 (1984) 2 (1)

A24 (1981-3) 1.3 + 0.7 (3) A29 (1984) 3.0 + 0 (3)

A7 (1990) 3 (1) A29 (1991) --1

A13 (1995-6) 2.0 + 0 (2) A8 (1998) 3 (1)

A10 (1997) 3 (1) A8 (1999) 0 (1)

MEAN 2.3 MEAN 2.3

SE 0.2 SE 0.2

N 15 N 13

1 Brood size was unknown.

92

raptors such as Flammulated Owls where males are the primary foragers. While

longer lifespan in males has not been substantiated, it is plausible for two reasons.

First, mean longevity on the study area from 1981-1999 was significantly greater for

males than females (3.2 + 0.6 yr vs 2.0 + 0.3 yr; Chapter 2), although this disparity

may in part result from female-biased dispersal (see below). Second, unpaired males

annually occupied 10-70% of territories, which suggests a shortage of females in the

breeding population and may reflect a longer mean lifespan by males (Chapter 2).

Other studies of monogamous species have reported or suggested male-biased sex

ratios in adults (Shields 1984, Breitwisch 1989 and sources therein, Burke and Nol

1998, Gibbs and Faaborg 1990, Payne and Payne 1990, Murphy 1996, but see

Kenward 1999).

Breeding Dispersal

Female Flammulated Owls had a higher breeding dispersal rate than males

(0.22 vs 0.02), a pattern also found in many other birds including raptors (e.g.,

Greenwood 1980, Marquiss and Newton 1982, Forero et al. 1999). In all cases of

female dispersal except two (the one divorce excluded) I was able to determine that

the original male did not return. However, 4 of 6 (67%) females whose mates were

known not to return remained on original territories, suggesting that other factors

were involved. Dispersal by females when mates do not return is likely adaptive

because opportunity to breed may be lost while waiting for a new male, especially

since most females apparently return from spring migration after males. Individuals

of other species, including raptors, also dispersed when mates did not return

93

(Greenwood and Harvey 1976, Newton and Marquiss 1982, Warkentin et al. 1991,

Montalvo and Potti 1992, Forero et al. 1999).

Frequency and distance of breeding dispersal can be underestimated in small

study areas (Barrowclough 1978, van Noordwijk 1984, Koenig et al. 1996), as they

may have been in my study. Owls dispersed one or two territories away from original

territories, but half (7 of 14) of all territories were within one mean territory-diameter

(439 + 77 m), and all territories were within two mean territory-diameters, of a study

boundary (Fig. 17). Thus, owls that may have dispersed to territories farther than two

territories were less likely to be detected. Moreover, females (but not males) whose

nests failed the previous year had lower return rates (9%) to the study area than

females whose previous nests were successful (54%), suggesting either higher

mortality or higher dispersal (beyond the study area) among females whose nests

failed. Females whose prior nests failed did not appear to be in poorer physical

condition than females whose nests were successful. That females dispersed rather

than died following failed nests is supported by Haas (1998), who found that

American Robins (Turdus migratorius) and Brown Thrashers (Toxostoma rufum)

subjected to experimental nesting failure returned at significantly lower rates than

birds that had nested successfully. Indeed, other studies have found that following

nesting failure females returned less frequently to territories or they dispersed farther

than males (e.g., Shields 1984, Gavin and Bollinger 1988, Payne and Payne 1993,

Beletsky and Orians 1991, Murphy 1996).

Consequently, dispersal distance by female Flammulated Owls may be

bimodal with females dispersing longer distances following unsuccessful nests and

94

shorter distances following successful nests. Dispersal to nearby territories, as found

in many other studies (e.g., Payne and Payne 1996, Hannon and Martin 1996,

Williams 1996, Forero et al. 1999), may be beneficial because dispersers can best

judge quality of resources and owls in adjacent territories (Hinde 1956, Greenwood

1980, Ens et al. 1996). Means by which owls assess territories or mates for potential

future occupancy or pairing is uncertain but may be accomplished by extra-territory

movements by males and females (Reynolds and Linkhart 1990, BDL, unpubl. data).

Although dispersal following nesting failure is beneficial if chances of future nesting

success are improved (Murphy 1996), benefits of dispersing to more distant

territories, where owls are unlikely to have knowledge of resources or potential

mates, are not clear.

Dispersing owls moved to territories where productivity over the 19 yr study

was significantly greater than on territories from which they dispersed. Since mean

brood size of owls after they dispersed did not increase on new territories, and

because mean total owlets produced by new mates was not significantly greater than

total owlets produced by original mates, my ability to detect consequences of

breeding dispersal may require reproductive data collected over lifetimes for

particular individuals. I reported previously that long-term productivity and

occupancy of territories by breeding pairs was positively correlated with percentage

of old ponderosa pine/Douglas-fir forests and negatively correlated with percentage

of young Douglas-fir/blue spruce forests, and that old ponderosa pine/Douglas-fir

forests were used significantly more for foraging by radio-tagged males (Linkhart and

Reynolds 1997, Linkhart et al. 1998, Chapter 5). Thus, owls dispersed to territories

95

containing more old ponderosa pine/Douglas-fir and less young Douglas-fir/blue

spruce. Other raptors and passerines also dispersed to higher-quality territories, as

inferred by movements to territories having higher historical nesting success, better

prey resources, or lower risk of predation (Baeyens 1981, Marquiss and Newton

1982, Weatherhood and Boak 1986, Beletsky and Orians 1987, Matthysen 1990,

Forero et al. 1999).

Several avian studies have found that younger individuals were more likely to

disperse than older individuals (e.g., Beletsky and Orians 1987, Payne and Payne

1993, Badyaev and Faust 1996). However, breeding dispersal was not associated

with younger owls in this study. Most (5 of 8 females and 1 male) dispersing

Flammulated Owls had at > 2 yr of breeding experience.

Mate Fidelity

The high mate fidelity in Flammulated Owls contrasts with the pattern of

lower mate fidelity in most other migratory birds, which presumably occurs because

of difficulty maintaining pair bonds during migration and asynchronous arrival times

on breeding grounds compared to residents (Wickler and Seibt 1983, Murphy 1996).

That I documented only one divorce in this study suggests that mate (and possibly

territory) familiarity conferred advantages to breeding Flammulated Owls. Studies of

raptors and other birds reported that mate fidelity was associated with higher

productivity, nest success, or earlier nesting (e.g., Newton 1982, Korpimaki 1988,

Orell et al. 1994, Murphy 1996), although other species showed no apparent benefit

by being faithful to mates (e.g., Freed 1987, Warkentin et al. 1991). Flammulated

Owls paired together for > 2 yr did not initiate incubation significantly earlier nor

96

were broods significantly larger than pairs breeding for the first time, although I could

not determine if owls gained breeding experience before their tenure on the study

area. Alternatively, owls may have long-term pair bonds simply because they are

constrained from other alternatives (Freed 1987); male choice appears to be restricted

by female availability while female choice may be limited to males available in

neighboring territories with previous breeding experience. High mate fidelity also

may be facilitated by high territory fidelity by males (Murphy 1996). Indeed, mean

tenure of males on territories (3.0 + 0.5 yr) was nearly twice mean tenure of females

on territories (1.6 + 0.2 yr), suggesting that returning females were likely to find their

original males on territories. That mean pair duration over the study (1.4 + 0.7 yr)

was approximately the same as mean tenure on territories by females suggests that

pair duration was limited by territory tenure of females.

Habitat Selection

Most male Flammulated Owls occupied a single territory their entire known

reproductive lives; only one breeding male changed territories in 62 male-years.

Moreover, males often continued to occupy original territories despite being unpaired

up to four consecutive years. Recall that mean tenure for males was calculated for

males that nested at least once. These males occupied a mean 58% (+ 3%) of all

territories annually (Chapter 2). Because I rarely captured unpaired males, I was

unable to determine their tenure on territories or extent to which they may have

dispersed. However, observation of several unpaired males, distinguished by unique

vocal characteristics (e.g., song pitch), indicated that most reoccupied their original

territories following return from migration (pers. observ.). An exception was an

97

unpaired male that occupied three different territories over four breeding seasons

(pers. observ.). Given that females (new and returning) only occupied half of all

territories annually and that breeding generally occurred on the same territories each

year (Chapter 5), many newly arriving males may settle on, and commit their

reproductive lives to, territories where breeding occurs irregularly. Consequently,

high site fidelity by males may counter predictions based on habitat selection models

that assume animals select habitats conferring highest reproductive success, and if

higher-quality habitats become available individuals should move to the new sites

(Fretwell and Lucas 1969). Thus, territory fidelity by Flammulated Owls may be

considered a suboptimal form of habitat selection with respect to territory quality

(Switzer 1993). It may be more profitable for a male of a long-lived species such as

the Flammulated Owl to await the probable arrival of a female in a territory where he

is familiar with the location of important nesting resources (see sources cited in

Introduction; Chapter 5), and possibly engage in extra-pair copulations when mates

are unavailable (Reynolds and Linkhart 1990). Males of other species including

raptors have been documented remaining on a single territory, even when higher-

quality territories apparently were available (Krebs 1971, Best 1977, Searcy 1979,

Bedard and LaPointe 1984, Janes 1984, Woolfenden and Fitzpatrick 1984, Lanyon

and Thompson 1986, Korpimaki 1988).

98

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Badyaev, A. V., and J. D. Faust. 1996. Nest site fidelity in female Wild Turkey:

potential causes and reproductive consequences. Condor 98:589-594. Baeyens, G. 1981. Functional aspects of serial monogamy: the Magpie pair-bond in

relation to its territorial system. Ardea 69:145-166. Barrowclough, G. F. 1978. Sampling bias in dispersal studies on finite areas. Bird-

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CHAPTER V

DETERMINING HABITAT QUALITY FROM LONG-TERM DEMOGRAPHICS IN

BREEDING FLAMMULATED OWLS

Abstract. – Basing the determination of habitat quality on long-term demography is

generally regarded as a valuable approach for understanding how animal populations

use space, but it has little empirical support. From 1981-1999, I measured

demographic performance of Flammulated Owls (Otus flammeolus) in a 511 ha study

area of mixed conifer forest in central Colorado. Boundaries of 12 owl territories

remained generally stable over the study, enabling me to determine demographic

performance for territories rather than for individuals. I used demographic

parameters that distinguished among territories to infer relative territory quality so

that habitat conditions could be compared across territories and with non-territory

habitat. Territories differed in total breeding yr, because some territories usually were

occupied by breeding pairs annually while other territories usually were occupied by

bachelor males. Productivity varied among territories, ranging from 0 to 35 owlets.

Mean territory tenure, which I used as an estimate of survival, and pair duration did

not differ among territories. Breeding dispersal resulted in females moving to

territories where productivity was significantly higher.

Productivity was positively correlated with territory area in ponderosa

pine/Douglas-fir forests, and with greater crown volume in the second-largest (33.0 –

48.2 cm) of four tree dbh (diameter at breast height) categories. Productivity was not

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correlated with density of cavity-trees. However, cavity trees clearly distinguished

territory from non-territory habitat, given that non-territory habitat contained < 10%

of mean cavity-tree density within territories. Few structural characteristics

distinguished combined territories from non-territory habitat. In comparisons of non-

territory habitat with three classes of territory distinguished by differing productivity,

only the high-productivity class (consisting of just one territory) contained greater

tree density and basal area in the larger dbh categories. Based on the fact that

moderate-productivity territories showed no differences in forest structure from

unoccupied habitat, and that low-productivity territories (usually occupied by

bachelor males) actually contained denser forests and smaller trees than non-territory

habitat, at least some portions of non-territory habitat may have been suitable for

territory establishment except for scarcity of cavity trees. Monitoring of predation on

artificial nests for 1 yr and relative prey abundance for 2 yr revealed no patterns

among selected territories differing in productivity, suggesting these factors were not

associated with habitat quality.

My results indicate that cavity-tree availability primarily determined where

owls established territories, while forest type and structure determined whether a

territory was more often occupied by breeding pairs or by bachelor males. High-

quality breeding habitat for Flammulated Owls in this study was characterized as

mature, relatively open stands of ponderosa pine/Douglas-fir that contained sufficient

cavity trees for nesting.

Habitat correlations with bachelor yr differed markedly from correlations with

productivity, indicating that inferring habitat quality based on abundance or duration

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of territory occupancy by males would be misleading. Breeding year may be a good

surrogate for productivity in future efforts to identify important breeding habitats for

this species, at least where other demographic parameters such as nesting success are

similar to mine.

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INTRODUCTION

Birds often occupy a range of habitats differing in resources necessary for

reproduction and survival (Fretwell and Lucas 1969, Cody 1985). These resources

include food, foraging sites, nesting sites, and places to avoid predators or

competitors (Steele 1993). Models predict that, in stable environments, birds settle

into high-quality habitats first and then progressively settle into lower-quality habitats

as high-productivity habitats are filled (Fretwell and Lucas 1969, Alatalo et al. 1985,

Pulliam 1988). This settlement pattern has been generally supported by field studies

(e.g., Brooke 1979, Zimmerman 1982, Moller 1983, Korpimaki 1988, Matthysen

1990).

Stable breeding populations depend on high-quality habitats because these

habitats confer sufficient reproduction and survival for populations to grow (Wiens

1986, Pulliam 1988). High-quality habitats also serve as sources of individuals

emigrating to low-quality or sink habitats, where reproduction is insufficient to

balance local mortality (Pulliam 1988). The distinction between high-quality and

low-quality habitats is important in conservation efforts because stable breeding

populations depend on high-quality habitats (Wiens 1986, Pulliam 1988, Howe et al.

1991). However, determining quality of breeding habitats has proven difficult and

procedures deemed best for distinguishing high-quality and low-quality habitats have

been little tested.

Most studies inferring the quality of avian breeding habitats have been based

on relative abundance (e.g., Anderson and Shugart 1974, Whitmore 1977, James and

Warner 1982, Wenny et al. 1993). This approach assumes that higher population

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densities reflect breeding habitats of higher quality. However, identification of

important breeding habitats based on species abundance is not always reliable and

may misrepresent suitability of habitats for breeding (Van Horne 1983, Maurer 1986,

Martin 1992). High population densities may occur in sink habitats when prey

populations are cyclically high (Korpimaki 1988), or when surplus individuals

emigrate from source habitats (Krebs 1971). That population density is not always

correlated with reproductive success was shown in studies where dense populations

actually produced fewer offspring than less dense populations (Korpimaki 1988,

Vickery et al. 1992, Porneluzi et al. 1993). Studies have shown that in some habitats,

particularly those that may be sub-optimal, the majority of singing males remain

unmated through the breeding season (Probst and Hayes 1987, Gibbs and Faaborg

1990, Linkhart and Reynolds 1997, Burke and Nol 1998).

Reliable identification of high-quality habitats requires measures of fitness

(direct or indirect quantification of survival and reproductive success) because choice

of habitat features that increase individual fitness should be favored over evolutionary

time (Fretwell and Lucas 1969, Van Horne 1983, Martin 1992). However, relatively

few studies have evaluated survival or reproductive success in inferring habitat

quality because quantifying demographic variables requires considerable time and

labor (Wiens 1973). In addition, measures of survival are complicated by

immigration and emigration of individuals among breeding sites (Raphael et al.

1996). Some studies have shown that indirect measures of fitness may act as

surrogates for direct measures, including nesting behaviors (Vickery et al. 1992),

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mate and site fidelity (Newton and Marquiss 1982, Haig and Oring 1988, Part 1994),

and breeding dispersal (Beletsky and Orians 1987, Matthysen 1990).

Long time periods are necessary to document patterns in demographic

variables of animals, as shown by studies of mammals (Peterson et al. 1984), birds

(Woolfenden and Fitzpatrick 1984), and reptiles (Gill et al. 1983). Short-term

measures of avian reproductive success may be biased by individual characteristics

such as age, condition, and experience (Coulson 1966, Nol and Smith 1987,

Korpimaki 1988, Ens et al. 1992). Stochastic environmental variation can also

compromise the reliability of short-term studies because these studies may fail to

capture the ecological and evolutionary consequences imposed by severe

environmental crunches (Grant 1986, Wiens 1986). Consequently, reliable inferences

of habitat quality require demographic data collected over long time periods (Van

Horne et al. 1997).

Choice of spatial scale is also important in investigations of habitat

relationships (Wiens 1986, Forman and Godron 1989, Kotliar and Wiens 1990).

While landscape-level studies are important for correlating avian population changes

with habitat (e.g., Martin 1981, Roth and Johnson 1993, Forsman et al. 1996),

territory-level studies may elicit the greatest understanding of habitat quality because

the quality and quantity of territory resources directly affect individual fitness

(Fretwell and Lucas 1969, Raphael et al. 1996). Identification of specific habitat

variables (e.g., nest-tree and foraging site characteristics; Matsuoka et al. 1997) in

territory-level studies also facilitate the development of species-specific management

plans.

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Few studies have linked demographic performance of individuals with

specific habitat features at the scale of individual territories. Most territory-level

studies have compared demographic performance with general habitat features, such

as vegetation types (Newton and Marquiss 1976, Fischer 1980, Zimmerman 1982,

Korpimaki 1988, Ens et al. 1992, Riddington and Gosler 1995). Only a few studies

(Janes 1984, Vickery et al. 1992, Breininger et al. 1995, Braden et al. 1997, Ortega

and Capen 1999) have statistically correlated demographic performance with specific

habitat variables such as perch sites or percentage of understory vegetation, and only

Janes (1984) reported demographic data collected from more than three yr. Thus,

little is known regarding the utility of assessing habitat quality by quantifying the

relationship between specific habitat characteristics on territories and long-term

demographic performance.

Here I identify the determinants of habitat quality for a breeding population of

Flammulated Owls by comparing habitat conditions in territories of varying quality as

inferred from demographic variables for a population in central Colorado studied

from 1981-1999. The owl is an obligate cavity-nester associated with montane

forests from the Rocky Mountains to the Pacific Coast, and from southern British

Columbia to Vera Cruz, Mexico (McCallum 1994a, Linkhart et al. 1998).

Flammulated Owls are insectivorous, feeding mostly on small lepidopterans

(Reynolds and Linkhart 1987), and are migratory, probably wintering in montane

forests of El Salvador, Guatemala, and Jalisco, Mexico (American Ornithologists’

Union 1983). The owls are listed as sensitive species in four western regions of the

USDA Forest Service (Verner 1994), because they are Neotropical migrants and are

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associated with mature forests (McCallum 1994a, Linkhart et al. 1998), and because

densities have declined following timber harvests (Marshall 1957, 1988, Phillips et al.

1964, Franzreb and Ohmart 1978).

My objectives were: (1) to describe demographic performance (territory

occupancy, reproductive success, territory tenure, pair duration, and breeding

dispersal) on territories, and identify variables that distinguished among territories;

and (2) to identify components of habitat quality by correlating habitat variables with

demographic performance on territories. I evaluated habitat quality based on two

questions: (a) Across territories, was demographic performance associated with

forest type and structure (e.g., tree density, basal area, and crown volume)? (b) Does

forest structure differ among territories and between territory and non-territory (i.e.,

unoccupied) habitat? I predicted that reproductive success would be positively

associated with area in ponderosa pine/Douglas-fir, a forest type most frequently used

by foraging males in an earlier study (Linkhart et al. 1998), and I also predicted that

highest-quality territories would contain the lowest density of trees and trees with

largest mean diameter, while non-territory habitat would be characterized by the

reverse. I also evaluated three possible limiting factors associated with the owl’s

habitat relationships, and predicted that highest-quality territories were characterized

by highest densities of cavity trees, lowest rates of nest predation, and greatest prey

abundance.

METHODS

Study Area

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Colorado. I established boundaries of the 511 ha study area after initial surveys

(1980) for territorial Flammulated Owls. After I confirmed the presence of owls,

boundaries were drawn around an area large enough to contain approximately 20

territorial males, based on an estimate of territory size for this species (274 m in

diameter; Marshall 1939). Forests within the study area generally consist of (1)

ponderosa pine (Pinus ponderosa) mixed with Douglas-fir (Pseudotsuga menziesii),

generally on ridgetops and south- and west-facing slopes, (2) quaking aspen (Populus

tremuloides) stands on lower slopes and bottoms of moist drainages, (3) quaking

aspen stands mixed with blue spruce (Picea pungens) in bottoms, lower slopes, and

benches in moist areas, and (4) Douglas-fir mixed with blue spruce, on higher slopes

in drainages and on north-facing slopes. Tree cutting on the study area has not

occurred since the 1880s, when a light harvest for railroad ties occurred. Snags and

trees with cavities were relatively abundant throughout the study area (Reynolds et al.

1985). The forest understory, consisting of over 100 species of grasses, forbs, and

shrubs, was poorly developed in all but the moist creek bottoms (Reynolds et al.

1985). Terrain was moderately steep (20-80% slope) and elevations ranged from

2,550-2,855 m. The study area is surrounded by forests composed of a similar mix of

forest types and ages.

Delineating Territories and Locating Nests

Each spring and summer from 1981-1999, I searched the entire study area for

territorial males (Reynolds and Linkhart 1984). I identified territory boundaries by


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telemetry in 1982-1983, which assisted in determining range in territory sizes and

identifying topographic characteristics of boundaries (Linkhart et al. 1998). Most

territory boundaries changed little from year to year (Fig. 18), despite male turnover

on each territory (see Results), as evidenced by the fact that males commonly sang

from the same groups of trees (and often the same trees) throughout their territories

annually, particularly along boundaries that were in close proximity to those of

neighboring males (Linkhart 1984, Linkhart et al. 1998). While I may have missed

some annual fluctuation along boundaries less defended by males due to absence of

adjacent neighbors, lack of annual change in most boundaries indicated that territories

were generally constant in time and space.

Each year I located all suitable nesting cavities (tree cavities with entrance

diameters > 4 cm, generally excavated by Northern Flickers [Colaptes auraetus] and

occasionally sapsuckers [Sphyrapicus spp.]) within territories and checked each for

nesting owls (Reynolds and Linkhart 1984). Unpaired males typically sang

throughout a breeding season, whereas singing in paired, nesting males dramatically

declined after egg-hatch (Reynolds and Linkhart 1987b). Due to time and effort spent

searching for owls and nests each year, I was confident that all nests were located. I

found most nests during incubation (late May and early June) and checked the status

of nests at least weekly (often two or three times per week) until the young fledged

(mid July). Breeding adults were captured at nests (occasionally on perches or day


Linkhart 1984). Sex of adults was determined by behavioral and morphological

characteristics (see Reynolds and Linkhart 1984). I banded owlets when they were

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Forest study area (white boundary), 1981-1999. Heavy white polygons

represent territories (A15 and A24) not occupied after 1984 and light white

polygons represent territories (A4, A8, A11, and A29) prior to boundary shifts.

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14-21 d old (fledging occurs at 22-24 d; Reynolds and Linkhart 1987b). A total 215

owls were banded over the study: 146 owlets (sex not determined) and 69 adults (29

males and 40 females). Because unpaired males were difficult to capture, all banded

males but one were owls that had > 1 nesting attempt.

Demographic Performance

Because territories generally were constant in time and space over the study,

despite individual turnover on each territory, I calculated cumulative demographic

performance over 19 yr for 12 territories. I omitted 2 territories (A15 and A24) from

calculations of demographic performance because they were not occupied after 1984.

For each territory I calculated the following variables: (1) occupied yr, the sum of all

yr (out of 19 yr possible; not necessarily consecutive) territories were occupied by

breeding pairs or bachelor males; (2) breeding yr, the total yr (not necessarily

consecutive) territories were occupied by breeding pairs; (4) bachelor yr, the total yr

(not necessarily consecutive) territories were occupied by unpaired males; (5) owlets,

the total number of banding-age owlets produced by pairs over the study; and (6)

nesting success, calculated by dividing the number of nests over the study that

fledged at least one young by the total number of nests in which eggs were layed. I

excluded 6 nests from calculations of nesting success whose outcome was uncertain

(A12 territory-1981; A18-1981; and A10-1990) or failed due to anthropogenic causes

(A18-1984, A29-1991, and A29-1994). In addition, fidelity and dispersal variables

were quantified for individual territories based on annual capture-recapture data on

adults banded over the study, including: (7) mean pair duration—the mean total yr in

which the same male and female remained paired, calculated by dividing the summed

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duration of all pair bonds by the total number of new pair bonds; and (8) mean

territory tenure—the mean yr that individuals of each sex remained on the same

territory, calculated by dividing total yr of occupancy by banded individuals on a

territory by total number of uniquely banded individuals on the territory. I used

territory tenure as an estimate of survival. Although I did not know the fate of owls

that failed to return to the study area, I assumed they died because movement between

territories (breeding dispersal) only occurred infrequently. Finally, I quantified (9)

breeding dispersal by summing the frequency of dispersal events on each territory.

In calculating fidelity variables, I presumed a male banded on a territory in

year X was present on the same territory in year X + 1, despite not recapturing him in

year X + 1, if: (1) a male was heard singing on the territory in year X + 1, and (2)

the same banded male was recaptured in a subsequent year on the same territory.

This assumption was based on the fact that I did not document any instances in which

a male left a territory and was recaptured in a subsequent year on the same territory. I

presumed a banded female that nested in year X did not return to her same original

territory in year X + 1 if no nest was found, since I did not document any instances in

which a female failed to nest in one year but nested on the same territory in the

previous and subsequent years.

My estimates of territory tenure and pair duration included adults banded in

1981 (n = 9 unknown histories) and adults banded and/or recaptured in 1999 (n = 10

unknown futures), because estimates of these variables for “fringe” individuals did

not significantly differ from “inclusive” individuals whose breeding lifespans began

after 1981 and terminated before 1999. Mean (+ standard error, SE) total yr of return

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for fringe males (n = 4 in 1981 and 5 in 1999) was 3.9 + 1.3 yr, and mean total yr of

return for inclusive males was 2.6 + 0.5 yr (n = 22; t = -0.94, df = 11, P = 0.37, two-

tailed). Likewise, mean total yr of return for fringe females (n = 5 in 1981 and 5 in

1999) was 2.4 + 0.6 yr and mean total yr of return for inclusive females was 1.7 + 0.3

yr (n = 31; t = -1.03, df = 14, P = 0.32, two-tailed). Mean pair duration for fringe

pairs (n = 5 in 1981 and 5 in 1999) was 1.6 + 0.2 yr and pair duration for inclusive

pairs was 1.4 + 0.1 SE yr (n = 44; t = -0.72, df = 12, P = 0.49, two-tailed).

I used Chi-square goodness-of-fit tests to determine if specific demographic

variables differed among the 12 territories, where the expected value in each case was

the variable mean across all territories. Degrees of freedom for these tests was 10

instead of 11 to account for estimation of the expected value equal to the variable

mean (Daniel 1990). I then used demographic variables that distinguished among

territories in correlations with habitat variables. In addition, I used a paired t-test

assuming unequal variances to compare productivity (total owlets over 19 yr) on

original territories where females nested before dispersing to productivity on new

territories where females nested after dispersing. For each female, I paired

productivity on the original territory with productivity on the new territory.

Vegetation Sampling

During summer 1998 I established sampling points 100 m apart on 11 east-

west transects set at 200 m intervals across the study area. To increase overall sample

sizes within territories and sample sizes among forest types of territories, I

supplemented the 1998 transects with additional 100 m interval transects within

territories in 1999. Overall, during 1998-1999 I established 209 territorial sampling

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points (18 + 2 territory-1) and 143 non-territorial sampling points. Locations of

sampling points were determined with Global Positioning System (GPS; Trimble),

and mapped in Arcview (ESRI 1995).

I quantified forest overstory (any tree > 2 m tall and > 2.5 cm dbh, diameter at

breast height) using an angle-gauge (Dilworth and Bell 1977), and identified 6-10

sample trees (mean 7.6 + 0.1) in a variable-radius circle around each sampling point.

For each sample tree I determined species, dbh, and crown volume (volume of live

foliage, based on shape, width, and height of the crown; sensu Mawson et al. 1976).

Based on all sample trees, at each sampling point I calculated estimates of crown

volume ha-1 (CRVL), basal area ha-1 (BA), and trees ha-1 (TPH; Dilworth and Bell

1977). I also calculated estimates of these variables according to 4 dbh size-

categories (2.5-17.7 cm, 17.8-32.9 cm, 33.0-48.2 cm, and > 48.3 cm; Table 1), based

on distribution of tree sizes across the study area.

I quantified forest understory (herbs, shrubs, and trees < 2 m tall and 2.5 cm

dbh) at sampling points using line-intercept (Canfield 1941) along two perpendicular

12.5 m tape lines (following cardinal directions) which crossed on the sampling point.

At 0.5 m intervals I recorded all species that ‘hit’ a 6 mm diameter vertical rod within

four height categories: 1-4 cm; 5-49 cm; 50-199 cm; and > 200 cm. I calculated

proportion of cover in each height category by dividing number of hits by total

number of sampling intervals (50; Table 4).

Determination of Forest Types

I used classification and regression tree analyses (CART; Breiman et al. 1984)

to develop a model of forest types on the study area based on data associated with the

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Table 4. Forest overstory and understory variables used in correlations with

demographic variables.

Variable Description

MN_DBH mean dbh (cm) of individual trees

BA basal area (m2 ha-1)

T_BAD1 basal area (m2 ha-1) in dbh category 1 (2.5-17.7 cm)



T_BAD4 basal area (m2 ha-1) in dbh category 4 (> 48.3 cm)

P_BAD1 proportion of BA in dbh category 1 (2.5-17.7 cm)



P_BAD4 proportion of BA in dbh category 4 (> 48.3 cm)

TPH trees ha-1

T_TPHD1 trees ha-1 in dbh category 1 (2.5-17.7 cm)



T_TPHD4 trees ha-1 in dbh category 4 (> 48.3 cm)

P_TPHD1 proportion of TPH in dbh category 1 (2.5-17.7 cm)



P_TPHD4 proportion of TPH in dbh category 4 (> 48.3 cm)

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MN_CRVL mean crown volume (m3) of individual trees

CRVL total crown volume (m3 ha-1)

T_CRVLD1 crown volume (m3 ha-1) of trees in dbh category 1 (2.5-17.7 cm)

T_CRVLD2 crown volume (m3 ha -1) of trees in dbh category 2 (17.8-32.9 cm)

T_CRVLD3 crown volume (m3 ha-1) of trees in dbh category 3 (33.0-48.2 cm)

T_CRVLD4 crown volume (m3 ha-1) of trees in dbh category 4 (> 48.3 cm)

P_CRVLD1 proportion of CRVL in dbh category 1 (2.5-17.7 cm)



P_CRVLD4 proportion of CRVL in dbh category 4 (> 48.3 cm)

UNVG_5 proportion of understory cover in height category 1-4 cm

UNVG5_50 proportion of understory cover in height category 5-49 cm

UNVG_50 proportion of understory cover in height category 50-199 cm

UNVG_200 proportion of understory cover in height category > 200 cm

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352 sampling points: field-identification of forest types; color-band reflectance

derived from thematic mapper (LANDSAT) satellite image (30 m resolution; data

gathered 14 September 1997) in GIS; and slope, aspect, and elevation derived from a

digital elevation model (DEM; 30 m resolution) in GIS. CART analyses resulted in

the delineation of six forest types on the study area: (1) ponderosa pine/Douglas-fir;

(2) Douglas-fir; (3) Douglas-fir/blue spruce; (4) quaking aspen; (5) quaking

aspen/blue spruce; and (6) Douglas-fir/limber pine (Figure 3). Overall model

accuracy was 83%, based on comparing field-assigned and analysis-assigned forest

types at sampling points. I then used ArcView (ESRI 1995) to map the six forest

types and determine area (ha) and percentage of area in each forest type for the study

area, and area (ha) and percentage of area of forest types in each territory.

Comparison of Structure Among Forest Types

To compare forest structure (overstory and understory variables; Table 4)

among disproportionately sampled forest types, I weighted individual sampling points

proportional to amount of area in each forest type and then pooled these weighted

values within each forest type and calculated from these a mean forest-type value for

each habitat variable. This is similar to estimation for stratified random sampling

(Cochran 1977), with the overall sample size maintained at the total number of

sampling points:

Wx= (a1/at)nt / n1 (Equation 1)

where, Wx was the weight factor for sampling points in forest type x, a1 was area

occupied by forest type x, at was the total area occupied by all forest types, nt was the

total number of sampling points, and n1 was the number of sampling points in forest

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type x. I then used ANOVA and Tukey’s Test (PROC GLM; SAS 1996) to compare

specific forest structure variables (Table 1) among the six forest types.

Comparisons Between Vegetation And Owl Demography

Determination of territory habitat was based on all sampling points within

combined territories. Boundaries of most territories remained fixed over the study

(Fig. 18). For four territories (A4, A8, A11, and A29) whose boundaries shifted

during 1983-1987, I evaluated habitat variables based on boundaries after the shifts

occurred, since new boundaries remained unchanged for the remainder of the study

and accounted for the majority (63-84%) of total study yr. I omitted two territories

(A15 and A24) from analyses of habitat quality because these territories were not

occupied after 1984 and much of the area within their boundaries was incorporated

into adjacent territories. I based determination of non-territory habitat on all

sampling points located outside of territory boundaries.

Relationship between forest type and structure and owl demography.—To assess the

relationship between forest type and demographic performance across territories, I

determined the area (ha) and proportion of total area of forest types within owl

territories using ArcView (ESRI 1996). I used Pearson product correlation analyses

(PROC REG; SAS 1996) to examine correlations among forest types and

demographic performance.

To evaluate relationships between forest structure (forest and understory

variables; Table 1) and demographic performance across territories, I weighted

individual sampling points proportional to amount of area in each forest type within a

particular territory using Equation 1. I then pooled these weighted values within each

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territory and calculated from these a mean territory value for each habitat variable. I

used Pearson product correlation analyses (PROC REG; SAS 1996) to examine

correlations between forest structure variables and demographic variables. I then

used stepwise regression analyses (PROC REG; SAS 1996) to evaluate the inter-

correlations among forest structure variables found significantly correlated with

demographic variables, and to address the increased probability of incurring Type 1

errors associated with multiple univariate comparisons.

Comparison of territory and non-territory habitat.—I compared forest structure in

territory vs non-territory habitat in two ways. First, I assessed whether forest

structure for combined territories differed from non-territory habitat. I weighted

sampling points to account for under- or over-sampling of each forest type within

combined territories or non-territory habitat using Equation 1. I used ANOVA

(PROC GLM; SAS 1996) to compare territory vs non-territory habitat for specific

variables (Table 4).

To further determine if forest structure differed between territory and non-

territory habitat, I compared non-territory habitat to three classes of territories, based

on the range in total owlets over the study (see Results): high-productivity (> 23

owlets), moderate-productivity (13 - 23 owlets), and low-productivity (< 8 owlets). I

defined these classes based on natural "breaks" in the range of total owlets; 12 owlets

separated high-productivity and moderate-productivity habitat classes, and 5 owlets

separated the moderate-productivity and low-productivity habitat classes. Although

this approach resulted in the high-productivity class containing only one territory

(A4) compared to the moderate-productivity (5 territories) and low-productivity (6

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territories) classes, I evaluated this territory separately because preliminary analyses

indicated its forest structure differed significantly from all other territories.

To compare forest structure in the three classes of owl habitat with non-

territory habitat, within each habitat class I weighted sampling points to account for

under- or over-sampling of each forest type using Equation 1. I used ANOVA and

Tukey’s Test (PROC GLM; SAS 1996) to compare specific forest structure variables

(Table 4) between territory and non-territory habitat.

Possible Limiting Factors Associated With Habitat Relationships

Density of Cavity Trees.—I annually located all snags (standing, dead trees) and live

trees with suitable cavities for nesting (hereafter, cavity trees) within territories.

Beginning in 1983, as part of a long-term investigation to determine the spatial and

temporal dynamics of cavity trees (Reynolds et al. 1985), I also mapped the location

of cavity trees outside of territories.

Quaking aspen cavity-trees, which accounted for the majority (> 80%) of owl

nests annually (unpubl. data), typically had shorter longevity (snags 1-10 yr, live trees

5-15 yr) than conifer cavity-trees (snags > 10 yr, live trees > 20 yr; pers. observ.).

However, the general location (i.e., forest patch) and relative density of deciduous

cavity-trees changed little from 1981-1999. Given the observed constancy in spatial

dynamics and density of conifer and deciduous cavity-trees over time, I used the

location of cavity trees in 1999 as the basis for inferring general location and density

of cavity trees over the19 yr study. I mapped the locations of all cavity trees found in

1999 on a DEM layer in ArcView (ESRI 1995), and determined the density of cavity

trees within territories and outside territories. I assessed correlation between density

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of cavity trees and demographic performance on territories with Pearson product

correlation analysis (PROC REG; SAS 1996).

Nest Predation.—Effects of nest predation were determined in two ways. First, I

monitored predation of artificial nests in six owl territories during summer 1999: two

territories producing the most owlets over the 19 yr study, two territories producing

the least owlets, and two territories producing an intermediate number of owlets. Red

Squirrels (Tamiasciurus hudsonicus) were inferred as the primary predator of owl

nests because I observed no other avian or mammalian predators of tree cavities over

the study (but see Linkhart and Reynolds 1994). I determined sites for six artificial

nests in each territory by placing two nests in each of the three most common forest

types: ponderosa pine/Douglas-fir, Douglas-fir, and quaking aspen/blue spruce.

Where possible, I used unoccupied, natural tree cavities with entrance diameters > 4

cm. In territories lacking sufficient natural cavities, I attached wooden nest boxes (40

x 20 x 20 cm, entrance 6 cm in diameter) to live trees 3-5 m above ground. Boxes

had a southern exposure, which approximated the orientation of most owl nests

(unpubl. data), and contained 1-3 cm of partially decomposed woody debris. Of 36

total artificial nests, 18 were in nest boxes. Most territories contained 3 nest boxes, 2

of which were generally in Douglas-fir forests because these forests contained the

fewest natural cavities. I placed two fresh Bobwhite Quail (Colinus virginianus) eggs

in each artificial nest beginning 10 June, when owl eggs were being incubated, and

monitored weekly predation rates until 25 July, when all but one owl brood had

fledged. Eggs lost to predation were replaced weekly, as well as eggs that remained

in nests beyond two weeks. Differences in predation rates of artificial nests among

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territories and among forest types were determined with chi-square (PROC GLM),

and correlations between predation rates and productivity on territories were

determined with Pearson product correlation analysis (PROC REG; SAS 1996).

Second, during summer 1999 I estimated relative density of Red Squirrels in

the six owl territories where I monitored predation of artificial nests (see above).

Since Red Squirrel territories typically contain one large midden, which contain

caches of conifer cones for winter food supplies (Gurnell 1984, Hurly and Lourie

1997), I used density of large middens as a measure of relative squirrel density in owl

territories. I mapped all large middens (dimensions > 2 x 3 m, depth > 20 cm) on a

DEM in Arcview (ESRI 1995). I assessed differences in midden density among

territories with ANOVA, and differences among forest types with Tukey’s Tests

(PROC GLM; SAS 1996). Correlations between midden density and nest-predation

rates, and between midden density and demographic performance, were determined

with Pearson product correlation analysis (PROC REG; SAS 1996).

Relative Prey Abundance.—I used black-light traps (Southwood 1981) to estimate

relative arthropod abundance in two owl territories, one producing the most owlets

(A4) and one producing the fewest owlets (A18) over the study. One black-light trap

was placed in the interior of the largest available forest patches of ponderosa

pine/Douglas-fir, Douglas-fir, and quaking aspen/blue spruce. Black-light traps were

placed between adjacent trees (3-4 m apart) and hung 1.5 m above ground adjacent to

a vertical white cloth sheet (1.5 x 2.0 m). On sampling nights (see below), arthropods

that landed on the sheet were counted every quarter-hour from 2100-2300 hr and

averaged over the 2 hr. I counted all flying arthropods, mostly lepidopterans, whose

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length from base of antennae to tip of wing was 15-29 mm. Lepidopterans of this

size were the primary prey delivered by males to nests (Linkhart et al. 1998, unpubl.

data).

I sampled arthropod populations in 1998 and 1999 during two owl nesting

stages: incubation (18 May-11 June), and nestling (23 June-23 July). I compared

arthropod abundance between the two territories by simultaneously sampling the

same forest type (e.g., ponderosa pine/Douglas-fir) in both territories. Three

sampling nights were required to make all three intra-forest type comparisons

between territories. I compared the 2 territories during 4 trap-nights (i.e., 4 nights in

each of the forest types) of the incubation stage (2 nights each in 1998 and 1999), and

5 trap-nights of the nestling stage (3 nights in 1998 and 2 nights in 1999).

Differences in relative prey abundance between territories were determined with

ANOVA (PROC GLM; SAS 1996). For these and all other statistical analyses, I

determined whether distributions of variables deviated significantly from normality

(PROC UNIVARIATE; SAS 1996), and if necessary performed data transformations

(log, square root, and arcsin) to achieve normality and reran the procedure. I use a

significance level of P = 0.05 and present means + standard error (SE).

RESULTS

Temporal/Spatial Constancy of Territories

I monitored occupancy on 14 owl territories from 1981-1999 (Fig. 1).

Territories were generally constant in time and space despite individual turnover on

each territory, with a few exceptions. First, the A24 territory was only occupied from

1981-1983. In 1984, a new male in an adjacent territory (A29) expanded the

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boundaries of his territory to include much of the western portion of A24 territory,

and these new boundaries did not change over the remainder of the study (Fig. 18).

Second, the male in A4 territory expanded the boundaries of his territory in 1983 to

include much of the eastern portion of A15 territory, which was only occupied in

1981-1982 and 1984 (Linkhart et al. 1998; Fig. 18). I did not know the boundaries of

either territory in 1984, when only A15 territory was occupied by a breeding pair, but

for the remainder of the study A4 territory contained the eastern half of A15 territory.

Finally, in 1988 the male in A8 territory expanded his territory north into the southern

portion of A11 territory, after the male in A11 territory apparently did not return from

migration, and northeast into the northern portion of A15 territory (Fig. 18). After

1988, A11 territory contained only the northern portion of the original A11 territory

(Fig. 18). In each of the above instances, shifts in territory boundaries occurred when

> 1 males in adjacent territories did not return from migration. Boundaries of all

other territories remained unchanged over the study. Males generally returned to

territories annually; territory fidelity was 98% and annual turnover was 10% (Chapter

4). Consequently, newly arriving males in the spring typically filled geographic voids

left by predecessors. The overall high stability of territory boundaries allowed me to

compare their habitat characteristics to their demographic performance over the entire

study. Below I report demographic performance for the 12 territories (omitting A15

and A24) active over the 19 yr.

Demographic Performance on Territories Territory Occupancy.—Territories differed in total yr (occupied yr) they were

occupied by owls, ranging from 3-19 yr (χ2 = 17.3, df = 10, P = 0.07; Table 5).

Table 5. Demography on owl territories from 1981-1999.

Territory Occupancy Reproductive Success

Territory

Occupied yr

Bachelor yr Breeding yra

No. success. nestsb

Nest success

(%)b

Owlets Mean owlets brood-1

Mean owlets

yr-1

A4 19 3 16 15 93.8 35 2.2 1.8

A8 16 5 11 9 81.8 23 2.1 1.2

A29 16 4 12 7 70c 17 1.7 0.9

A10 13 4 9 5 62.5c 13 1.4 0.7

A11 14 6 8 7 87.5 17 2.1 0.9

A13 12 5 7 7 100 14 2.0 0.7

A12 14 10 4 3 100c 7 1.8 0.4

A27 9 7 2 2 100 5 2.5 0.3

A20 6 3 3 3 100 8 2.7 0.4

A7 9 6 3 1 33.3 3 1 0.2

A18 13 10 3 1 100c 2 1 0.1

A2 4 3 1 0 0 0 0 0

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MEAN 12.1 5.5 6.6 5.0 77.4 12.0 1.7 0.6

SE 1.3 0.7 1.4 1.2 9.2 2.9 0.2 0.1 a Identical to nesting attempts, since owls only attempted to breed a maximum of once yr-1 and did not renest if nests failed. b Nests that fledged at least one owlet c Excludes nesting attempts where outcome of nest was unknown or nest failed due to anthropogenic causes: A29-2 attempts; A10-1

attempt; A12-1 attempt; A18-2 attempts

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Territory tenure- Males Territory tenure-Females Pair bonding Territory

Total Males

Mean tenure (yr) Total Females

Mean tenure (yr)

Total unique pair bonds

Mean duration (yr)

A4 5 3.4 9 1.6 10 1.4

A8 4 2.8 4 2.8 7 1.4

A29 2 7.0 5 2.0 5 2.0

A10 3 3.7 5 1.4 5 1.4

A11 2 5.5 5 1.6 5 1.6

A13 4 2.3 6 1.2 6 1.2

A12 1 3.0 2 1.5 2 1.5

A27 1 6.0 2 1.0 2 1.0

A20 2 1.5 2 1.5 2 1.5

A7 1 1.0 3 1.0 1 1.0

A18 1 1.0 1 1.0 1 1.0

A2 1 1.0 1 1.0 1 1.0

MEAN 2.2 3.2 3.8 1.5 3.9 1.3

SE 0.4 0.6 0.7 0.1 0.8 0.1

Territories were occupied by breeding pairs (breeding yr) only 35% of total yr (6.6 +

1.1 yr; not necessarily consecutive), and breeding yr differed among territories (χ2 =

36.9, df = 10, P < 0.001; Table 5). Similarly, territories were occupied by bachelor

males (bachelor yr) 29% of total yr (5.5 + 0.6 yr; not necessarily consecutive), but

bachelor yr did not differ among territories (χ2 = 12.2, df = 10, P = 0.27; Table 5).

Some territories usually were occupied by breeding pairs annually while some

territories usually were occupied by bachelor males. Three territories (A4, A8, and

A29) had > 12 breeding yr and five territories (A12, A7, A18, A27, and A2) had more

bachelor yr than breeding yr (Table 5).

Reproductive Success.—Pairs on territories had a maximum of one nesting attempt

annually. Because I never observed renesting by owls, even when nests failed during

early incubation, mean nesting attempts territory-1 (6.1 + 1.0) over the study was

identical to mean breeding yr. Nesting success over the study was high; 82% of all

nests were successful (60 of 73 nests, excluding 6 nests whose outcome was uncertain

or failed due to anthropogenic causes). The 13 nest failures, which occurred in 8

different territories, were all attributed to nest predation. Four territories never had

nest failure and three territories had < 33% nest success, although all of these

territories except one (A13) had < 3 nest attempts (Table 5).

Total owlets territory-1 (12.0 + 2.9) differed among territories (χ2 = 93.3, df =

10, P < 0.001; Table 5). The most productive territory (A4; 35 owlets) produced 60%

more owlets than the next most productive territory (A8; 22 owlets), while the least

productive territory (A2) failed to produce any owlets (Table 5). Of 144 total owlets

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produced by all 12 territories over the study, 25% (3 territories) produced 52% of

owlets, and 50% (6 territories) produced 83% of owlets (Fig. 19).

Territories had a mean brood size of 1.7 + 0.2 owlets, including successful

(nests fledging > 1 owlet) and unsuccessful nests, and mean brood size did not differ

among territories (χ2 = 7.42, df = 10, P = 0.69; Table 5). Excepting one brood of 1

owlet and one brood of 4 owlets (both in A4 territory), all broods contained either 2

or 3 owlets. Overall, territories produced a mean 0.6 + 0.1 owlets yr-1 over the 19 yr

study (Table 5).

Pair Duration.—Mean pair duration territory-1 was 1.3 + 0.1 yr, and did not differ

among territories (χ2 = 0.8, df = 10, P = 0.99; Table 5). Only one divorce was

documented over the study; a female who had nested successfully once with her

original mate in A10 territory in 1997 was found nesting with a new male in A8

territory in 1999. Territories had a mean 3.9 + 0.8 unique pair bonds, and differences

among territories were attributable to breeding yr (Spearman’s p = 0.86, P < 0.001;

Table 5) since duration of most pair bonds was 1 yr (74%; Chapter 4).

Territory Tenure.—Mean territory tenure, which I used as an estimate of survival, for

males was 3.2 + 0.6 yr (Table 5), and did not differ among territories (χ2 = 14.16, d.f.

= 10, P = 0.17). Three territories (A27, A29, and A11) had mean tenure by males >

5.5 yr, and each of these was occupied by < 2 males. Mean territory tenure for

females was 1.5 + 0.1 yr (Table 5), and this also did not differ among territories (χ2 =

2.0, df = 10, P = 0.99). Mean longest tenure by females on a territory was 2.8 yr, and

this territory (A8) was occupied by 4 females (Table 5).

Breeding Dispersal.—Breeding dispersal by females occurred on eight occasions

136

Figure 19. Contribution of individual territories to total owlets produced by

combined territories from 1981-1999.

0

20

40

60

80

100

0 20 40 60 80 100

Cumulative Percent of Territories

Cum

ulat

ive

perc

ent o

f ow

lets

137

over the study, and usually occurred when males on original territories did not return

after spring migration (Chapter 4). Four territories (A4, A8, A29, and A12) were

recipients of 2 dispersing females each, although one pair of owls also dispersed from

the latter territory including the female immigrant. Females dispersed to territories

having significantly higher productivity (paired t = -2.53, df = 7, P = 0.04; Table 6).

The only dispersing male also moved from a territory having lower productivity (A12

territory; 7 owlets) to a territory having higher productivity (A4 territory; 35 owlets).

Distribution and Structure of Forest Types

Forests of ponderosa pine/Douglas-fir and Douglas-fir accounted for the

greatest percentage of the study area (53% and 23%) while each of the other forest

types occupied < 10% (Table 7, Fig. 20). Forest composition of combined territories

was similar to non-territory habitat; each forest type differed by < 4% between the

two areas (Table 7). However, among territories, percentage of area in ponderosa

pine/Douglas-fir was greatest in A4 and least in A12, while the percentage of area in

Douglas-fir was greatest in A18 and least in A4 (Table 7). All other forest types

accounted for a mean < 10% of territories (Table 7).

Overstory structure of ponderosa pine/Douglas-fir forests differed

significantly from most other forest types (Table 8). Ponderosa pine/Douglas-fir

forests contained less basal area (BA), lower tree density (TPH), and larger mean

crown volume (MN_CRVL), than all other forest types except quaking aspen, and

contained trees with larger mean dbh (MN_DBH) than Douglas-fir, Douglas-fir/blue

spruce, and quaking aspen/blue spruce (Table 8). Compared to Douglas-fir forests in

the two smallest dbh categories (2.5-17.7 cm and 17.8-32.9 cm), ponderosa

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Table 6. Comparison of owlets produced over 19 yr on territories where females

nested before dispersal (“original territory”) to owlets produced on territories to

which those same females dispersed (“new territory”).

Original territory New territory Net

difference in

Territory (yr)

Owlets Territory (yr) Owlets owlets (%)

A15 (1982) 7 A4 (1983) 35 400

A29 (1983) 17 A12 (1984) 7 -59

A24 (1983) 4 A29 (1984) 17 325

A15 (1984) 7 A12 (1985) 7 0

A12 (1986) 7 A4 (1987) 35 400

A7 (1990) 3 A29 (1991) 17 467

A13 (1996) 14 A8 (1998) 23 64

A10 (1997) 13 A8 (1999) 23 77

MEAN 9.0 MEAN 20.5 MEAN 209

SE 1.8 SE 3.8

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Table 7. Area and percentage of forest types in owl territories, non-territory habitat,

and over the entire study area.

Pipo/Psme1 Psme2 Psme/Pipu3 Potr4 Potr/Pipu5 Psme/Pifl6

Terr7 ha % ha % ha % ha % ha % ha % Totalha)

A2 4.1 62 1.2 18 0.2 3 1.1 16 0 0 0.1 1 6.6

A4 18.7 88 1.4 6 0 0 0.6 3 0.4 2 0.3 1 21.3

A7 6.0 55 1.5 14 0 0 0 0 2.3 20 1.3 11 11.1

A8 14.0 79 2.8 16 0 0 0.3 2 0.5 3 0.2 1 17.6

A10 11.3 61 3.6 19 1.3 7 0 0 1.9 10 0.5 3 18.5

A11 6.1 49 3.2 25 0.5 4 0 0 1.5 12 1.3 10 12.5

A12 1.7 26 1.8 27 2.0 30 0.6 10 0.5 7 0 0 6.6

A13 7.9 38 5.3 26 5.3 26 0.1 <1 1.3 6 0.7 4 20.6

A18 5.9 33 8.3 47 1.8 10 0 0 1.1 6 0.8 5 17.8

A20 7.7 42 5.8 32 1.2 6 0 0 2.4 13 1.3 7 18.3

A27 1.9 35 1.5 28 0.4 7 0 0 1.6 30 0 0 5.4

A29 18.6 75 3.1 12 0 0 0 0 3.0 12 0.2 1 24.8

Mean 8.6 56 3.3 22 1.0 7 0.2 1 1.4 10 0.5 4 15.1

N 159 50 77 24 29 9 3 1 23 7 26 8 319

SA8 271 53 120 23 42 8 6 1 40 8 33 7 511

1 ponderosa pine/Douglas-fir; 2 Douglas-fir; 3 Douglas-fir/blue spruce; 4 quaking

aspen; 5 quaking aspen/blue spruce; 6 Douglas-fir/limber pine; 7 territory; 8 study

area

140

Figure 20. Distribution of owl territories, cavity trees, and forest types on the

Manitou Experimental Forest study area. A15 and A24 territories (‘temp

territ’; temporary territories) were not occupied after 1984, and A40 (black

ellipse) was an unstudied territory on the edge of the study area. Legend:

‘‘terr cavit’ = cavity trees within owl territories; ‘non-terr cavit’ = cavity trees

outside of owl territories; ‘p pine’ = ponderosa pine; ‘D-fir’ = Douglas-fir;

‘spruce’ = blue spruce; ‘aspen’ = quaking aspen; and ‘l pine’ = limber pine.

Table 8. Comparison of forest structure variables (mean + SE) among forest types. Different letters denote significant differences

among forest types (Tukey’s tests, df = 346, p = 0.05).

Forest Type

Variablea (units) Ponderosa

pine/Douglas-fir

Douglas-fir Douglas-

fir/blue spruce

Quaking aspen Quaking aspen

/blue spruce

Douglas-

fir/limber pine

MN_DBH (cm) 31.0 + 0.6 a 24.3 + 0.6 b 22.9 + 1.0 b 22.4 + 2.1 ab 23.5 + 1.0 b 27.7 + 2.2 ab

BA (m2 ha-1) 22.4 + 1.0 a 31.9 + 1.2 b 34.2 + 2.2 b 24.0 + 1.3 ab 31.8 + 1.9 b 31.8 + 4.5 b

T_BAD1 (m2 ha-1) 3.8 + 0.5 a 10.4 + 1.0 b 11.3 + 1.5 b 6.4 + 1.9 ab 9.9 + 1.5 b 6.3 + 1.7 ab

T_BAD2 (m2 ha-1) 9.8 + 0.7 a 14.9 + 0.8 b 17.6 + 1.7 b 13.4 + 2.6 ab 15.0 + 1.7 ab 15.9 + 4.1 b

T_BAD3 (m2 ha-1) 7.4 + 0.4 a 6.1 + 0.5 ab 4.9 + 1.1 b 3.8 + 1.9 ab 5.0 + 0.8 ab 8.0 + 1.9 ab

T_BAD4 (m2 ha-1) 1.5 + 0.2 a 1.2 + 0.1 b 0.4 + 0.2 abc 0.5 + 0.3 abc 3.0 + 0.6 ac 1.7 + 1.0 abc

P_BAD1 0.15 + 0.02 a 0.31 + 0.02 b 0.33 + 0.04 b 0.27 + 0.08 ab 0.31 + 0.04 b 0.22 + 0.06 ab

P_BAD2 0.40 + 0.02 0.47 + 0.02 0.52 + 0.03 0.54 + 0.10 0.46 + 0.05 0.44 + 0.08

P_BAD3 0.35 + 0.02 a 0.20 + 0.02 b 0.14 + 0.03 b 0.16 + 0.07 ab 0.15 + 0.02 b 0.27 + 0.06 ab

P_BAD4 0.09 + 0.01 a 0.02 + 0.01 b 0.04 + 0.01 b 0.02 + 0.02 ab 0.05 + 0.02 ab 0.07 + 0.04 ab

TPH (trees ha-1) 825 + 79 a 2268 + 295 b 2334 + 386 b 1694 + 447 ab 2027 + 303 b 1354 + 249 b

T_TPHD1 (trees ha-1) 554 + 77 a 1895 + 293 b 1642 + 320 b 1340 + 477 ab 1638 + 320 b 914 + 239 ab

142

T_TPHD2 (trees ha-1) 203 + 15 a 317 + 20 b 598 + 189 b 318 + 51 ab 338 + 39 ab 364 + 105 ab

T_TPHD3 (trees ha-1) 62 + 4 54 + 5 93 + 49 33 + 17 43 + 7 69 + 15

T_TPHD4 (trees ha-1) 6 + 0.8 a 2 + 0.6 b 1 + 0.7 ab 2 + 2 ab 8 + 2 ab 7 + 5 ab

P_TPHD1 0.38 + 0.03 a 0.64 + 0.03 b 0.65 + 0.05 b 0.50 + 0.13 ab 0.60 + 0.06 b 0.50 + 0.10 ab

P_TPHD2 0.39 + 0.02 0.29 + 0.03 0.29 + 0.04 0.43 + 0.11 0.34 + 0.05 0.33 + 0.08

P_TPHD3 0.20 + 0.02 a 0.07 + 0.01 b 0.06 + 0.03 b 0.07 + 0.05 ab 0.03 + 0.01 b 0.15 + 0.07 ab

P_TPHD4 0.03 + 0.01 a <0.01 b <0.01 ab <0.01 ab <0.01 ab 0.02 + 0.01 ab

MN_CRVL (m3 tree-1) 141.2 + 8.4 a 68.8 + 4.8 b 69.8 + 13.3 b 77.2 + 20.8 ab 64.3 + 7.3 b 83.0 + 11.6 b

CRVL (m3 ha-1 x 103) 84.2 + 7.5 114.0 + 11.9 142.6 + 44.5 135.4 + 72.7 152.6 + 43.8 94.8 + 22.8

T_CRVLD1 (m3 ha-1x 103) 3.0 + 0.6 a 8.6 + 1.6 b 7.7 + 1.5 b 4.4 + 1.9 ab 6.4 + 1.3 b 6.1 + 2.1 ab

T_CRVLD2 (m3 ha-1x 103) 24.8 + 3.0 a 51.5 + 6.1 b 5.5 + 8.1 b 28.0 + 10.6 ab 32.8 + 6.2 ab 32.5 + 7.9 ab

T_CRVLD3 (m3 ha-1x 103) 33.5 + 3.2 47.0 + 7.6 71.7 + 40.7 64.8 + 47.0 52.5 + 13.7 47.0 + 16.0

T_CRVLD4 (m3 ha-1x 103) 23.0 + 5.2 a 6.9 + 2.9 b 7.4 + 4.8 b 38.2 + 27.2 ab 60.9 + 31.7 ab 8.5 + 5.2 ab

P_CRVLD1 0.04 + 0.01 a 0.09 + 0.02 b 0.08 + 0.02 b 0.08 + 0.05 ab 0.13 + 0.04 b 0.05 + 0.02 ab

P_CRVLD2 0.31 + 0.02 a 0.50 + 0.03 b 0.56 + 0.05 b 0.49 + 0.13 ab 0.44 + 0.05 ab 0.40 + 0.09 ab

P_CRVLD3 0.46 + 0.02 a 0.36 + 0.03 ab 0.29 + 0.05 b 0.33 + 0.12 ab 0.27 + 0.04 b 0.45 + 0.08 ab

P_CRVLD4 0.18 + 0.02 a 0.05 + 0.02 b 0.06 + 0.03 ab 0.11 + 0.09 ab 0.14 + 0.04 ab 0.09 + 0.05 ab

UNVG_5 0.14 + 0.01 ad 0.27 + 0.02 bc 0.37 + 0.03 b 0.25 + 0.1abcd 0.29 + 0.03 bc 0.20 + 0.04 cd

UNVG5_50 0.33 + 0.02 a 0.31 + 0.02 a 0.33 + 0.04 a 0.78 + 0.07 b 0.69 + 0.04 b 0.25 + 0.05 a

UNVG_50 0.08 + 0.01 0.09 + 0.01 0.07 + 0.01 0.10 + 0.05 0.15 + 0.02 0.06 + 0.02

143

UNVG_200 <0.01 b <0.01 b <0.01 b <0.01 ab 0.04 + 0.01 a <0.01 b

a Refer to Table 4 for meaning of variable abbreviations.

pine/Douglas-fir contained less basal area (T_BAD1, P_BAD1, T_BAD2, and

P_BAD2), lower tree density (T_TPHD1, P_TPHD1, and T_TPHD2), and less crown

volume (T_CRVLD1, P_CRVLD1, and T_CRVLD2, P_CRVLD2; Table 8). In the

two largest dbh categories (33.0-48.2 cm), ponderosa pine/Douglas-fir contained

greater basal area (P_BAD3, T_BAD4, and P_BAD4), greater tree density

(P_TPHD3, T_BAD4, and P_BAD4), and greater crown volume (P_CRVLD3,

T_CRVLD4, and P_CRVLD4) than Douglas-fir forests (Table 8). Similar to the

pattern described above, overstory structure of ponderosa pine/Douglas-fir forests

also differed from Douglas-fir/blue spruce and quaking aspen/blue spruce, but

primarily in the smallest and second-largest dbh categories (Table 8). Quaking aspen

and Douglas-fir/limber pine showed few differences in overstory structure compared

to ponderosa pine/Douglas-fir, probably because these forest types were least

sampled and had highest SE (Table 8). Proportion of understory vegetation 1-4 cm

tall (UNVG_5) was least in ponderosa pine/Douglas-fir compared to Douglas-fir,

Douglas-fir/blue spruce, and quaking aspen/blue spruce, while quaking aspen and

quaking aspen/blue spruce contained the highest proportion of understory vegetation

5-49 cm tall (UNVG_50; Table 8).

Comparisons Between Vegetation And Owl Demography

For comparisons with vegetation variables, I used two demographic variables

that differed significantly among the 12 territories: breeding yr, which was

equivalent to total nesting attempts, and owlets. While these variables were highly

correlated (r = 0.95) and resulted in similar territory rankings (Table 5), I retained

both variables in analyses to assess the efficacy of breeding yr as a surrogate for

145

owlets in future analyses of habitat quality. In addition to these variables, I also

included bachelor yr and occupied yr in comparisons with vegetation variables.

Bachelor yr, which was negatively correlated with owlets (-0.41) and breeding yr (-

0.40) and resulted in nearly opposite territory rankings (Table 5), was included to

compare habitat conditions in breeding vs non-breeding territories. Occupied yr was

included to determine if habitat quality could be inferred from duration of occupancy.

Comparisons of territory and non-territory habitat were based solely on owlets.

Correlation Between Forest Type And Demography.—Across territories, breeding yr

was positively correlated with total area and proportion of area in ponderosa

pine/Douglas-fir, and negatively correlated with proportion of area in Douglas-fir

(Table 9). Owlets showed the same, but slightly weaker, correlations (Table 9). In

contrast, bachelor yr was negatively correlated with total area and proportion of area

in ponderosa pine/Douglas-fir, and positively correlated with proportion of area in

Douglas-fir (Table 9). Occupied yr was correlated only with proportion of area in

ponderosa pine/Douglas-fir (Table 9). Significant correlations between forest types

and demographic variables generally showed strong linearity (Fig. 21).

Correlation Between Forest Structure And Demography.—Across territories, owlets

were positively correlated with basal area (P_BAD3) and crown volume (T_CRVLD3

and P_CRVLD3) in the second-largest dbh category (33.0-48.2 cm), and showed a

trend (P = 0.07-0.09) of negative correlations with several forest structure variables in

the two smallest dbh categories (2.5-17.7 cm and 17.8-32.9 cm; Table 10).

Associated with these overstory variables, partial correlation analyses identified a

single structural dimension associated with owlets that was dominated by crown

146

Figure 21. Correlations between demographic variables and forest types across owl

territories (n = 12).

y = 0.729x + 0.2794

R2 = 0.8171

0

2

4

6

8

10

12

14

16

18

0.0 5.0 10.0 15.0 20.0

Area (ha) in ponderosa pine/Douglas-fir

Bre

edin

g y

r

y = -0.086x + 10.101

R2 = 0.477

0

2

4

6

8

10

12

0.0 20.0 40.0 60.0 80.0 100.0

Percent of territory area in ponderosa pine/Douglas-fir

Bac

hel

or

yr

y = 1.3951x - 0.0644

R2 = 0.6491

0

5

10

15

20

25

30

35

40

0.0 5.0 10.0 15.0 20.0

Area (ha) in ponderosa pine/Douglas-fir

Ow

lets

Table 9. Correlations among demographic variables and forest types within owl territories (df = 11 for all correlations). Significant

correlations (p < 0.05) are indicated with an asterisk.

Owlets Breeding yr Bachelor yr Occupied yr

Forest Types r P r P r P r P

Pipo/psme1 (area ha) 0.81 0.002* 0.90 < 0.001* -0.56 0.06 0.66 0.02*

Pipo/psme(prop. of total7) 0.70 0.01* 0.76 0.004* -0.69 0.01* 0.43 0.16

Psme2 (area ha) -0.19 0.55 -0.29 0.64 0.26 0.42 -0.02 0.35

Psme (prop. of total) -0.58 0.05* -0.62 0.03* 0.65 0.02* -0.30 0.35

Psme/pipu3 (area ha) -0.19 0.67 -0.16 0.62 0.24 0.46 -0.02 0.91

Psme/pipu (prop. of total) -0.25 0.43 -0.29 0.36 0.54 0.07 -0.04 0.98

Potr4(area ha) 0.04 0.91 -0.03 0.93 -0.17 0.61 -0.12 0.71

Potr (prop. of total) -0.28 0.38 -0.31 0.34 -0.05 0.89 -0.35 0.26

Potr/pipu5 (area ha) -0.14 0.67 -0.01 0.97 -0.16 0.63 -0.10 0.76

Potr/pipu (prop. of total) -0.35 0.26 -0.37 0.24 0.19 0.55 -0.29 0.37

Psme/pifl6 (area ha) -0.15 0.65 -0.18 0.57 -0.05 0.88 -0.22 0.48

Psme/pifl (prop. of total) -0.20 0.53 -0.23 0.47 <0.01 0.98 -0.25 0.43

1 ponderosa pine/Douglas-fir; 2 Douglas-fir; 3 Douglas-fir/blue spruce; 4 quaking aspen; 5 quaking aspen/blue spruce; 6 Douglas-fir/limber pine; 7 proportion of total area (ha) in territory

148

Table 10. Correlations among demographic and forest structure variables within owl territories (df = 11 for all analyses). Significant

correlations (p < 0.05) are indicated with an asterisk.

Owlets Breeding yr Bachelor yr Occupied yr

Variablea r P r P r P r P

MN_DBH 0.54 0.07 0.51 0.09 -0.52 0.08 0.25 0.43

BA -0.21 0.51 -0.18 0.58 0.78 0.003* 0.25 0.43

T_BAD1 -0.38 0.22 -0.35 0.26 0.70 0.01* 0.02 0.96

T_BAD2 -0.34 0.28 -0.28 0.38 0.77 0.003* 0.13 0.68

T_BAD3 0.56 0.06 0.51 0.09 -0.38 0.22 0.34 0.28

T_BAD4 0.19 0.55 0.15 0.65 -0.39 0.21 -0.06 0.86

P_BAD1 -0.51 0.09 -0.45 0.14 0.52 0.08 -0.19 0.55

P_BAD2 -0.46 0.13 -0.42 0.18 0.53 0.08 -0.15 0.64

P_BAD3 0.58 0.05* 0.52 0.09 -0.51 0.09 0.27 0.40

P_BAD4 0.18 0.58 0.17 0.60 -0.53 0.08 -0.12 0.71

TPH -0.26 0.42 -0.30 0.34 0.56 0.06 <0.01 0.99

T_TPHD1 -0.19 0.55 -0.23 0.47 0.48 0.11 0.03 0.93

T_TPHD2 -0.53 0.08 -0.57 0.05* 0.78 0.003* -0.17 0.59

T_TPHD3 -0.02 0.95 -0.12 0.71 0.02 0.96 -0.12 0.71

T_TPHD4 -0.06 0.85 -0.13 0.69 <0.01 0.99 -0.14 0.67

149

P_TPHD1 -0.55 0.07 -0.50 0.10 0.34 0.28 -0.35 0.27

P_TPHD2 -0.30 0.35 0.33 0.30 0.11 0.73 0.42 0.18

P_TPHD3 0.55 0.06 0.48 0.12 -0.42 0.18 0.28 0.38

P_TPHD4 0.22 0.49 0.22 0.49 -0.50 0.10 -0.05 0.86

MN_CRVL 0.23 0.47 0.18 0.58 -0.50 0.10 -0.09 0.78

CRVL 0.05 0.87 -0.09 0.78 0.22 0.49 0.03 0.93

T_CRVLD1 -0.44 0.15 -0.45 0.15 0.67 0.02* -0.10 0.75

T_CRVLD2 -0.51 0.09 -0.55 0.07 0.69 0.01* -0.20 0.52

T_CRVLD3 0.65 0.02* 0.50 0.10 -0.44 0.16 0.29 0.34

T_CRVLD4 -0.03 0.93 -0.08 0.80 0.05 0.87 -0.06 0.86

P_CRVLD1 -0.25 0.44 -0.09 0.79 0.26 0.42 0.05 0.88

P_CRVLD2 -0.51 0.09 -0.48 0.11 0.64 0.02* -0.16 0.61

P_CRVLD3 0.67 0.02* 0.65 0.02* -0.71 0.01* 0.30 0.34

P_CRVLD4 -0.04 0.90 -0.14 0.67 -0.10 0.75 -0.21 0.52

UNVG_5 -0.50 0.10 -0.50 0.10 0.25 0.42 -0.39 0.21

UNVG5_50 -0.09 0.78 -0.09 0.78 -0.11 0.73 -0.16 0.61

UNVG_50 0.07 0.83 0.11 0.73 -0.11 0.73 0.06 0.85

UNVG_200 -0.08 0.80 -0.02 0.94 -0.11 0.73 -0.08 0.79


volume (Step 1 variable = P_CRVLD3, partial r = 0.67, and Step 2 variable =

T_CRVLD3, partial r = 0.09). Compared with owlets, breeding yr showed similar

correlations but they were generally weaker (Table 10). In contrast, bachelor yr was

positively correlated with basal area (BA; Table 10). In the two smallest dbh

categories, bachelor yr was also positively correlated with basal area (T_BAD1, and

T_BAD2), tree density (T_TPHD2), and crown volume (T_CRVLD1, T_CRVLD2,

and P_CRVLD2), and was negatively correlated with crown volume (P_CRVLD3) in

the second-largest dbh category (Table 10). Associated with all significant overstory

variables, partial correlation analyses identified a single structural dimension

correlated with bachelor yr that was dominated by basal area (Step 1 variable = BA,

partial r = 0.78; Step 2 variable = T_TPHD2, partial r = 0.11). Occupied yr was not

correlated with any forest structure variables (Table 10).

Comparison Between Territory and Non-territory Habitat.—Territory habitat

(territories combined) differed from non-territory habitat in having less total basal

area (BA), and in containing less basal area (T_BAD2) and lower tree density

(T_TPHD2 and P_TPHD2) in the second-smallest dbh category (Table 11). Territory

habitat also contained a higher proportion of understory vegetation 5-49 cm tall

(UNVG_50) than non-territory habitat (Table 11).

Comparisons Among Territory Classes and Non-territory Habitat.—Compared with

non-territory habitat, the high-productivity class (A4 territory) was characterized as

having trees with larger overall mean dbh (MN_DBH), and in the second-smallest

dbh category, having less basal area (T_BAD2 and P_BAD2), lower tree density

(T_TPHD2), and less crown volume (T_CRVLD2 and P_CRVLD2; Table 12). The

151

Table 11. Comparison of forest structure variables (mean + SE) in territory vs non-

territory habitat (ANOVA tests, df = 351). Significant correlations (P < 0.05) are

indicated with an asterisk.

Variablea (units) Territory Non-territory F P

MN_DBH (cm) 28.5 + 0.6 27.3 + 0.6 1.76 0.18

BA (m2 ha-1) 25.2 + 0.8 28.4 + 1.2 4.97 0.03*

T_BAD1 (m2 ha-1) 6.7 + 0.6 6.1 + 0.6 2.92 0.09

T_BAD2 (m2 ha-1) 11.1 + 0.6 13.9 + 0.9 7.49 0.007*

T_BAD3 (m2 ha-1) 6.5 + 0.3 7.1 + 0.6 1.02 0.31

T_BAD4 (m2 ha-1) 1.1 + < 0.1 1.4 + < 0.1 1.70 0.19

P_BAD1 0.23 + 0.02 0.21 + 0.02 0.78 0.18

P_BAD2 0.41 + 0.02 0.46 + 0.02 3.10 0.08

P_BAD3 0.30 + 0.02 0.26 + 0.02 3.06 0.08

P_BAD4 0.06 + 0.01 0.06 + 0.01 0.02 0.89

TPH (trees ha-1) 1372.8 + 130.6 1426.2 + 153.6 0.26 0.61

T_TPHD1 (trees ha-1) 1072.5 + 127.2 1015.4 + 145.2 2.54 0.11

T_TPHD2 (trees ha-1) 241.1 + 14.8 333.2 + 43.7 5.19 0.02*

T_TPHD3 (trees ha-1) 54.8 + 3.0 71.4 + 11.1 2.89 0.09

T_TPHD4 (trees ha-1) 4.5 + 0.5 5.9 + 1.0 1.48 0.22

P_TPHD1 0.50 + 0.02 0.46 + 0.03 2.02 0.16

P_TPHD2 0.32 + 0.02 0.40 + 0.03 5.14 0.02*

P_TPHD3 0.16 + 0.02 0.12 + 0.01 2.52 0.14

152

P_TPHD4 0.02 + 0.006 0.02 + 0.004 0.82 0.37

MN_CRVL (m3 tree-1) 117.3 + 7.4 99 + 6.1 1.93 0.17

CRVL (m3 ha-1 x 103) 92.8 + 0.7 116.6 + 16.3 1.01 0.32

T_CRVLD1 (m3 ha-1 x 103) 5.0 + 0.6 5.1 + 1.0 0.50 0.48

T_CRVLD2 (m3 ha-1 x 103) 33.8 + 3.2 33.8 + 4.0 0.14 0.71

T_CRVLD3 (m3 ha-1 x 103) 38.9 + 3.8 48.6 + 10.0 0.03 0.86

T_CRVLD4 (m3 ha-1 x 103) 14.9 + 2.8 29.0 + 10.0 2.55 0.11

P_CRVLD1 0.07 + 0.01 0.05 + 0.01 0.75 0.39

P_CRVLD2 0.37 + 0.02 0.42 + 0.03 2.02 0.16

P_CRVLD3 0.43 + 0.02 0.39 + 0.03 1.92 0.17

P_CRVLD4 0.13 + 0.02 0.13 + 0.02 0.00 0.98

UNVG_5 0.20 + 0.01 0.20 + 0.01 0.06 0.81

UNVG5_50 0.40 + 0.02 0.30 + 0.02 13.08 0.0003*

UNVG_200 0.08 + 0.01 0.09 + 0.01 0.35 0.55

UNVG_300 <0.01 + <0.01 <0.01 + <0.01 0.47 0.49


Table 12. Comparison of forest structure variables (mean + SE) among non-territory

and three classes of territory habitat. Different letters denote significant differences

among habitats (Tukey’s tests, df = 348, p = 0.05).

Class of Habitat2

Variable1 (units) High Moderate

Low Non-territory

MN_DBH (cm) 35.0 + 2.5 a 29.0 + 0.7 b 26.0 + 0.8 c 27.0 + 0.6 bc

BA (m2 ha-1) 20.4 + 1.5 a 25.7 + 1.2 ab 26.2 + 1.2 ab 28.4 + 1.2 b

T_BAD1 (m2 ha-1) 3.2 + 0.9 ac 6.8 + 0.8 bc 8.3 + 0.9 b 6.1 + 0.7 c

T_BAD2 (m2 ha-1) 7.6 + 1.4 a 11.1 + 3.5 ab 12.2 + 0.8 ab 13.9 + 0.9 b

T_BAD3 (m2 ha-1) 7.6 + 1.0 ab 7.0 + 0.4 a 4.7 + 0.4 b 7.1 + 0.6 a

T_BAD4 (m2 ha-1) 2.1 + 0.5 1.0 + 0.2 1.0 + 0.2 1.4 + 0.2

P_BAD1 0.14 + 0.04 a 0.23 + 0.02 ab 0.28 + 0.02 b 0.21 + 0.02 a

P_BAD2 0.32 + 0.05 a 0.40 + 0.02 ab 0.45 + 0.02 ab 0.46 + 0.02 b

P_BAD3 0.42 + 0.05 a 0.32 + 0.02 ac 0.21 + 0.02 bc 0.26 + 0.02 c

P_BAD4 0.11 + 0.03 0.06 + 0.01 0.05 + 0.01 0.06 + 0.01

TPH (trees ha-1) 926 + 247 a 1341 + 196 ab 1704 + 191 b 1426 + 154 ab

T_TPHD1 (trees ha-1) 694 + 247 ad 1033 + 190 bc 1346 + 187 b 1015 + 145 cd

T_TPHD2 (trees ha-1) 163 + 32 a 244 + 21 ab 305 + 26 b 333 + 44 b

T_TPHD3 (trees ha-1) 60 + 8 59 + 4 48 + 5 71 + 11

T_TPHD4 (trees ha-1) 9 + 2 4 + 1 5 + 1 6 + 1

P_TPHD1 0.29 + 0.08 ad 0.50 + 0.04 bc 0.60 + 0.03 b 0.46 + 0.03 cd

P_TPHD2 0.34 + 0.06 0.32 + 0.03 0.30 + 0.03 0.40 + 0.03

P_TPHD3 0.32 + 0.06 a 0.16 + 0.02 b 0.09 + 0.02 b 0.12 + 0.01 b

P_TPHD4 0.06 + 0.02 a 0.02 + 0.01 b 0.02 + 0.01 b 0.02 + 0.003 b

MN_CRVL (m3 tree-1) 175.0 + 23.8 a 111.5 + 8.8 ab 106.6 + 12.4 b 99.0 + 6.1 b

CRVL (m3 ha-1 x 103) 105.5 + 15.2 89.5 + 5.5 101.2 + 9.3 116.7 + 16.3

T_CRVLD1 (m3 ha-1 x 103) 2.7 + 1.0 ad 5.2 + 0.8 bc 6.4 + 1.50 b 5.1 + 1.0 cd

T_CRVLD2 (m3 ha-1 x 103) 15.3 + 4.2 a 32.0 + 4.3 ac 43.3 + 5.2 b 33.9 + 3.7 bc

T_CRVLD3 (m3 ha-1 x 103) 63.7 + 23.8 39.3 + 5.0 35.4 + 4.8 48.6 + 9.9

T_CRVLD4 (m3 ha-1 x 103) 23.8 + 11.7 12.9 + 3.6 16.1 + 4.5 29.0 + 10.0

P_CRVLD1 0.03 + 0.02 0.07 + 0.01 0.08 + 0.01 0.05 + 0.01

P_CRVLD2 0.23 + 0.05 a 0.33 + 0.03 ac 0.47 + 0.03 b 0.42 + 0.03 bc

P_CRVLD3 0.53 + 0.05 a 0.47 + 0.03 a 0.32 + 0.03 b 0.39 + 0.03 ab

P_CRVLD4 0.20 + 0.04 0.12 + 0.02 0.13 + 0.03 0.13 + 0.02

UNVG_5 0.15 + 0.03 a 0.19 + 0.01 a 0.24 + 0.02 b 0.20 + 0.01 ab

UNVG5_50 0.42 + 0.05 ab? 0.40 + 0.03 a 0.40 + 0.03 a 0.30 + 0.02 b

154

UNVG_50 0.09 + 0.02 0.08 + 0.01 0.06 + 0.01 0.09 + 0.01

UNVG_200 0.01 + .001 0.01 + 0.002 0.01 + 0.001 0.01 + 0.004

1 Refer to Table 4 for meaning of variable abbreviations.

2 High = high-productivity (territories producing > 23 owlets); moderate = moderate-

productivity (territories producing 13-23 owlets); low = low-productivity (territories

producing < 9 owlets);

155

high-productivity class also had a greater proportion of basal area (P_BAD3) in the

second-largest dbh category, and greater tree density (P_TPHD3 and P_TPHD4) in

the two largest dbh categories, than non-territory habitat (Table 12). Notably, the

moderate-productivity class was not different than non-territory habitat for any

forest structure variable (Table 12). Compared with non-territory habitat, the low-

productivity class differed in the smallest dbh category by containing greater basal

area (T_BAD1 and P_BAD1), higher tree density (T_TPHD1 and P_TPHD1), and

greater crown volume (T_CRVLD1), and differed in the second-largest dbh category

by containing less basal area (T_BAD3; Table 12). The low-productivity class also

contained a greater proportion of understory vegetation 1-4 cm tall (UNVG_5) than

non-territory habitat (Table 12).

Possible Limiting Factors Associated With Habitat Relationships

Density of Cavity Trees.—Territories contained a mean 7.7 + 1.0 cavity trees (range 3

– 13; Table 13), but density of cavity trees was not correlated with owlets (R2 = 0.01,

F = 0.05, df = 11, P = 0.83). Mean density of cavity trees across territories was 0.6 +

0.3 ha-1 (range 0.3 – 1.0 ha–1; Table 13). However, non-territory habitat contained

only 0.05 cavity trees ha-1, less than 10% of mean cavity-tree density within

territories (Table 13, Fig. 20). Moreover, nearly 30% (5 of 17) of all non-territory

cavities occurred in a single cluster within the boundaries of an unstudied territory

(A40) on the edge of the study area (Fig. 20). Cavity trees generally had a clustered

distribution across the study area because most cavity trees were quaking aspen (74

%; 89 of 120) that occurred in discrete patches along drainage bottoms. As a result,

several portions of the study area contained no cavity trees (Fig. 20).

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Table 13. Density of cavity trees among owl territories and in non-territory habitat.

Territory Total cavity trees Territory area (ha) Density of cavity trees

(trees ha-1)

A2 3 6.6 0.46

A4 9 21.3 0.42

A7 4 11.1 0.36

A8 8 17.6 0.45

A10 13 18.5 0.70

A11 13 12.5 1.04

A12 6 6.6 0.91

A13 11 20.6 0.53

A18 7 17.8 0.39

A20 6 18.3 0.33

A27 5 5.4 0.92

A29 7 24.8 0.28

MEAN + SE 7.7 + 1.0 15.1 + 1.9 0.6 + 0.1

NON-TERRITORY 17 318.51 0.05

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Predation on Artificial Nests.—Predation rates on artificial nests differed among

territory classes (χ2 = 15.5, df = 2, P < 0.001), but there was no clear pattern.

Predation rates were highest in moderate-productivity territories (42%; 35 of 84 nest-

weeks), lowest in low-productivity territories (12%; 10 of 84), and intermediate in

high-productivity territories (21%; 15 of 74). As a result, predation rates were not

correlated with owlets (R2 = 0.03, F = 0.11, df = 5, P = 0.76). Predation rates were

24% (20 of 84 nest-weeks) in ponderosa pine/Douglas-fir, 30% (22 of 73)

in Douglas-fir, and 23% (18 of 77) in quaking aspen/blue spruce. Predation rates did

not differ among forest types (χ2 = 1.08, df = 2, P = 0.58).

Density of Red Squirrel middens did not differ among territory classes (F =

0.39, df = 5, P = 0.71). Mean density of middens was 0.8 + 0.1 ha-1 in high-

productivity territories, 1.1 + 0.1 ha-1 in moderate-productivity territories, and 1.1 +

0.1 ha-1 in low-productivity territories. However, frequency of middens differed

among forest types (χ2 = 14.75, df = 4, P =0.005). Frequency of middens was highest

in Douglas-fir (35 middens, 28 expected based on proportion of area in Douglas-fir),

Douglas-fir/blue spruce (17 middens, 7.2 expected), quaking aspen (11 middens, 3.3

expected), and quaking aspen/blue spruce (4 middens, 0.1 expected), and least in

ponderosa pine/Douglas-fir (50 middens, 67.6 expected) and Douglas-fir/limber pine

(no middens, 0.1 expected). Midden density was not correlated with owlets (R2 =

0.14, F = 0.63, df = 5, P = 0.47) or with predation rates at artificial nests (R2 = 0.03, F

= 0.13, df = 5, P = 0.74).

Relative Prey Abundance.—Arthropod abundance did not differ between the high-

productivity territory (A4) and the low-productivity territory (A18; Table 14).

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Table 14. Comparison of relative arthropod abundance (mean + SE) between a high-

productivity territory (A4) and a low-productivity territory (A18) during 1998-1999

(statistical significance based on ANOVA).

No. arthropods (mean + SE)

trap-night-1

Nesting stage Forest type1 A4 territory A18 territory df F p

Incubation Pipo/psme 14.0 + 3.4 12.8 + 4.3 7 0.05 0.83

Psme 13.8 + 4.7 11.8 + 4.8 7 0.09 0.78

Potr/pipu 8.9 + 2.2 11.4 + 4.5 7 0.24 0.65

Nestling Pipo/psme 11.0 + 1.8 13.4 + 2.1 9 0.77 0.40

Psme 21.4 + 5.7 20.4 + 2.9 9 0.03 0.88

Potr/pipu 24.1 + 7.6 21.7 + 2.2 8 0.07 0.80

1 Pipo/psme = ponderosa pine/Douglas-fir; Psme = Douglas-fir; Potr/pipu = quaking

aspen/blue spruce

159

DISCUSSION

Habitat Quality

Territory quality of Flammulated Owls in this study was associated with

mature forests of ponderosa pine/Douglas-fir. Owlets and breeding yr (equivalent to

total nesting attempts), which differed markedly among territories, were positively

correlated with territory area and proportion of area in ponderosa pine/Douglas-fir

forests. Territory quality was also associated with overstory structure in these forests,

based on the fact that owlets were positively correlated with greater crown volume in

the second-largest (33.0-48.2 cm) dbh category. In contrast, Douglas-fir forests were

sub-optimal habitat for breeding owls. Owlets and breeding yr were negatively

correlated with proportion of area in these forests, which contained greater tree

density, basal area, and crown volume in the two smallest dbh categories (2.5-17.7

and 17.8-32.9 cm) than ponderosa pine/Douglas-fir. Moreover, bachelor yr (total yr

territories were occupied by unpaired males) was positively correlated with this forest

type and with basal area. Previous studies found that unpaired males of other species

including raptors occupied territories in sub-optimal habitats (Newton and Marquiss

1976, Korpimaki 1988, Burke and Nol 1998).

Density of cavity trees was not correlated with reproductive success,

indicating that abundance of cavity trees was not associated with territory quality. All

territories contained > 3 cavity trees and had > 1 breeding attempt over the study,

suggesting that relatively few cavity trees were sufficient for nesting. However,

density of cavity trees clearly distinguished territory from non-territory habitat. Non-

territory habitat not only had a cavity-tree density that was < 10% of mean density

160

within territories, but because cavity-trees generally had a clustered distribution

across the study area, most of the non-territory habitat was characterized as

containing no cavity-trees.

The apparent importance of cavity trees in territory occupancy by males was

underscored by the few differences in other aspects of forest structure that

distinguished territory from non-territory habitat overall. Compared to non-territory

habitat, combined territories contained fewer trees and less basal area in the second-

smallest dbh category, and more understory cover 5-49 cm tall. However, in

comparisons of the three classes of territory productivity (high-, moderate-, and low-

productivity) with non-territory habitat, only the high-productivity class (which

contained only A4 territory) was distinguished by having less basal area and lower

tree density in the smaller dbh categories and the converse in the larger dbh

categories. These data indicate that differences between combined territories and

non-territory habitat were attributable to the unique characterisitics of A4 territory,

and to differences between the two statistical analyses (compared with the two-class

analyses, the four-class analyses had a larger mean squared error and a higher critical

value for evaluation). Based on the fact that moderate-productivity territories showed

no differences in overstory structure from non-territory habitat, and that low-

productivity territories actually contained greater basal area and tree density in the

smaller dbh categories than non-territory habitat, availability of cavity trees appeared

fundamentally important for territory establishment by males. These data also

suggest that at least some portions of non-territory habitat, which overall contained

similar proportions of each forest type to combined territories, were potentially

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suitable for occupancy by males except for scarcity of cavity trees. Thus, despite the

fact that snags and cavity trees were relatively abundant on the study area, which had

not been harvested since the 1800s (Reynolds et al. 1985), cavity-tree availability

clearly affected owl distribution and density, as it has with many other secondary-

cavity nesters (e.g., Brawn and Balda 1988, Newton 1998 and sources cited therein).

In summary, my results indicate that habitat quality was determined by two

primary factors. First, cavity-tree availability determined where owls established

territories, and second, forest type and structure determined whether a territory was

more often occupied by breeding pairs or by bachelor males. High-quality breeding

habitat for Flammulated Owls in this study was characterized as mature, relatively

open stands of ponderosa pine/Douglas-fir that contained sufficient cavity trees for

nesting.

Uniqueness of A4 Territory

A4 territory was by far the most productive (35 owlets, 16 breeding yr) of all

territories, and contained significantly greater basal area, crown volume, and tree

density in the second-largest dbh category than the moderate and low-productivity

classes. Was the habitat in A4 territory optimal for Flammulated Owls? Given its

uniqueness I cannot be certain, but several observations indicate this may be true.

First, correlations between reproductive success on A4 territory and forest type and

forest structure represented a positive linear extension of patterns across all other

territories. Second, duration of this study was likely sufficient to dilute chance effects

associated with quality of individual owls. In fact, A4 territory was occupied by 8

unique females and 5 unique males over the 19 yr study, both of which were more

162

than any other territory. Finally, A4 territory was the recipient of the most dispersals

(3; 2 females and 1 male), suggesting that this territory was preferable for breeding.

Possible Factors Underlying Habitat Relationships

Nest predation is a primary cause of nesting mortality for many bird species

(e.g., Skutch 1949, Ricklefs 1969), and is believed to be an important factor in the

evolution of life-history characteristics and habitat selection (Slagsvold 1982,

Sonerud 1985a, Martin 1988, Bosque and Bosque 1995). However, my data suggest

that nest predation was not associated with territory quality in Flammulated Owls. In

addition to the fact that nest success across territories was high (82% over the study),

as it is in most cavity-nesters compared with open nesting birds (Ricklefs 1969,

Wilcove 1985), I found no differences in predation rates among owl territories, and

no differences among forest types where nests were located. Density of Red Squirrel

middens, which I used as a measure of relative squirrel density in territories, was

highest in Douglas-fir forests where trees had highest density. This is consistent with

other studies that found denser stands of coniferous forests were preferred habitat for

Red Squirrels (Rusch and Reeder 1978, Gurnell 1984). However, density of middens

was not correlated with owl productivity, providing no evidence that Red Squirrels

affected habitat selection by owls. These results contrast with those of Korpimaki

(1993) and Sonerud (1985b), who found that rates of nest predation were high in

populations of cavity-nesting Tengmalm’s Owl (Aegolius funereus), and that owls

preferentially nested in areas where nest boxes had lowest predation rates.

Prey abundance affects the quality of breeding habitats for many birds

including raptors (e.g., Janes 1984, Korpimaki 1988, Burke and Nol 1998). However,

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I found no evidence that arthropod abundance over the 2 yr sampling period differed

between a high-productivity and low-productivity territory. Moreover, annual

constancy in owl reproductive variables (Chapter 2) suggests that any stochasticity

associated with arthropod abundance during the breeding season has had little effect

on long-term productivity. Still, research on prey abundance is needed over more yr

to better assess the relationship between prey and productivity, in addition to two

other aspects of prey abundance. Because topographic temperature gradients often

become established after dark (pers. observ.), study is needed to determine if flying

arthropods follow warmer temperatures by migrating to higher slope positions, where

ponderosa pine/Douglas-fir forests typically occur. In addition, because energetic

demands by females peak during egg-formation (generally late May in Colorado),

when nights are cold and arthropod activity is low, more study is needed to determine

if arthropod abundance during this possible ‘bottleneck’ period is correlated with owl

productivity.

Positive correlations between productivity and ponderosa pine/Douglas-fir

forests likely reflect the importance of these forests and their structure in the

behavioral ecology of Flammulated Owls. I previously reported that male owls,

which provide almost all the food for their mates and owlets until fledging, foraged

significantly more often in forests of ponderosa pine/Douglas-fir than in other forest

types (Linkhart et al. 1998). This forest type may be important for males because the

characteristically large, open tree crowns facilitate their gleaning and hover-gleaning

foraging tactics within and on the surface of tree crowns (Linkhart et al. 1998,

Reynolds and Linkhart 1987). Thus, as long as arthropod density is not limiting,

164

relative arthropod abundance among forest types or among territories may not be as

important as vegetation structure that facilitates capture of arthropods. Structure

associated with forests of ponderosa pine/Douglas-fir is also likely important to other

behaviors; I reported previously that owls preferentially used older (larger) ponderosa

pine and Douglas-fir trees for singing and day-roosting, probably because these trees

provided more protective cover against inclement weather and predators than younger

trees (Linkhart et al. 1998).

Bases for Inferring Territory Quality

Many studies have inferred habitat quality based on relative abundance (see

sources cited in Introduction) or duration of occupancy (e.g., Moller 1983, Bunzel

and Druke 1989, Newton 1989, Matthysen 1990). In my study occupied yr, which

was equivalent to duration of occupancy by breeding pairs and bachelor males, was

correlated only with territory area in ponderosa pine/Douglas-fir forest and showed

no correlation with any forest structure variables. This was because bachelor males

that occupied up to 70% of territories annually (Chapter 2) were positively associated

with sub-optimal habitat conditions. Therefore, inferring habitat quality based on

density or occupancy alone may be unreliable if mating status or reproductive success

of individual males is not known, as has been previously suggested (e.g., Van Horne

1983, Robinson 1992).

Breeding yr, which was identical to total nesting attempts in this study, was

nearly equivalent to total owlets fledged in habitat correlations because (1) nesting

success was high (82%), since relatively few nests were lost to predators, (2) range

of clutch size was very small (clutches almost always contained either 2 or 3 eggs),

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among the least of North American strigiforms (Chapter 2), and (3) most eggs

hatched and survived to fledging (Chapter 2). Consequently, breeding yr may be a

good surrogate for owlets in inferences of habitat quality in other portion of the

Flammulated Owl’s range, if breeding yr is associated with productivity as it was in

my study. Pairing status was used in assessments of habitat quality in other studies,

although the equivalency of this variable to productivity was not known (e.g., Burke

and Nol 1998).

Mean tenure on individual territories, which I used as an estimate of survival,

did not differ among territories for either sex suggesting that territory quality did not

influence individual survival. These data should be viewed cautiously for males,

however, since they were based primarily on males that nested more than once and

not on bachelor males (capturing bachelor males was difficult). Nonetheless, tenure

of males that were unpaired up to 5 yr between nesting attempts was similar to males

who nested annually (Chapter 4), suggesting that survival did not differ between

bachelor and breeding males. For males, this may indicate that survival is more

influenced outside of the breeding season, such as during migration, when many

Neotropical migrants species suffer mortality (Gill 1999). For females, survival may

be affected more by the energetic cost associated with both migration and egg-laying

(Chapter 2) than by selection of breeding territory. Tenure or turnover on territories

was influenced by territory quality in other studies of habitat quality (Marquiss and

Newton 1982, Korpimaki 1988). While the number of unique pair bonds on

territories ranged from 1 (A24, A18, and A2 territories) to 10 (A4 territory), the

frequency of this variable was primarily a function of the total number of breeding

166

pairs (= breeding yr) and was not a reflection of mate fidelity. Mean bond duration

did not differ among territories, likely because bond duration was limited by tenure of

females on territories (Chapter 4) rather than territory quality. Other studies have

found that mate fidelity differed among territories, suggesting that mate fidelity was

associated with habitat quality (Newton and Marquiss 1982, Haig and Oring 1988,

Part 1994).

The fact that most territories eventually produced owlets indicates that long-

term demographic data are needed to make accurate inferences regarding habitat

quality. While low-productivity territories were typically occupied by bachelors

annually, occasionally these territories were occupied by males that bred for 1-3

consecutive years. Short-term studies that captured only the years in which breeding

occurred on these territories would have identified relative territory quality much

differently than I have over the 19 yr study, resulting in inaccurate interpretations of

habitat quality. In addition, because most Flammulated Owls remained on territories

their entire known reproductive lives (up to 12 yr; Chapter 4), and because lifetime

reproduction was primarily a function of longevity (Chapter 3), long-term study was

required to assess contributions of individuals. Other studies have shown that long

time periods are necessary to evaluate effects of atypical individuals and extreme

environmental conditions (e.g., Woolfenden and Fitzpatrick 1984, Van Horne et al.

1997).

Stability of Territories

Considering the marked variation in reproductive success among territories,

frequent changes in territory boundaries and usurpation of territory ownership might

167

be expected if males attempted to improve their fitness (Janes 1984). However,

territory boundaries did not often change, possibly due to stable densities of cavity

trees, since other studies have found that territory boundaries shifted in response to

changing cavity resources (van Balen et al. 1982, East and Perrins 1988). But why

would males occupy low-quality territories their entire known reproductive lives

without dispersing to higher-quality territories? One possibility is that it is more

profitable for a male of a long-lived species such as the Flammulated Owl to await

the probable arrival of a female in a territory where he is familiar with the location of

sites for feeding, nesting, and escaping predators (Greenwood 1980). In addition,

males may "hedge their bets" by engaging in extra-pair copulations (EPCs; Reynolds

and Linkhart 1990, pers. observ.). Other raptor studies have found that territory

boundaries remained fixed or changed little year-to-year despite turnover of territory

occupants (Southern 1970, Newton 1976, Janes 1984, Nicholls and Fuller 1987), or

that males remained on the same territories even when higher-quality territories were

apparently available (e.g., Janes 1984, Woolfenden and Fitzpatrick 1984, Korpimaki

1988).

Distribution of Territories on the Landscape

Several authors reported finding clusters of singing male Flammulated Owls

separated by relatively large areas of apparently unoccupied habitat (e.g., Marcot and

Hill 1980, Howie and Ritcey 1987), a phenomenon that led some previous researchers

to postulate that the owls may be semi-colonial (see Winter 1974). Such clusters do

not provide direct evidence for coloniality, because observations are not based on

locations of nests but rather on responsiveness of singing males, which may or may

168

not be nesting. Compared to nesting males, unpaired males are more likely to be

detected because they sing with greater duration through the night and through the

breeding season (Linkhart et al. 1998). Aggregations of nesting territories, where

they exist, may reflect either surrounding areas of suitable habitat that are unoccupied

(Winter 1974), or surrounding habitat that appears suitable for breeding, but is in fact

suboptimal (Howie and Ritcey 1987, Reynolds and Linkhart 1992). My data support

the latter hypothesis, because male owls in this study only established territories

where suitable cavity trees for nesting were available, leaving unoccupied some

surrounding areas of habitat that appeared potentially suitable for breeding (i.e.,

forests consisting of mature, relatively open ponderosa pine/Douglas-fir), but lacked

cavity trees. Although density of cavity trees was not correlated with reproductive

success, territories were generally aggregated around clusters of cavities, which

occurred primarily in large quaking aspen trees in bottom areas, and secondarily in

large conifers on ridge tops (Reynolds et al. 1985, pers. observ.).

Forest Management

The correlation of productivity on territories with higher densities of larger-

diameter trees suggests that Flammulated Owls are adapted to forests that were

historically maintained by fire. Fire supression in many western forests, which were

characterized by open stands of large-diameter trees prior to European settlement, has

resulted in higher tree densities especially in the smaller diameter classes and has

resulted in conversion of many pine forests to fir forests (Cooper 1960, Covington

and Moore 1992). Tree density in smaller diameter classes was negatively correlated

with productivity on territories, suggesting that fire suppression may be resulting in

169

sub-optimal habitat for Flammulated Owls. Research is needed to determine the

effects on owl productivity of prescribed burns and/or selective logging that return

forest structure to pre-settlement conditions.

Further research is required to determine how and if patterns evident on this

511 ha study area are applicable elsewhere. Generally, breeding Flammulated Owls

have been associated with mature montane forests throughout their range (McCallum

1994a). However, Flammulated Owls may not be restricted to breeding in forests of

ponderosa pine/Douglas-fir, as indicated by studies that found owls breed in nest

boxes at relatively high densities in quaking aspen stands of northern Utah (Marti

1997), and that owls breed in pure forests of Douglas-fir in Montana (Powers et al.

1996). In order to determine the importance of floristics and forest structure, and to

assess the effects of forest management activities on breeding Flammulated Owls

over their range, researchers need to undertake comparative demographic studies of

owls in different forest types and across multiple forest management regimes.

170

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180

CHAPTER VI

CONCLUSIONS

Several demographic characteristics indicated that Flammulated Owls have a

life history more typical of larger birds, which generally have lower fecundity and

longer breeding lifespans, higher nesting success, and are longer-lived than smaller

birds (Newton 1998, Ricklefs 2000). First, fecundity of Flammulated Owls is among

the lowest and least variable of North American and European strigiforms, despite the

fact that they are one of the smallest (Johnsgard 1988). Second, male Flammulated

Owls had longer breeding lifespans compared to other species of strigiforms for

which comparable data exist (Korpimaki 1992, Marti 1997). Third, at least 75% of

nests were successful in 16 of 19 years for an overall nesting success rate of 82% over

the study. Among North American strigiforms, only Spotted Owls (S. occidentalis)

have a higher reported nesting success (85%; Forsman et al. 1984). Fourth, I never

observed replacement clutches or multiple broods in Flammulated Owls. Among

raptors in temperate regions, several small species and some larger species are known

to lay a replacement clutch if the first clutch is lost at an early stage (Newton 1979,

Johnsgard 1988). Finally, male Flammulated Owls have maximum longevity (at least

12 yr) greater than other small (< 150 g) owls and longevity comparable to many

larger owls (Glutz and Bauer 1980, Clapp et al. 1983, Klimkiewicz and Futcher 1989,

Klimkiewicz, pers. comm).

181

Data on lifetime reproductive success (LRS) indicated that a small percentage

of adult Flammulated Owls accounted for the majority of the total offspring produced

in the population, which is consistent with LRS data from other avian species

(Clutton-Brock 1988, Newton 1989, Wiklund 1995). Seventeen percent of females

and 27% of males accounted for 50% of all Flammulated Owl offspring, while in

other species the percentages for females range from 15% (Red-billed Gulls; Mills

1989) to 31% (Kingfishers; Bunzel and Druke 1989), and the percentages for males

range from 14% (Indigo Buntings; Payne 1989) to 30% in (Kingfishers; Bunzel and

Druke 1989). Males had longer breeding lifespans and more mates over their

lifetimes than females. These differences may result from males having greater

longevity than females (see below). Total breeding years were strongly correlated

with lifetime productivity for females and males, because clutch sizes varied little and

nesting success was high. Among all bird species, breeding lifespan has emerged as

the major demographic determinant of LRS (Newton 1989).

Male Flammulated Owls had significantly longer tenure on territories than

females, probably because males had greater territory fidelity (98% vs 78%) and an

apparently longer lifespan. Male-biased site fidelity is widespread among raptors

(e.g., Newton and Marquiss 1982, Forero et al. 1999), because in resource-defense

mating systems (Emlen and Oring 1977) males may have more to gain by being

faithful to breeding territories than females (Greenwood 1980). While longer lifespan

in male Flammulated Owls has not been substantiated, it is plausible for two reasons.

First, mean longevity over 19 yr was significantly greater for males than females,

although this disparity may in part reflect female-biased dispersal. Second, unpaired

182

males annually occupied 10-70% of territories, which suggests a shortage of females

in the breeding population. The apparent lack of females may be tied to energetics.

Flammulated Owls lay clutches representing a greater proportion of their mass (55-

60%) than other North American strigiforms (Johnsgard 1988). Coupled with the

high energetic cost of long-distance migration immediately prior to egg-laying, these

data suggest that females may be predisposed to higher mortality rates than males.

Female Flammulated Owls had a higher rate of breeding dispersal than males

(0.22 vs 0.02), a pattern that is well-documented in other birds (e.g., Greenwood

1980, Marquiss and Newton 1982, Forero et al. 1999). Dispersing females moved to

territories where productivity over the 19 yr study was significantly greater than on

territories from which they dispersed, suggesting that females were capable of

distinguishing relative quality among territories.

Fidelity and dispersal data suggest that dispersal distance by female owls may

be bimodal with females moving to adjacent territories following successful nests and

more distant territories (> 2 km) following unsuccessful nests. Dispersal by females

to nearby territories, as was found in many other species (e.g., Payne and Payne 1996,

Hannon and Martin 1996, Williams 1996, Forero et al. 1999), may be beneficial

because dispersers can best judge the quality of resources and potential mates in

adjacent territories (Hinde 1956, Greenwood 1980, Ens et al. 1996). Although

dispersal following nesting failure is beneficial if chances of future nesting success

are improved (Murphy 1996), benefits of dispersing to more distant territories, where

owls are unlikely to have knowledge of resources or potential mates, are not clear.

183

Most male Flammulated Owls occupied a single territory their entire known

reproductive lives. Whether or not males were paired annually did not appear to limit

their tenure or likelihood of changing territories. Males often continued to occupy

original territories despite being unpaired up to four consecutive years, even when

more productive territories apparently were available. Thus, high site fidelity by

males appeared to counter predictions based on habitat selection models that assume

animals select habitats conferring highest reproductive success, and that if higher-

quality habitats become available individuals should move to the new sites (Fretwell

and Lucas 1969). Consequently, territory fidelity by Flammulated Owls may be

considered a suboptimal form of habitat selection with respect to territory quality

(Switzer 1993). Males of other species including raptors have been documented

remaining on a single territory, even when higher-quality territories apparently were

available (e.g., Janes 1984, Woolfenden and Fitzpatrick 1984, Korpimaki 1988).

Productivity of owls over the 19 yr study was positively correlated with

territory area in ponderosa pine/Douglas-fir forests, and with greater crown volume in

the second-largest of four tree-diameter categories. Productivity was not correlated

with density of cavity-trees. However, cavity trees clearly distinguished territory

from non-territory habitat, since non-territory habitat contained < 10% of mean

cavity-tree density within territories. Few structural characteristics distinguished

combined territories from non-territory habitat. In comparisons of non-territory

habitat with three classes of territory distinguished by differing productivity, only the

high-productivity class contained greater tree density and basal area in the larger dbh

categories. Based on the fact that moderate-productivity territories showed no

184

differences in forest structure from unoccupied habitat, and that low-productivity

territories (usually occupied by bachelor males) actually contained denser forests and

smaller trees than non-territory habitat, at least some portions of non-territory habitat

may have been suitable for establishment of territories except for scarcity of cavity

trees. Monitoring of predation on artificial nests for 1 yr and relative prey abundance

for 2 yr revealed no patterns among selected territories differing in productivity,

suggesting these factors were not associated with habitat quality.

My results indicate that habitat quality was determined by two primary

factors. First, cavity-tree availability determined where owls established territories,

and second, forest type and structure determined whether a territory was more often

occupied by breeding pairs or by bachelor males. High-quality breeding habitat for

Flammulated Owls in this study was characterized as mature, relatively open stands

of ponderosa pine/Douglas-fir that contained sufficient cavity trees for nesting.

Many studies have inferred habitat quality based on relative abundance or

duration of occupancy (e.g., Bunzel and Druke 1989, Newton 1989). In my study

occupied yr, which was equivalent to duration of occupancy by breeding pairs and

bachelor males, was correlated only with territory area in ponderosa pine/Douglas-fir

forest and showed no correlation with any forest structure variables. This was

because bachelor males that occupied up to 70% of territories annually were

positively associated with sub-optimal habitat conditions. Therefore, inferring habitat

quality based on density or occupancy alone may be unreliable or misleading if

mating status or reproductive success of individual males is not known, as has been

previously suggested (e.g., Van Horne 1983, Robinson 1992). Breeding yr may be a

185

good surrogate for productivity in future efforts to identify important breeding

habitats for this species, at least where other demographic parameters such as nesting

success are similar to mine.

Habitat correlations in this study suggest that environmental changes,

including fire suppression, may affect long-term viability of owl populations. The

open structure and species composition of ponderosa pine/Douglas-fir forests

throughout the western United States were historically maintained by frequent, low-

intensity ground fires (Cooper 1960). Fire suppression has resulted in increased tree

densities in ponderosa pine and mixed-conifer forests, has converted many pine

forests to fir forests, and has changed fire type from low-intensity to catastrophic,

habitat-destroying, crown fire (Barrett et al. 1980, Gordon 1980). In this study,

younger, dense stands of Douglas-fir were associated with sub-optimal breeding

habitat. In addition, many ponderosa pine and mixed-conifer forests within the range

of the Flammulated Owl have been harvested. These habitat changes have resulted in

declines of Flammulated Owls in some areas (e.g., Marshall 1957, 1988; Franzreb

and Ohmart 1978). Species exhibiting characteristics of K-selection (sensu Pianka

1970), such as Flammulated Owls, generally can be expected to respond slowly to

environmental perturbations because of their low fecundity and low density (Newton

1998). In order to understand effects of changes on structure and species composition

of owl breeding habitat, and on long-term impact to the reproduction and survival of

owl populations, researchers need to undertake comparative demographic studies of

owls across multiple forest management regimes and forest types.

186

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Documents

LIFE HISTORY CHARACTERISTICS AND HABITAT QUALITY OF ... · quality in a Colorado population of Flammulated Owls ( Otus flammeolus ) in a 19-yr study. The owl is a small, monogamous