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EFFECTS OF ALIEN SHRUBS ON CATERPILLARS AND EFFECTS OF ALIEN SHRUBS ON CATERPILLARS AND

SHRUBLAND-DEPENDENT PASSERINES WITHIN THREE SHRUBLAND-DEPENDENT PASSERINES WITHIN THREE

TRANSMISSION LINE RIGHTS-OF-WAY IN SOUTHEASTERN NEW TRANSMISSION LINE RIGHTS-OF-WAY IN SOUTHEASTERN NEW

HAMPSHIRE HAMPSHIRE

Matthew Tarr University of New Hampshire, Durham

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EFFECTS OF ALIEN SHRUBS ON CATERPILLARS AND SHRUBLAND-DEPENDENT PASSERINES WITHIN THREE TRANSMISSION LINE

RIGHTS-OF-WAY IN SOUTHEASTERN NEW HAMPSHIRE

BY

MATTHEW DAVID TARR B.S. Wildlife Ecology, University of New Hampshire, 1996 M.S. Wildlife Ecology, University of New Hampshire, 1998

DISSERTATION

Submitted to the University of New Hampshire in Partial Fulfillment of

the Requirements for the Degree of

Doctor of Philosophy

In

Natural Resources and Environmental Studies

September, 2017

ii

ALL RIGHTS RESERVED © 2017

Matthew David Tarr

iii

This dissertation has been examined and approved in partial fulfillment of the requirements for

the degree of Doctor of Philosophy in Natural Resources and Environmental Studies.

Dissertation Director, Dr. John A, Litvaitis, Professor Emeritus Wildlife and Conservation Biology, UNH

Dr. Adrienne I. Kovach, Assistant Professor Wildlife and Conservation Biology, UNH

Dr. Thomas D. Lee, Associate Professor Environmental Conservation and Sustainability, UNH

Dr. Leonard R. Reitsma, Professor Zoology, Plymouth State University

Dr. Doug Tallamy, Professor Entomology, University of Delaware

On June 1, 2017

Original approval signatures are on file with the University of New Hampshire

Graduate School.

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ACKNOWLEDGEMNTS

This project was supported by generous financial contributions from the New

Hampshire Fish and Game Department, Eversource Energy, the Avian Powerline

Interaction Committee, the American Wildlife Conservation Foundation, and the

University of New Hampshire Hamel Center for Undergraduate Research, and was

approved by the University of New Hampshire Institutional Animal Care and Use

Committee (IACUC Protocol #110902).

This project would not have been possible without the exceptional work of my

dedicated field research technicians: Randy Shoe, Jack Hernandez, Emily Johnson,

Tricia Thai, Danette Perez, and Peter Goode, who worked tirelessly to assist me in all

aspects of field data collection and in sorting and identifying insects. I am especially

indebted to Logan Cline, Sarah Dudek, and Andrew Rossi who helped me trial all of

my research methods during a 2012 pilot study, and who returned in 2013 to help me

train technicians and coordinate and conduct all field research activities.

I am grateful to all of the exceptionally talented people who were generous with their

time and provided me with their guidance when I was in the early stages of planning

this project. I am particularly grateful to Mariko Yamasaki, Pam Hunt, Dave King, and

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Scott Schlossberg for sharing their extensive knowledge of shrubland systems in New

England and for providing suggestions for improving my study. Thank you to Don

Chandler and Doug Tallamy for your time and patience during the many discussions

we had regarding the multiple insect sampling methods I considered for this study.

Thank you to Mike Akresh, all of the banders and extractors at Appledore Island

Migration Station, particularly Sara Morris and Anthony Hill, and to Len Reitsma, for

all of your guidance and instruction in mist netting, handling, and banding birds.

Thank you especially to Len for allowing me to work as a subpermittee under your

master permit during my project.

Thank you to Kurt Nelson for all of your assistance helping me gain access to

transmission line study sites, for coordinating maintenance activities for and around my

project, and for being an effective liaison with Eversource. I am greatly appreciative to

the entire John Brown and Son’s brontosaurus crew for all of your interest in my project

and for mowing sites to meet the needs of the project. Thank you to Joanne Curran-

Celentano and to Karen Semo for your guidance and for all of your work analyzing

plasma carotenoid samples from my birds.

I am grateful and indebted to my PhD committee: John Litvaitis, Adrienne Kovach,

Tom Lee, Len Reitsma, and Doug Tallamy for your willingness to share your time,

expertise, and guidance throughout all aspects of this project and for your constructive

comments on rough and early versions of this dissertation.

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Finally, thank you to my entire family, especially Sue and David Blanchette, my wife

Pam, and my daughter Cailin, for your unwavering patience, support, and love - you

are my source of strength and inspiration and I am blessed to have you each in my life.

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LIST OF TABLES

TABLE PAGE

Table 1.1. Comparing mean ± SE (range) estimated caterpillar abundance (mg dry matter), shrub diversity, and alien and native shrub composition estimated in common yellowthroat territories among three transmission line rights-of-way in Rockingham and Strafford County, New Hampshire 2013…………………………………………………………………………............ 19

Table 1.2. Pairwise correlations between caterpillar abundance/m2 territory and vegetation metrics estimated in 36 common yellowthroat territories. Rockingham and Strafford County, New Hampshire, 2013…….. 22

Table 1.3. The 15 species of shrubland-dependent passerines that regularly breed in transmission line rights-of-way (ROW) in Strafford and Rockingham County, New Hampshire and those that were observed within each ROW study site during the 2013 nestling season……………….. 29

Table 2.1. Mean [± SE (range)] territory size, adult provisioning variables, and nestling diet composition measured in common yellowthroat territories among three transmission line rights-of-way in Rockingham and Strafford County, New Hampshire, 2013…………………………………. 52

Table 2.2. Comparing the average [± SE (range)] annual productivity, nestling growth, and nestling condition estimated in common yellowthroat territories in three transmission line rights-of-way (ALIEN, MIXED, NATIVE) in Rockingham and Strafford County, New Hampshire, 2013……………………………………………………………………………….... 58

Table 2.3. Comparing mean [± SE (range)] nestling common yellowthroat plasma carotenoid and carotenoid-based plumage variables estimated within common yellowthroat territories among three transmission line rights-of-way (ALIEN, MIXED, NATIVE) in Rockingham and Strafford County, New Hampshire, 2013………………….. 60

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Table 2.4. Pairwise correlations (Peterson’s r) between plasma carotenoids and growth, condition, and plumage color variables of 85 common yellowthroat nestlings raised in three transmission line rights-of-way in Rockingham and Strafford Co. New Hampshire 2013………………………… 63

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LIST OF FIGURES

FIGURE PAGE

Figure 1.1. Mean weight of smooth-bodied (i.e., lacking hairs or spines) caterpillars collected between early May and late-July from the dominant native and alien (glossy buckthorn and autumn olive) shrub species growing in three transmission line rights-of way in Rockingham and Strafford County, New Hampshire, 2013………………………………………. 12

Figure 1.2. Estimated mean number of smooth-bodied (i.e., lacking hairs or spines) caterpillars per m3 of foliage of each of the dominant native and alien (glossy buckthorn and autumn olive) shrub species sampled at three transmission line rights-of way in Rockingham and Strafford County, New Hampshire, 2013…………………………………………………………………... 14

Figure 1.3. Estimated mean biomass (mg dry matter) of smooth-bodied (i.e., lacking hairs or spines) caterpillars per m3 of foliage of each of the dominant native and alien (glossy buckthorn and autumn olive) shrub species sampled at three transmission line rights-of way in Rockingham and Strafford County, New Hampshire, 2013…………………………………. 16

Figure 1.4. NMDS ordination of 36 common yellowthroat territories occurring in three transmission line rights-of-way (ALIEN, MIXED, NATIVE), based on the vegetation composition within each territory………………….. 18

Figure 1.5. Comparing the proportion of alien shrubs, the proportion of native shrubs, shrub diversity, and estimated caterpillar abundance within common yellowthroat territories located at the ALIEN, MIXED, and NATIVE study sites. Rockingham and Strafford County, New Hampshire, 2013……………………………………………………………………………….... 20

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Figure 2.1. Comparing average adult common yellowthroat provisioning variables and the proportion of nestling yellowthroat diets (percent of total estimated prey biomass, see text) comprised by caterpillars estimated at the ALIEN, MIXED, and NATIVE study sites. Rockingham and Strafford County, New Hampshire, 2013………………………………………………….. 53

Figure 2.2. The proportion of the most common prey items composing diets of nestling common yellowthroats raised in three transmission line rights-of-way in Rockingham and Strafford County, New Hampshire, 2013……………………………………………………………………………….... 55

Figure 2.3. Comparing average growth and size variables of nestling common yellowthroats raised in territories located at the ALIEN, MIXED, and NATIVE study sites. Rockingham and Strafford County, New Hampshire 2013.………………………………………………………….. ……… 59

Figure 2.4. Comparing average plasma carotenoid concentrations and HSB (Hue, Saturation, Brightness) plumage scores of nestling common yellowthroats raised in territories located at the ALIEN, MIXED, and NATIVE study sites. Rockingham and Strafford, County New Hampshire, 2013……. 61

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS……………………………………………………….. iv

LIST OF TABLES………………………………………………………………….. vii

LIST OF FIGURES………………………………………………………………… ix

ABSTRACT………………………………………………………………………… xiv

CHAPTER PAGE

I. INFLUENCE OF NATIVE AND ALIEN SHRUBS ON CATERPILLAR ABUNDANCE IN TERRITORIES OF BREEDING SHRUBLAND- DEPENDENT BIRDS……………………………………………………………. 1

Introduction………………………………………………………………… 1

Methods……………………………………………………………………... 5

Study system………………………………………………………... 5

Caterpillar sampling……………………………………………….. 7

Estimating caterpillar abundance in bird territories……………. 8

Data analysis………………………………………………………… 10

Results………………………………………………………………………… 11

Caterpillar weight and abundance on native and alien shrub species………………………………………………………………… 11

Estimated caterpillar abundance within common yellowthroat territories……………………………………………………………… 15

Discussion…………………………………………………………………….. 23

Implications to shrubland-dependent passerines………………… 28

A tipping-point of native plant abundance in shrublands………. 32

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Management considerations………………………………………… 34

II. EFFECTS OF ALIEN SHRUBS ON COMMON YELLOWTHROAT PRODUCTIVITY AND NESTLING DIET COMPOSITION, GROWTH, AND PLUMAGE COLOR…………………………………………………..... 37

Introduction………………………………………………………….......... 37

Methods……………………………………………………………………. 42

Study system………………………………………………………. 42

Nestling growth and annual productivity……………………… 44

Nestling plasma carotenoids……………………………………... 46

Nestling plumage color…………………………………………… 47

Adult provisioning and nestling diet composition…………….. 48

Data analysis……………………………………………………….. 49

Adult provisioning and nestling diet variables………… 49

Nestling growth, condition, plasma carotenoids and plumage color……………………………………………… 50

Results……………………………………………………………………… 51

Adult provisioning and nestling diet composition……………. 51

Annual productivity, nestling growth, and nestling condition………………………………………………………….... 56

Nestling plasma carotenoids and carotenoid-based plumage color………………………………………………………………… 57

Discussion…………………………………………………………………. 64

Nestling diet composition and adult provisioning……………. 64

Productivity, nestling growth, plasma carotenoids, and plumage color……………………………………………………… 66

LIST OF REFERENCES…………………………………………………………… 74

APPENDICIES…………………………………………………………………….. 87

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APPENDIX A. A MODIFICATION TO JOHNSON’S (2000) “BRANCH- CLIPPING” METHOD FOR ESTIMATING CATERPILLAR ABUNDANCE ON LIVE TREE/SHRUB FOLIAGE…………………………… 88

APPENDIX B. THE TOTAL NUMBER OF 0.04 M3 NET SAMPLES COLLECTED TO ESTIMATE CATERPILLAR ABUNDANCE ON EACH SHRUB SPECIES AT EACH TRANSMISSION LINE RIGHT-OF-WAY STUDY SITE. ROCKINGHAM AND STRAFFORD COUNTY, NH. 2013….. 98

APPENDIX C. A MASS-LENGTH REGRESSION FOR ESTIMATING CATERPILLAR BIOMASS……………………………………………………….. 99

APPENDIX D. APPROVAL FOR UNIVERSITY OF NEW HAMPSHIRE INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (IACUC) PROTOCOL #110902……………………………………………………………… 104

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ABSTRACT

EFFECTS OF ALIEN SHRUBS ON CATERPILLARS AND SHRUBLAND-DEPENDENT PASSERINES WITHIN THREE TRANSMISSION LINE

RIGHTS-OF-WAY IN SOUTHEASTERN NEW HAMPSHIRE

by

Matthew David Tarr University of New Hampshire, September, 2017

By altering plant species composition and reducing plant species richness, invasion of

non-native (alien) plants can reduce caterpillar abundance in bird habitats and simplify

the bird community, but whether these cascading effects of alien plants extend further

to affect bird productivity has not been quantified. This study is the first to quantify

how reduced caterpillar abundance associated with alien plant invasion affects the

reproductive success of a breeding passerine, the common yellowthroat (Geothlypis

trichas trichas). The study was conducted in three transmission line rights-of-way where

plant composition in yellowthroat territories was dominated by a near monoculture of

alien shrubs (“ALIEN” site), by a mixture of native and alien shrubs (“MIXED” site), or by

only native shrubs (“NATIVE” site). At each site, I estimated total caterpillar abundance

in each yellowthroat territory based on the native and alien shrub species composition. I

then determined if differences in shrub composition in territories affected the types of

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arthropods that adult yellowthroats fed nestlings, or if differences affected yellowthroat

productivity or nestling growth, plasma carotenoid concentrations, and carotenoid-

based plumage color.

Caterpillar abundance was lowest on the alien shrubs autumn olive (Eleagnus umbellata)

and glossy buckthorn (Frangula alnus) and on native red maple (Acer rubrum) and

northern arrowwood (Viburnum dentatum). Caterpillar abundance was greatest on

native birch (Betula spp.) speckled alder (Alnus incana spp. rugosa), willow (Salix spp.),

meadowsweet (Spiraea latifolia), and sweetfern (Comptonia peregrina). Differences in the

relative abundance of the specific native and alien shrub species composing territories

resulted in territories at the MIXED site supporting the greatest total estimated caterpillar

abundance. Territories at the ALIEN site supported the lowest caterpillar abundance that

was 62% and 75% lower than that at the NATIVE and MIXED sites, respectively.

Shrubland bird richness at the MIXED and NATIVE sites was ≥ 2x that at the ALIEN site,

indicating that most shrubland birds avoided the ALIEN site for breeding. Differences in

caterpillar abundance among sites did not result in difference in yellowthroat

productivity, but adults at the ALIEN site fed nestlings a lower proportion of caterpillars

and increased their foraging effort to feed nestlings a greater proportion of alternative

prey (e.g., bees/flies, butterflies/moths, odonates, and spiders) than adults at the other

sites. Caterpillar availability to birds seemed to be reduced only where alien shrubs

grew abundant enough to reduce both the abundance and diversity of native shrubs

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within a shrubland. Overall, nestling growth, plasma carotenoids, and plumage color

score were greatest at the NATIVE site where nestlings were fed a large proportion of

caterpillars and an intermediate proportion of alternative prey, and lowest at the MIXED

site where nestling diets were composed of a large proportion of caterpillars and the

lowest proportion of alternative prey. Conversion of shrublands from native shrubs to

near-monocultures of alien shrubs may equate to habitat loss for shrubland-dependent

passerines that are unable to adapt to conditions of low shrub diversity and low

caterpillar abundance. Species such as common yellowthroats that readily feed

nestlings grasshoppers and spiders may experience no reduction in annual

reproductive success in habitats where alien shrubs reduce caterpillar abundance, but

elevated foraging costs of adults may result in negative fitness effects that can only be

identified by monitoring the same individual birds over multiple breeding seasons.

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CHAPTER I

INFLUENCE OF NATIVE AND ALIEN SHRUBS ON CATERPILLAR ABUNDANCE IN TERRITORIES OF BREEDING SHRUBLAND-DEPENDENT BIRDS

Introduction

Invasion by non-native (alien) plants can alter plant composition and structure, leading

to potentially negative cascading effects to both arthropods and birds within human-

altered communities (Ortega et al. 2006, Baiser et al. 2008, George et al. 2013).

Understanding the interactions among alien plants, arthropods, and birds can provide

information critical for setting habitat-management priorities and for conserving bird

populations.

Among the arthropods, caterpillars (i.e., larval Lepidoptera and Hymenoptera:

Symphyta) may be particularly sensitive to alien plant invasions (Tallamy 2004,

Tallamy and Shropshire 2009) because most are host specialists that feed only on plants

with which they have evolved (Ehrlich and Raven 1964, Bernays and Graham 1988,

Forister et al. 2015). Host specialization among caterpillars is primarily the result of

plant secondary chemicals that function as toxins, repellents, and/or feeding stimuli to

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caterpillars (Feeny 1976, Rhodes and Cates 1976, Futuyma 1983). As a result, alien

plants that employ novel chemical defenses (e.g., glossy buckthorn Frangula alnus,

autumn olive Eleagnus umbellata, honeysuckle Lonicera spp.) typically host few

caterpillars (Graves and Shapiro 2003, Tallamy and Shropshire 2009, Burghardt et al.

2010), and the caterpillars that do occur on them are often smaller and experience lower

survival than those on native plants (Tallamy 2004, Tallamy et al. 2010, Fickenscher et

al. 2014). Natural communities dominated by alien plants can therefore, support a lower

abundance and diversity of caterpillars than those composed predominately by native

plants (Burghardt et al. 2008, Fickenscher et al. 2014).

Lower native plant abundance and richness associated with alien plant invasion also

can lead to an altered bird community and/or a reduction in breeding bird richness, due

to altered habitat structure (Wilson and Belcher 1989, Whitt et al 1999, Williams and

Karl 2002) and reduced abundance of the arthropods that birds require for raising

young (Hickman et al. 2006, Gan et al. 2010, George et al. 2013). Greenburg’s “breeding

currency hypothesis” (Greenburg 1995, Johnson et al. 2005, Newell et al. 2014) predicts

lower breeding bird richness in habitats supporting a low abundance of large

caterpillars or grasshoppers (Orthoptera), specifically, and at least three studies have

confirmed this prediction in habitats dominated by alien plants (Burghardt et al. 2008,

George et al. 2013, Narango et al. in press). Low caterpillar abundance could also have

important fitness consequences for birds that remain in habitats dominated by alien

3

plants, because caterpillar availability is correlated positively with bird reproductive

success (Rodenhouse and Holmes 1992, Eeva et al. 1997, Sillanpää et al. 2009).

In the northeastern United States, the alien shrubs glossy buckthorn, autumn olive, and

honeysuckle are common components of many shrub-dominated habitats (shrublands)

that are breeding habitat for > 30 species of shrubland-dependent passerines

(Schlossberg and King 2007). The majority of these birds are considered species of

conservation concern due to population declines associated with habitat loss (Rich et al.

2004, Schlossberg and King 2007), and all would be expected to rely on caterpillars

(and/or grasshoppers) as the primary prey items they feed to nestlings and fledglings

(Greenburg 1995). If alien shrubs reduce caterpillar abundance to the point that bird

productivity is diminished, they may be contributing to shrubland bird declines, and

controlling alien shrubs may be an important bird conservation strategy.

Although a previous study (Fickenscher et al. 2014) documented lower caterpillar

abundance in shrublands dominated by alien shrubs compared to those dominated by

native shrubs, that study did not investigate how low caterpillar abundance on alien

shrubs might be offset by large caterpillar abundance on co-occurring native shrubs

(sensu Laiolo 2002, Marshal and Cooper 2004, Bereczki et al. 2014), such that caterpillar

availability to birds is not restricted until alien shrubs become abundant enough to

reduce plant diversity and bird foraging options. Because most temperate passerines

forage primarily within their territory while raising nestlings (Morse 1989), and

4

caterpillar abundance varies among each alien and native plant species (Tallamy and

Shropshire 2009, Singer et al. 2012, Fickenscher et al. 2015), the relative abundance of the

specific plants composing each bird territory should determine the overall caterpillar

abundance for each bird species occupying a shrubland (Robinson and Holmes 1984,

Holmes and Schultz 1988, Marshall and Cooper 2004).

To determine how differences in caterpillar abundance on native and alien shrubs may

influence caterpillar abundance for birds, I conducted a study in shrublands where

plant composition in bird territories spanned a gradient from a near monoculture of

alien shrubs to territories composed only of native shrubs. My first objective was to

quantify caterpillar abundance on the most abundant native and alien shrub species

composing each shrubland. My second objective was to estimate total caterpillar

abundance in bird territories based on the alien and native shrub species composition. I

predicted that caterpillar abundance would be lowest on the alien shrubs and that

caterpillar abundance would decline in bird territories as the proportion of alien shrubs

composing bird territories increased. I report how a mix of alien and native shrubs

within territories may affect estimated caterpillar abundance within each shrubland,

and how differences in plant species composition and caterpillar abundance among

shrublands were related to bird species richness.

5

Methods

Study system

This study was conducted May-August 2013 on three shrub-dominated transmission

line rights-of-way (ROW) located in Rockingham and Strafford County, New

Hampshire. Vegetation within each ROW had been maintained with an industrial

forestry mower for > 15 years prior to this study and was last mowed in March 2012,

such that by the beginning of the study each site was composed of > 50% young tree

and shrub cover < 3.5 m tall (hereafter “shrubs”), with the remainder composed

predominantly of ferns, forbs, and grasses. Because site types could not be replicated

due to the intensive nest monitoring effort required for a larger study investigating the

effects of alien shrubs on bird reproductive success (Chapter 2), three sites were selected

that were as similar as possible to one another (e.g., in their growing conditions and

surrounding landscape) except for their level of invasion by autumn olive and glossy

buckthorn. The “NATIVE” site was 6.4 ha and contained no autumn olive and only trace

amounts of glossy buckthorn. The most abundant shrubs at the NATIVE site were young

aspens (Populus tremuloides and P. grandidentata), birches (Betula populifolia and B.

papyrifera), blackberry (Rubus allegheniensis), blueberries (Vaccinium corymbosum and V.

angustifolium), cherries (Prunus pensylvanica and P. serotina), meadowsweet (Spirea alba

var. latifolia), red maple (Acer rubrum), oaks (predominantly Quercus rubra but also Q.

velutina and Q. alba), speckled alder (Alnus incana ssp. rugosa), sweetfern (Comptonia

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peregrina), willows (Salix spp.), and winterberry holly (Ilex verticillata). The “MIXED” site

was 13.9 ha and was dominated by a low to moderate abundance of glossy buckthorn,

and native aspens, birches, blackberry, blueberries, meadowsweet, oaks, red maple,

speckled alder, sweetfern, willows, and winterberry holly, as well as lesser amounts of

autumn olive and native arrowwood (Viburnum dentatum), cherries, and dogwoods

(Cornus amomum and C. racemosa). The “ALIEN” site was 6.9 ha and was dominated by

autumn olive and glossy buckthorn that each formed near monocultures in many areas.

Native shrubs at the ALIEN site included blackberry, blueberries, meadowsweet, and

speckled alder.

There are 15 species of shrubland-dependent passerines that regularly nest within ROW

in the study area, but a pilot study conducted in 2012 determined that common

yellowthroats (Geothlypis trichas trichas) were the most abundant foliage-gleaning bird at

all three study sites, and they were the only one to occur in abundance at the ALIEN site

(unpublished data). Therefore, yellowthroats were selected as the representative

shrubland bird species for determining how alien and native shrubs can influence

caterpillar abundance in bird territories. Yellowthroats are territorial and socially

monogamous during the breeding season; both parents feed nestlings a variety of

arthropods, including caterpillars and grasshoppers (Hofslund 1959, Guzy and

Ritchison 1999, Chapter 2), and they forage nearly entirely within their territory while

feeding nestlings (personal observation). In 2013, each site was visited every 1-3 days

7

and at each visit, all shrubland-dependent bird species observed either by sight or

sound were recorded as present.

Caterpillar sampling

Caterpillars were sampled from the most abundant shrub species at each site in mid-

June, early July, and late-July, encompassing the period of the nesting season of most

shrubland birds in the study area. Each ROW was divided into 18 - 57 caterpillar

sampling areas of approximately equal area (0.4 ha) using ArcMap (Ver. 10.2.2,

www.esri.com), and for each sampling date, a new random point was generated within

each area. Points were located in the field with a GPS unit and from each point the

nearest two individuals of each of the most abundant shrubs that characterized the site

were sampled. All caterpillar sampling occurred between 1000 and 1400 hrs. when

foliage was dry and all samples were collected from foliage occurring between 1 m - 3.5

m above ground level. Caterpillars were captured using a sweep net design and a

technique adapted from Johnson (2000; Appendix A), in which a sweep net was used to

first enclose ± 0.04 m3 of shrub foliage in a single pass that was then shaken to dislodge

caterpillars into the net. Caterpillars were immediately transferred into plastic bags that

were stored in a cooler until caterpillars could be sorted to order (Lepidoptera or

Hymenoptera: Symphyta) and family (for Lepidoptera), site, shrub species, and sample

date. Caterpillars were measured (length, mm), dried for 72 hours at 75° C, and weighed

(0.1 mg) with an analytic balance to determine their biomass (mg dry matter). Because

8

the majority of net samples contained zero caterpillars, high standard error of samples

precluded identifying differences in caterpillar abundance among shrub species when

data were analyzed by site and sampling date. Therefore, all samples from each shrub

species were pooled to estimate the average weight of caterpillars collected from each

species, as well as the number and biomass of caterpillars collected on average per m3 of

foliage from each species. Spiny and hairy caterpillars belonging to the Lepidoptera

families Arctiidae, Lymantriidae, Noctuidae, and Nymphalidae were excluded from

these estimates because adult birds are unlikely to feed these caterpillars to nestlings

(Slagsvold and Lifjeld 1985, Newell et al. 2014, Chapter 2).

Estimating caterpillar abundance in bird territories

At each site, all male common yellowthroats were captured using mist nets and marked

with a unique combination of three colored leg bands. Territory boundaries of each

male were estimated from ≥ 30 locations (± 5 m accuracy) of the bird collected with a

GPS unit beginning 3 days after banding and continuing every 1-3 days until nestlings

in the territory fledged. Territories were mapped as minimum convex polygons

generated by overlaying locations of each male on a current digital aerial photo of the

study site in ArcMap.

Live foliage volume of all shrub species and herbaceous plants comprising each

territory was estimated within 5 d after nestlings fledged. Vegetation was sampled at

points along a 3.5 m x 3.5 m sampling grid established in each territory and the total

9

foliage volume (m3) of each substrate was estimated following the methods detailed by

Mills et al. (1991) in which a graduated pole is held vertically at each point to essentially

sample live foliage occurring within a series of 0.1 m dia. x 0.1 m tall cylinders stacked

along the height of the pole. For each territory, the proportion of the total foliage

volume composed by each shrub species, ferns, forbs, and grasses was calculated. The

percent alien shrubs in each territory was considered as the proportion of the total

shrub foliage volume composed by alien shrubs. Shannon’s diversity and Simpson’s

diversity were calculated for each territory based on the proportion of the total foliage

volume composed by each shrub species. A single estimate of caterpillar abundance

(mg dry matter) per m2 of each territory was calculated based on the total volume of

each shrub species comprising the territory, multiplied by each shrub species’

corresponding estimate of caterpillar abundance/m3 of foliage.

With only two exceptions, I sampled caterpillars from all shrub species composing ≥ 3%

of the total shrub foliage volume estimated within territories. I did not sample

caterpillars from blackberry that accounted for an average of 7.6%, 6.2%, and 20.4% of

the total shrub foliage volume in territories at the ALIEN site, MIXED site, and NATIVE

site, respectively, because there was no practical way to sample these heavily-thorned

plants with the nets I was using, and other typical methods for sampling caterpillars

(e.g., beating onto a sheet, timed counts) would not have allowed me to extrapolate my

estimate of caterpillar abundance to the foliage volume sampled. I did not sample

10

caterpillars from cherry that appeared too sparse to sample, but accounted for 7.2% of

the average total shrub foliage volume in territories at the NATIVE site; cherry was

absent from the ALIEN site and occurred only in trace amounts at the MIXED site.

Data analysis

Unless otherwise noted, all statistical analyses were performed using JMP Pro (Ver. 12,

SAS, www.JMP.com) with a P < 0.05 level of significance. The average weight of

caterpillars and the average number and biomass (a function of average number and

weight) of caterpillars per m3 foliage, was compared among shrub species using

Kruskal-Wallis followed by Wilcoxon tests among all pairs, with a test for false

discovery rate of significance (Glickman et al. 2014). The vegetation composition among

territories across all study sites was compared with a non-metric multidimensional

scaling ordination (NMS) in PC-ORD (McCune and Mefford 2011), constructed of a

main matrix composed of all territories and the log (generalized) proportion of the total

foliage volume of ferns, forbs, grass, and each shrub species occurring in > 5% of

territories and accounting for > 5% of the total foliage volume in each territory. The

second matrix consisted of all territories and territory size, Shannon’s diversity, and

Simpson’s diversity. The ordination was executed using a random starting

configuration for 250 runs with real data, a stability criterion of 0.000001 for 10

iterations with a maximum of 500 iterations, Sorenson as the distance measure, and no

penalty for tie handling. The average percent total foliage volume of each shrub species,

11

Shannon’s and Simpson’s diversity, and estimated caterpillar abundance in territories

was compared among study sites using ANOVA and Tukey’s HSD. Data from all

territories were pooled and Pearson’s r was used to test for significant correlations

between estimated caterpillar abundance and the percent total foliage volume of each

shrub species within territories.

Results

Caterpillar weight and abundance on native and alien shrub species

A total of 4315 caterpillar net samples were collected from 67 to 247 individual shrubs

of each species, depending on the distribution and abundance of each species at each

site (Appendix B). A total of 587 caterpillars were captured in 304 (7%) samples and

most samples (88%) captured only one caterpillar. The most numerous smooth-bodied

caterpillars in samples were hymenoptera larvae (n = 198) and lepidopteran larvae in

the families Geometridae (n = 101), Notodontidae (n = 60), and Lycaenidae (n = 27).

Overall, average caterpillar weight on alien shrubs (1.55 mg, SE = 3.96) was lower (χ2 =

6.76, df = 1, P = 0.009) that than on native shrubs (6.35 mg, SE = 0.93), but average

caterpillar weight was different among shrub species (χ2 = 55.2, df = 14, P < 0.0001).

Specifically, the lightest caterpillars occurred on glossy buckthorn, aspen, arrowwood,

autumn olive, sweetfern, and oak, and the heaviest were on birch, willow, red maple,

blueberry, and speckled alder (Fig 1.1). Caterpillars on birch were heavier than those on

glossy buckthorn (P < 0.0001), aspen (P < 0.0001), sweetfern (P < 0.0001), oak (P =

Figure 1.1. Mean weight of smooth-bodied (i.e., lacking hairs or spines) caterpillars collected between early May and late-July from the dominant native and alien (glossy buckthorn and autumn olive) shrub species growing in three transmission line rights-of way in Rockingham and Strafford County, New Hampshire, 2013. Numbers in parenthesis are the number of caterpillars composing the sample for each shrub species. Bars with different letters have significantly different caterpillar weights (P < 0.05).

a c e

a

b c

a b

a c

b c

b c

a b

a b b

c

b d e

a b a

b b d b

d

13

0.0002), winterberry holly (P = 0.0013), meadowsweet (P < 0.0001), and willow (P =

0.0013); those on blueberry were heavier than those on glossy buckthorn (P = 0.0008),

aspen (P = 0.0002), and sweetfern (P = 0.006); those on speckled alder were heavier than

those on glossy buckthorn (P = 0.003) and aspen (P = 0.0002). The average weight of

caterpillars collected at the ALIEN site (3.4 mg, SE = 3.1, n = 33) was 33% and 64% lighter

than that of those at the MIXED site (5.1 mg, SE = 1.1, n = 264) and the NATIVE site (9.5

mg, SE = 1.8, n = 106), respectively.

Overall, the average number of caterpillars/m3 of foliage was lower (χ2 = 15.87, df = 1, P

< 0.0001) on the alien shrubs (0.8 mg, SE = 0.43) than on the native shrubs (2.4 mg, SE =

0.16), but the number of caterpillars/m3 of foliage differed among shrub species (χ2 =

71.5, df = 14, P < 0.001). Specifically, the fewest number occurred on red maple, autumn

olive, glossy buckthorn, and dogwood and the greatest number occurred on sweetfern,

aspen, willow, birch, meadowsweet, and speckled alder (Fig 1.2). The number of

caterpillars on red maple was less (P < 0.05) than that on all other shrubs except

arrowwood, blueberry, dogwood, glossy buckthorn, and autumn olive; that on autumn

olive was less than that on sweetfern (P = 0.002), willow (P = 0.01), and meadowsweet (P

= 0.01); that on glossy buckthorn was less than that on sweetfern (P < 0.0001), aspen (P <

0.0001), willow (P < 0.0001), meadowsweet (P < 0.0001), and speckled alder (P = 0.001);

that on blueberry was less than that on sweetfern (P < 0.0001), aspen (P = 0.02), willow

(P = 0.002), and meadowsweet (P = 0.002) ; that on red oak was less than that on

Figure 1.2. Estimated mean number of smooth-bodied (i.e., lacking hairs or spines) caterpillars per m3 of foliage of each of the dominant native and alien (glossy buckthorn and autumn olive) shrub species sampled at three transmission line rights-of way in Rockingham and Strafford County, New Hampshire, 2013. Bars with different letters have significantly different caterpillar abundance (P < 0.05).

a g k

a d g i

a b g j

a f i l

a c g i

b c d h i

b c d e f I k

a e f i

b c d e f i

c d e f h i j

f g

b c d e l

e f h

d e f h

f h

15

meadowsweet (P = 0.01); that on meadowsweet was less than that on birch (P = 0.02);

that on birch was less than that on sweetfern (P = 0.002).

The average caterpillar biomass/m3 foliage was lower (χ2 = 16.`6, df = 1, P < 0.0001) on

the alien shrubs (1.3 mg, SE = 3.3) than on the native shrubs (9.8 mg, SE = 1.3), but the

average caterpillar biomass/m3 foliage differed among shrub species (χ2 = 70.9, df = 14, P

< 0.001). The lowest biomass was on glossy buckthorn, red maple, autumn olive,

arrowwood, oak, and dogwood, and the greatest was on birch, speckled alder, willow,

meadowsweet, and sweetfern (Fig 1.3). The average caterpillar biomass on glossy

buckthorn was less than that on speckled alder (P < 0.0007), willow (P < 0.0001),

meadowsweet (P < 0.0001), sweetfern (P < 0.0001), and aspen (P < 0.0001); that on red

maple was less than that on birch (P = 0.008), speckled alder (P = 0.002), willow (P <

0.0001), meadowsweet (P < 0.0001), sweetfern (P < 0.0001), aspen (P < 0.0001), and

maleberry (P = 0.005); that on autumn olive was less than that on sweetfern (P = 0.002);

that on red oak was less than that on meadowsweet (P = 0.009) and sweetfern (P =

0.001); that on blueberry was less than that on willow (P = 0.004), meadowsweet (P =

0.002), and sweetfern (P = 0.0002); that on winterberry holly was less than that on

sweetfern (P = 0.002); that on sweetfern was less than that on birch (P = 0.002).

Estimated caterpillar abundance within common yellowthroat territories

I mapped and determined the vegetation composition within 36 yellowthroat territories

among all three sites. Territories ordinated by site, reflecting a pattern of greatest alien

Figure 1.3. Estimated mean biomass (mg dry matter) of smooth-bodied (i.e., lacking hairs or spines) caterpillars per m3 of foliage of each of the dominant native and alien (glossy buckthorn and autumn olive) shrub species sampled at three transmission line rights-of way in Rockingham and Strafford County, New Hampshire, 2013. Bars with different letters have significantly different caterpillar biomass (P < 0.05).

a b

a b c d

a b e

a b e

a b e

b c e

a b c d

c bde

de

a b c

e d e

c de

a

b d

17

shrub composition and lowest shrub diversity at the ALIEN site, and lowest alien shrub

composition and high shrub diversity at the NATIVE site (Fig 1.4). Further, vegetation

composition in territories at the NATIVE and MIXED site was more similar than to that in

territories at the ALIEN site. The NMS ordination was influenced most strongly by the

plant substrates correlating with axis-1 that explained 79% of the effect in the ordination

(Fig 1.4). The plant substrates with the strongest significant negative effect with axis-1

were the percent total foliage volume of all alien shrubs (r2 = 0.73), glossy buckthorn (r2

= 0.66), and autumn olive (r2 = 0.38). The plant substrates with the strongest significant

positive effect with axis-1 are the percent total foliage volume of ferns (r2 = 0.8), red

maple (r2 = 0.68), all native shrubs (r2 = 0.65), cherry (r2 = 0.62), birch (r2 = 0.39), and

sweetfern (r2 = 0.39), and both Shannon’s diversity (r2 = 0.5) and Simpson’s diversity (r2 =

0.48). The average percent total foliage volume of alien shrubs and the percent total

foliage volume of glossy buckthorn differed among sites (F2, 33 = 124.7.0, P < 0.001 and F2,

33 = 49.6, P < 0.001, respectively) and were greatest in territories at the ALIEN site,

intermediate in territories at the MIXED site, and lowest in territories at the NATIVE site

(Table 1.1, Fig. 1.5). The average percent total foliage volume of autumn olive was

greater (F2, 33 = 5.1, P = 0.01) in territories at the ALIEN site than in those at the other sites

where autumn olive was either uncommon (MIXED site) or absent (NATIVE site). The

average percent total foliage volume of native shrubs in territories at the MIXED and

NATIVE sites differed by < 3.5% and was greater (F2, 33 = 43.9, P < 0.001) than in territories

Figure 1.4. NMDS ordination of 36 common yellowthroat territories occurring in three transmission line rights-of-way (ALIEN, MIXED, NATIVE), based on the vegetation composition within each territory (n = number of territories at each site). Axis -1 explains 79% of the effect in the ordination and Axis-2 explains 9%. Plant substrates correlated with Axis-1 are the percent total foliage volume of each substrate. Rockingham and Strafford County, New Hampshire. 2013.

Correlated (+) with AXIS-1 ferns r2 = 0.8

red maple r2 = 0.68 all native shrubs r2 = 0.65

cherry r2 = 0.62 Shannon’s Diversity r2 = 0.5 Simpson’s Diversity r2 0.48

birch r2 = 0.32 sweetfern r2 = 0.39

Correlated (-) with AXIS-1 all alien shrubs r2 = 0.73

glossy buckthorn r2 = 0.66 autumn olive r2 = 0.38

Correlated (+) with AXIS-2 arrowwood r2 = 0.59

all herbaceous plants r2 = 0.48 winterberry holly r2 = 0.47

grass r2 = 0.47

Correlated (-) with AXIS-2 blackberry r2 = 0.48

ALIEN (6 territories) MIXED (20 territories) NATIVE (10 territories)

caterpillar abundance/m² territory 0.8 ± 0.3 (0.5 to 1.1) a 3.2 ± 0.2 (1.7 to 4.6) b 2.1 ± 0.3 (1.0 to 3.4) c

Vegetation MetricShannon's diveristy 0.99 ± 0.08 (0.54 to 1.42) a 2.12 ± 0.04 (1.99 to 2.61) b 2.16 ± 0.06 (1.93 to 2.54) bSimpson's diversity 0.48 ± 0.03 (0.26 to 0.66) a 0.84 ± 0.2 (0.76 to 0.91) b 0.85 ± 0.2 (0.81 to 0.91) b

Total Shrub Foliage Volume (%) Alien shrubs 75.3 ± 3.8 (55.3 to 85.0) a 27.9 ± 2.2 (5.7 to 43.3) b 2.0 ± 2.9 (0.3 to 7.6) c

Total Foliage Volume (%): Alien shrubs 41.8 ± 2.4 (33.1 to 49.8) a 14.1 ± 1.3 (2.2 to 25.5) b 0.9 ± 1.9 (0.1 to 3.0) c autumn olive 4.7 ± 1.3 (0.0 to 21.1 ) a 0.2 ± 0.7 (0.0 to 2.5 ) b 0.0 ± 1.0 (0.0 to 0.0 ) b glossy buckthorn 37.1 ± 2.9 (19.2 to 48.8) a 13.9 ± 1.6 (2.2 to 25.5) b 0.9 ± 2.2 (0.1 to 3.0) c

Native shrubs 9.0 ± 3.0 (5.1 to 17.2) a 36.4 ± 1.6 (24.9 to 56.2) b 39.8 ± 0.3 (29.1 to 52.5) baspen 0.0 ± 1.2 (0.0 to 0.0) 2.8 ± 0.7 (0.0 to 13.3) 1.4± 0.9 (0.0 to 3.6) arrowwood 0.1 ± 0.3 (0.0 to 0.6) 0.5 ± 0.1 (0.0 to 2.5) 0.1 ± 0.2 (0.0 to 0.8) birch <0.1 ± 1.1 (0.0 to 0.0) a 5.1 ± 0.6 (0.6 to 10.7) b 3.6 ± 0.9 (0.3 to 10.1) bblackberry 4.3 ± 1.3 (1.9 to 6.8) a 3.3 ± 0.7 (0.1 to 13.9) a 8.4 ± 1.0 (4.9 to 16.1) bblueberry 0.0 ± 0.8 (0.0 to 0.0) a 2.5 ± 0.4 (0.0 to 7.3) b 0.3 ± 0.6 (0.0 to 1.6) acherry 0.0 ± 0.4 (0.0 to 0.0) a 0.4 ± 0.2 (0.0 to 1.7) a 2.9 ± 0.3 (0.2 to 5.8) bdogwood 0.1 ± 0.4 (0.0 to 0.3) 1.1 ± 0.2 (0.0 to 4.3) 0.3 ± 0.3 (0.0 to 1.2) maleberry 0.0 ± 0.3 (0.0 to 0.0) 0.6 ± 0.2 (0.0 to 3.0) 0.2 ± 0.2 (0.0 to 1.5) meadowsweet 0.9 ± 0.8 (0.1 to 1.8) a 3.3 ± 0.5 (0.5 to 8.4) b 3.4 ± 0.6 (0.8 to 8.8) aboak 0.0 ± 1.1 (0.0 to 0.0) a 3.1 ± 0.6 (0.0 to 12.8) b 1.8 ± 0.8 (0.3 to 5.3) abred maple <0.1 ± 1.4 (0.0 to 0.1) a 3.4 ± 0.8 (0.0 to 13.2) a 7.8 ± 1.1 (2.7 to 16.0) bspeckled alder 0.6 ± 0.6 (0.0 to 1.6) 1.3 ± 0.4 (0.0 to 6.3) 0.7 ± 0.5 (0.0 to 3.8) sweetfern 0.0 ± 1.0 (0.0 to 0.0) a 1.0 ± 0.6 (0.0 to 5.3) a 4.3 ± 0.8 (0.0 to 10.5) bwillow 0.7 ± 0.7 (0.0 to 3.1) ab 2.1 ± 0.4 (0.0 to 6.7) a 0.3 ± 0.5 (0.0 to 1.4) bwinterberry holly 0.5 ± 1.3 (0.0 to 1.9) 3.1 ± 0.7 (0.0 to 15.9) 1.7 ± 1.0 (0.0 to 6.7)

Site¹

Table 1.1. Comparing mean ± SE (range) estimated caterpillar abundance (mg dry matter), shrub diversity, and alien and native shrub composition estimated in common yellowthroat territories among three transmission line rights-of-way in Rockingham and Strafford County, New Hampshire, 2013.

1 Columns with different letters are significantly different

A. a

b

c

B.

a

b b

C.

a

b b D.

a

b

c

Figure 1.5. Comparing the proportion of alien shrubs, the proportion of native shrubs, shrub diversity, and estimated caterpillar abundance within common yellowthroat territories located at the ALIEN, MIXED, and NATIVE study sites. Rockingham and Strafford County, New Hampshire, 2013. Within each sub-figure, bars with different letters differ significantly (P < 0.05).

21

at the ALIEN site (Table 1.1, Fig. 1.5). Similarly, average Shannon’s diversity and

Simpson’s diversity was nearly identical between territories at the MIXED and NATIVE

sites and greater (Shannon’s = F2, 33 = 94.9, P < 0.001, Simpson’s = F2, 33 = 72.7, P < 0.001)

than in territories at the ALIEN site (Table 1.1, Fig. 1.5). The average percent total foliage

volume of birch was lower (F2, 33 = 8.1, P = 0.001) in territories at the ALIEN site than at

the other sites. The percent total foliage volume of both sweetfern and red maple were

greater (F2, 33 = 7.1, P = 0.003 and F2, 33 = 10.6, P = 0.0003, respectively) in territories at the

NATIVE site than at the other sites.

Based on differences in shrub composition among territories, the estimated caterpillar

abundance in territories differed among study sites (F2, 33 = 21.2, P < 0.001) and was

lowest at the ALIEN site (0.8 mg/m2, SE = 0.3), greatest at the MIXED site (3.2 mg/m2, SE =

0.2) and intermediate at the NATIVE site (2.1 mg/m2, SE = 0.3; Table 1.1, Fig. 1.5).

Estimated caterpillar abundance/m2 territory was positively correlated with Shannon’s

and Simpson’s diversity, and the percent total foliage volume of native shrubs, birch,

blueberry, meadowsweet, oak, and willow (Table 1.2). Estimated caterpillar

abundance/m2 territory was negatively correlated with percent total shrub foliage

volume of alien shrubs, and marginally negatively significantly correlated (P = 0.05)

with the percent total foliage volume of alien shrubs.

22

Vegetation Metric Pearson's r r² PShannon's diversity 0.58 0.34 < 0.001Simpson's diversity 0.59 0.35 < 0.001

Total Shrub Foliage Volume (%): Alien shrubs -0.35 0.12 0.04

Total Foliage Volume (%): Alien shrubs -0.33 0.11 0.05

Native shrubs 0.63 0.40 < .0001 birch 0.84 0.70 < .0001 blueberry 0.55 0.32 < .001 meadowsweet 0.42 0.18 0.01 oak 0.38 0.15 0.03 willow 0.36 0.13 0.04

Caterpillar abundance/m² territory

Table 1.2. Pairwise correlations between caterpillar abundance/m2 territory and vegetation metrics estimated in 36 common yellowthroat territories. Rockingham and Strafford County, New Hampshire, 2013.

1 Proportion of the total shrub foliage volume composed of alien shrubs

23

Discussion

Caterpillar biomass in territories was associated with the relative abundance of the

specific native and alien shrub species within each bird territory, rather than the

presence or absence of alien shrubs within the territory. Glossy buckthorn and autumn

olive supported low caterpillar biomass, and although certain native shrubs supported

large caterpillar biomass (e.g., birch, speckled alder, willow, meadowsweet, sweetfern,

aspen), others (e.g., red maple, arrowwood) did not (Figs 1.2 and 1.3). As a result,

territories composed predominantly of native red maple or arrowwood could have

supported caterpillar abundance as low as, or lower than, territories composed

predominantly of alien shrubs. However, in territories where native shrub diversity

was high, low caterpillar biomass on autumn olive, glossy buckthorn, red maple, and/or

arrowwood was offset by high caterpillar biomass on other native shrub species, even if

those other shrubs accounted for a relatively lower proportion of the foliage volume

within the territory. For example, although glossy buckthorn supported the lowest

caterpillar biomass (Fig 1.3) and accounted for nearly 50% of the foliage volume in some

territories (Table 1.1), the percent total foliage volume of glossy buckthorn did not have

a significant influence on my estimate of caterpillar abundance in territories (Table 1.2);

comparatively, birch supported the greatest caterpillar biomass (Fig 1.3) and it

explained 70% of the variability in caterpillar abundance in territories (Table 1.2),

despite accounting for < 11% of the foliage volume in territories (Table 1.1). Although

24

the proportion of alien shrubs were negatively associated with caterpillar abundance in

territories, alien shrubs seem less influential (r2 ≤ 0.11) than native shrubs (r2 = 0.40) in

determining caterpillar abundance (Table 1.2). Overall, the presence of the native

shrubs birch, blueberry, meadowsweet, and oak seemed to have the greatest influence

on caterpillar abundance in territories (Table 1.2), and the presence of one or more of

these species within a territory may have been especially important for offsetting low

caterpillar abundance on other shrub species and for influencing caterpillar availability

for birds. Previous studies have identified positive correlations between native plant

diversity and caterpillar abundance in bird habitats (Burghardt et al. 2008, Bereczki et

al. 2014), and my estimates of total caterpillar abundance in territories were also

significantly and positively correlated with shrub diversity in territories (Table 1.2).

The ability of multiple native shrubs to offset low caterpillar abundance on alien shrubs

was particularly apparent at the MIXED site, where territories were composed of a

significantly greater proportion of alien shrubs, but they still supported greater

caterpillar abundance than territories at the native site (Table 1.1). Importantly,

although alien shrubs at the MIXED site accounted for as much as 43% of the total shrub

foliage volume in territories, these shrubs did not grow dense enough on these sites to

reduce native shrub diversity or the percent total foliage volume of native shrubs below

those at the NATIVE site (Table 1.1). At the MIXED site, low caterpillar abundance on the

alien shrubs and on native arrowwood and red maple seems to have been offset by

25

greater caterpillar abundance on other native shrubs growing at the site (particularly,

aspen, birch, meadowsweet, speckled alder, sweetfern, willow, and winterberry holly,

Fig 1.3), resulting in territories at the MIXED site supporting about 35% greater

caterpillar biomass than those at the NATIVE site. Comparatively, in territories at the

ALIEN site, autumn olive and glossy buckthorn formed near monocultures that reduced

the abundance and diversity of native shrubs below those in territories at either the

MIXED or NATIVE sites (Fig 1.4, Table 1.1). As a result, territories at the ALIEN site had

caterpillar abundance about 62% and 75% lower than that within territories at the

NATIVE and MIXED sites, respectively.

Caterpillars were not completely absent from autumn olive and glossy buckthorn; these

caterpillars likely originated from miscues by ovipositing female butterflies and moths

that resulted in some caterpillars hatching on these non-host plants (Futuyma and

Gould 1979, Graves and Shapiro 2003). However, the caterpillars I collected from alien

shrubs were among the smallest I collected from all shrub species (Fig 1.1) and this is

expected from caterpillars exposed to chemicals produced by non-host plants (Rhoades

and Cates 1976, Scribner and Feeny 1979), and has been observed in caterpillars raised

experimentally on autumn olive and glossy buckthorn, specifically (Tallamy et al. 2010,

Fickenscher et al. 2014). Due to the predominance of autumn olive and glossy

buckthorn at the ALIEN site, common yellowthroat territories here contained both fewer

26

caterpillars and caterpillars of smaller average size than territories at either the MIXED or

NATIVE sites.

Red maple has been reported previously to support either high caterpillar abundance

(Futuyma and Gould 1979, Graber and Graber 1983) or low caterpillar abundance

(Butler and Strazanac 2000, Singer at al. 2012, Newell et a. 2014). Although oaks

typically support abundant caterpillars (Futuyma and Gould 1979, Butler and Strazanac

2000, Singer et al. 2012), caterpillars occurring on oaks are often smaller and weigh less

than those occurring on shorter-lived woody plants (Feeny 1970). Low caterpillar

abundance on red maple maples and low weight of caterpillars on oak have been

attributed to the tannins in these plants (Feeny 1976, Futuyma and Gould 1979) that

deter caterpillars and slow their growth (Feeny 1976, Futuyma 1983), and this likely

explains the low abundance of caterpillars I measured on these species.

While I found low caterpillar abundance on blueberry, Fickenscher et al. (2014)

previously measured high caterpillar abundance on blueberry (V. corymbosum) within

my study area and they ranked this species second only to aspen (Q. tremuloides) in its

value as a caterpillar host plant. Differences in caterpillar abundance on blueberry

growing in my study area between my study and that of Fickenscher et al. (2014) could

simply reflect the natural among-year variability in caterpillar abundance that should

be expected on any shrub species (Feeny 1970, Holmes and Schultz 1988, van Asch and

Visser 2007), or they could indicate my sampling method was less effective at extracting

27

caterpillars from blueberry. Specifically, the foliage on blueberry is arranged at the end

of stiff branches and on rounded shrubs that made it difficult for me to fill the sampling

net completely with foliage, perhaps resulting in a lower volume of blueberry foliage

sampled compared to other shrubs, leading to a downwardly biased estimate of

caterpillar abundance on blueberry. Fickenscher et al.’s (2014) method of beating foliage

onto a sheet may have been more effective at extracting caterpillars from blueberry, but

it precluded them from easily determining caterpillar abundance based on the volume

of foliage sampled.

It is also important to note that estimates of caterpillar abundance on each shrub species

are likely influenced to some unknown degree by the foraging activity of birds at each

site, such that caterpillars collected in net samples represent only those that were not yet

collected by foraging birds. For example, if birds foraged preferentially for caterpillars

on red maple or oak (Narango et al. in press), this could depress caterpillar numbers on

these shrubs (Holmes et al. 1979) and result in my sampling efforts underestimating the

actual number of caterpillars occurring on these native plants. Because birds regularly

avoid foraging in plant substrates where caterpillars are predictably scarce (Eeva et al.

1997, Newel et al. 2014, Narango et al. in press), the low caterpillar abundance I

estimated on glossy buckthorn and autumn olive likely provides an accurate estimate of

the caterpillars produced by these plants. Accounting for the effect that bird foraging

behavior had on my estimates of caterpillar abundance would require identifying the

28

substrates birds use/avoid and from which they capture caterpillars successfully, but I

was able to collect few usable data from observations of birds foraging in these sites

where a large number of shrub species grew densely intermingled (unpublished data).

Implications to shrubland-dependent passerines

Estimates of caterpillar abundance on different plant species provide only rough

estimates of caterpillar availability to birds (Royama 1970, Bell and Ford 1970, Holmes

and Schultz 1988) because both the branching pattern and leaf position on branches of

each plant species determine which plants birds can capture prey from efficiently

(Morse 1976, Holmes and Robinson 1981, Whelan 2001). As a result, foliage-gleaning

birds show strong preferences for the particular tree and shrub species they forage on

based on differences in plant foliage structure (Holmes et al. 1979, Holmes and

Robinson 1981), and the distribution of preferred foraging substrates not only can

influence caterpillar availability to birds (Morse 1976, Holmes and Schultz 1988), but

also is predicted to influence the distribution and abundance of bird species (Robinson

and Holmes 1984, Holmes and Schultz 1988, Gabbe et al. 2002). Daily field observations

of breeding birds present at each site in my study support this prediction, as more than

half of the 15 species of shrubland-dependent passerines that regularly nest in ROW in

the study area were absent from the ALIEN site where alien shrubs comprised ≥ 55% of

the shrub foliage volume and native shrub diversity was lowest (Table 1.3).

Comparatively, all but two of the 15 species were present at the NATIVE site and all were

29

Study site Alde

r fly

catc

her

Amer

ican

gol

dfin

ch

Blac

k-an

d-w

hite

war

bler

Blue

-win

ged

war

bler

Ceda

r wax

win

g

Ches

tnut

-sid

ed w

arbl

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Com

mon

yel

low

thro

at

East

ern

tohw

ee

Fiel

d sp

arro

w

Gra

y ca

tbird

Indi

go b

untin

g

Nor

ther

n ca

rdin

al

Prai

rie w

arbl

er

Song

spar

row

Yello

w w

arbl

er

# bird spp. Size (ha)ALIEN - Y - - Y - Y - - Y Y Y - Y - 7 6.9MIXED Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y 15 13.9NATIVE Y Y Y - Y Y Y Y Y Y Y Y Y Y - 13 6.4

2013 Breeding Bird Composition¹

Table 1.3. The 15 species of shrubland-dependent passerines that regularly breed in transmission line rights-of-way (ROW) in Strafford and Rockingham County, New Hampshire and those that were observed within each ROW study site during the 2013 nestling season.

1 “Y” in column indicates the species was observed within the ROW at least once during regular bird observations collected at each site.

30

present at the MIXED site where native shrub diversity remained high, despite

differences in alien shrub composition between sites (Tables 1.1 and 1.3).

Importantly, because each of the 15 shrubland-dependent bird species has its own

preferred foraging methods (e.g., fly-catching, foliage gleaning, ground gleaning) and

should have preferred substrates from which it can capture prey efficiently (Holmes et

al. 1979, Holmes and Robinson 1981, Apigian and Allen-Diaz 2006), each should be

expected to respond in its own way to low caterpillar abundance in habitats where alien

shrubs reduce native shrub diversity. For example, bird species that primarily glean

prey from shrub foliage (e.g., blue-winged warbler, chestnut-sided warbler, prairie

warbler, yellow warbler) should have the strongest preference for foraging on

particular shrub species and therefore, should be the species most sensitive to the loss of

preferred foraging substrates (Robinson and Holmes 1984, Holmes and Schultz 1988,

Flanders et al. 2006). These species might respond to low caterpillar abundance by

switching their diets to include a greater proportion of non-caterpillar prey items (De

Reed 1982, Cooper et al. 1990, Rodenhouse and Holmes 1992); they might not change

their diet and suffer negative fitness consequences associated with consuming less food

(Sample et al. 1993); or they may simply avoid habitats where preferred foraging

substrates are lacking or absent (Robinson and Holmes 1984, Holmes and Schultz 1988,

Gabbe et al. 2002). In my study, shrubland bird richness at both the MIXED and NATIVE

sites was ≥ 2x that at the ALIEN site (Table 1.3), indicating that most shrubland bird

31

species simply avoided breeding at the ALIEN site. For these species, avoidance may be

the most effective strategy for circumventing poor fitness consequences associated with

settling in habitats composed of a limited number of plant species to which they are

either intolerant or unable to exploit efficiently. These results suggest that near-

monocultures of alien shrubs at the ALIEN site resulted in a loss of breeding habitat for

some shrubland-dependent bird species (e.g., prairie warbler, eastern towhee, field

sparrow); this loss of habitat may be contributing to the population declines observed

for these species.

Other shrubland birds such as common yellowthroats, song sparrows, and gray

catbirds that tolerate a range of plant structure conditions within shrublands (personal

observation) and/or incorporate a large proportion of grasshoppers (yellowthroats and

song sparrows) or fruits (gray catbirds) into nestling diets (Bent 1963, Gleditsch and

Carlo 2014), may be flexible enough in their foraging preferences to rear young

successfully in habitats where glossy buckthorn or autumn olive are the predominant

shrub species and where caterpillars occur in low abundance (sensu Sample et al. 1993,

Eeva et al 1997, Gabbe et al. 2002). Following this expectation, adult yellowthroats at the

ALIEN site did not experience lower productivity compared to those at the other sites,

but they did respond to low caterpillar abundance at the ALIEN site by feeding nestlings

a greater proportion of non-caterpillar prey items than adults at the other sites (Chapter

2).

32

Greater shrubland bird richness at the MIXED site compared to the NATIVE site (Table

1.3) is likely due, in part, to the fact that the MIXED site was large and included micro-

habitats suitable for blue-winged warblers and yellow warblers, habitats that were

absent at the NATIVE site. Flanders et al (2006) observed no difference in the abundance

of shrub-foraging birds between habitats dominated by native grasses and those

invaded with alien grasses; they attributed the similarity in bird communities between

their sites to the fact that alien grasses did not reduce the abundance of native shrubs

supporting prey items for the shrub-foraging birds. My results are analogous to those of

Flanders et al. (2006), because although a simple visual assessment of the MIXED site

would likely result in the observer concluding that the site was “invaded” by glossy

buckthorn, overall, native shrub diversity and abundance at the MIXED site were still

high and not different from those at the NATIVE site where glossy buckthorn accounted

for ≤ 3% of the shrub foliage volume (Table 1.1). As a result, the MIXED and NATIVE sites

supported a nearly identical community of shrubland birds.

A tipping-point of native plant abundance in shrublands

The results of my study support those of others (e.g., Flanders et al. 2006, Hickman et al.

2006, Burghardt et al. 2008, George et al. 2013, Narango et al. in press) and indicate there

is likely a native: alien shrub tipping-point within shrublands that can be identified as

the point at which alien shrubs become abundant enough to reduce native shrub

richness and abundance within a shrubland. Once this tipping point is surpassed, the

33

bird community becomes simplified due to a simplification in habitat structure and/or

lower arthropod abundance associated with the alien shrubs. Whether the change in

habitat structure or the low arthropod abundance is the factor most responsible for

altering the bird community has not been elucidated. However, birds settling in

heavily-invaded habitats are typically exposed to lower arthropod abundance

compared to those settling in habitats composed of a diversity of native plants (e.g.,

Flanders et al. 2006, Burghardt et al. 2008, Gan et al. 2010). In my study, this native:

alien shrub tipping-point was apparently exceeded at the ALIEN site where alien shrubs

accounted for ≥ 55% of the total shrub foliage volume in territories, but not at the MIXED

site where alien shrubs accounted for up to 43% of the total shrub foliage volume and

both caterpillar abundance and shrubland bird richness exceeded those at the NATIVE

site. Unfortunately, identifying a specific native: alien shrub tipping-point that reliably

predicts the relative abundance of caterpillars within all New England shrublands is

probably impossible, due to the variety of shrub species that occur within the region

and my results indicating that high caterpillar abundance on one shrub species can

offset the effects of low caterpillar abundance on others. What seems to be clear, is that

habitats composed of near monocultures of glossy buckthorn and autumn olive support

a predictably low abundance of caterpillars (Fickenscher et al. 2014, this study).

34

Management considerations

Decisions on whether to invest resources for controlling invasive shrubs within bird

habitats are best made on a site-by-site basis and should be based on a thorough

assessment of overall management objectives at each site, as well as, an honest

assessment of both the expected habitat benefit and long-term success of efforts to

reduce alien shrub abundance in each shrubland (Litvaitis et al. 2013). Efforts to reduce

alien shrub abundance should employ a holistic approach that addresses the underlying

mechanisms responsible for each site becoming invaded (Orion 2015), considers the

potential habitat value of all plant species occurring at each site and the consequences of

their removal (Schlaepfer et al. 2011, Litvaitis et al. 2013, Shackelford et al. 2013), and

employs control methods that avoid predisposing the site to further invasion, while

maintaining the capacity of the site to support vigorous native plants capable of

resisting current and future invasion (Hobbs and Huenneke 1992, Clark 2003, Carroll

2011, Lockwood et al. 2013). The results of my study support those of others and

indicate that shrublands and shrubland bird territories composed by high native shrub

richness are those most likely to support a stable abundance of caterpillars, since

differences in caterpillar abundance among host species are most likely to offset one

another where plant richness is high (Laiolo 2002, Marshall and Cooper 2004, Bereczki

et al. 2014), and habitats composed of a variety of native plant species are those most

35

likely to support foraging substrates preferred by a variety of foliage-gleaning

shrubland birds (Holmes et al. 1979, Robinson and Holmes 1984, Gabbe et al. 2002).

The results of my study suggest that removing alien shrubs may not be warranted for

managing bird habitat in shrublands where overall shrub richness is high and native

shrubs account for the majority of the shrub species composition (i.e., foliage volume or

cover); removing alien shrubs from these shrublands may actually reduce foraging

options for birds, particularly if the alien shrubs produce flowers that attract abundant

pollinating insects (Drost et al. 2001, Pendelton et al. 2011), or produce fruit that birds

use for rearing nestlings (Gleditsch and Carlo 2014). Fruits produced by alien shrubs

may also enhance the habitat value of shrublands for adult and juvenile birds during

the post-fledging period of the breeding season (Pagen et al. 2000, Marshall et al. 2003,

McDermott and Wood 2010), as well as the overwinter survival of resident birds

(McCarty et al. 2002, Greenberg and Walter 2010, Gleditsch and Carlo 2011). However,

these potential benefits provided by a limited number of alien shrubs within a

shrubland should be considered carefully with the potential for these shrubs to increase

in abundance and reduce native shrub richness and abundance over time.

Regularly monitoring existing shrublands and maintaining native shrub dominance

within a shrubland by selectively removing individual alien shrubs, may be an effective

strategy for maintaining the habitat value of a shrubland for shrubland birds. By

removing alien shrubs in a manner that both minimizes soil disturbance and maintains

36

as much established native shrub cover as possible, managers can increase the

likelihood that native shrubs will provide competition to reduce alien shrub

regeneration and expand into the areas where alien shrubs have been removed (Hobbs

1989, Clark 2003, Carroll 2011).

37

CHAPTER II

EFFECTS OF ALIEN SHRUBS ON COMMON YELLOWTHROAT PRODUCTIVITY AND NESTLING DIET COMPOSITION, GROWTH, AND PLUMAGE COLOR

Introduction

For most temperate-breeding passerines, annual reproductive success is determined

primarily by the number of large caterpillars (i.e., larval Lepidoptera, Hymenoptera:

Symphyta) that adults deliver to nestlings (Greenberg 1995, Schwagmeyer and Mock

2008, Newell et al. 2014) and plant species composition is the most important habitat

factor determining caterpillar size and availability for birds (Robinson and Holmes

1984, Naef-Daenzer and Keller 1999, Marshall and Cooper 2004). By altering plant

species composition and reducing plant richness, invasion of non-native (alien) plants

can reduce caterpillar size and abundance in bird habitats and simplify the bird

community (Burghardt et al. 2008, Narango et al. in press, Chapter 1). Whether these

cascading effects of alien plants extend further to affect bird productivity has not been

quantified. This information may be critical for conserving bird populations in a variety

of terrestrial and wetland habitats where alien plants are becoming increasingly

ubiquitous (Lockwood et al. 2013).

38

In the northeastern United States, the alien shrubs autumn olive (Eleagnus umbellata)

glossy buckthorn (Frangula alnus), and honeysuckle (Lonicera spp.) have become

common components of many shrub-dominated habitats (shrublands) that are required

breeding habitat for > 30 species of shrubland-dependent passerines, many of which are

species of regional conservation concern due to long-term population declines (Rich et

al. 2004, Schlossberg and King 2007). Compared to most native shrubs, these alien

shrubs typically support a lower abundance of caterpillars due to their unique chemical

defenses that are toxic to and repel caterpillars that have not evolved with them

(Bernays and Graham 1988, Tallamy and Shropshire 2009, Fickenscher et al. 2014). As a

result, shrublands dominated by autumn olive and glossy buckthorn can support lower

caterpillar abundance than those dominated by a diversity of native shrubs

(Fickenscher et al. 2014, Chapter 1). Perhaps due to low caterpillar abundance, loss of

preferred foraging substrates, and/or altered habitat structure, many species of

shrubland-dependent birds seem to avoid breeding in shrublands where alien shrubs

have significantly reduced native shrub abundance and diversity (Chapter 1). Others,

including common yellowthroats (Geothlypis trichas), regularly breed in shrublands

where alien shrubs form near monocultures (McChesney 2012, Chapter 1).

I conducted a study to determine if low caterpillar abundance in shrublands invaded by

alien shrubs affects common yellowthroat (G. t. trichas) reproductive success or nestling

health, or if adult birds offset low caterpillar abundance by feeding nestlings other

39

arthropods [e.g., spiders (Aranae), flies (Diptera), bees (Hymenoptera)] that are often

abundant in habitats dominated by alien plants (Drost et al. 2001, Gibson et al. 2013,

Fickenscher et al. 2014).

Compared to other arthropods, caterpillars are superior food items for nestlings

because they are high in fat, protein, and water, and they are composed of a low

proportion of indigestible chitin (Redford and Dorea 1984, Kaspari and Joern 1993,

Brodmann and Reyer 1999). Large caterpillars are especially preferred as prey because

they allow adult birds to maximize the amount of energy and nutrients they can deliver

efficiently to growing nestlings (Greenberg 1995, Schwagmeyer and Mock 2008, Newell

et al. 2014). For insectivorous birds, caterpillars are also one of the best sources of

carotenoids (Partali et al. 1987, Isaksson and Anderson 2007, Sillanpää et al. 2008), that

may function as antioxidants in birds (Olson and Owens 1998, von Schantz et al. 1999,

Isaksson et al. 2005) and enhance both immunocompetence (Blount et al. 2003, Saino et

al. 2003, Pike et al. 2007) and nestling growth (Eeva et al. 2009). Further, the carotenoid

lutein is also the sole pigment that common yellowthroats use to pigment their yellow

plumage (McGraw et al. 2003). Because there is a tradeoff in how carotenoids are

allocated in the body, the intensity (i.e., saturation and brightness) of a bird’s

carotenoid-based plumage is expected to correlate positively with both the amount of

caterpillars in the diet (Slagsvold and Lifjeld 1985, Eeva et al. 1998, Hõrak et al. 2000)

40

and the general health of the bird (von Schantz et al. 1999, Freeman-Gallant et al. 2011,

Henschen et al. 2016).

In habitats where caterpillar abundance has been reduced by insecticides or pollution,

breeding passerines can experience food limitation leading to reduced nestling growth

rates (Gibb and Betts 1963, Goodbred and Holmes 1996, Naef-Daenzer et al. 2000),

smaller size of nestlings at fledging (Van Noordwijk et al. 1995, Eeva et al. 1998, Naef-

Daenzer and Keller 1999), and lower adult productivity due to reduced nestling

survival (Pascual and Peris 1992, Eeva et al. 2009, Sillanpää et al. 2009) or because adults

make fewer nesting attempts (Rodenhouse and Holmes 1992). In some instances, adult

birds can maintain a large proportion of caterpillars in nestling diets even if caterpillar

abundance is low, by increasing their territory size (Morse et al. 1976, Seki and Takano

1998, Marshall and Cooper 2004), increasing the amount of time they spend foraging

(Seki and Takano 1998, Apigian and Allen-Diaz 2006, Isaksson and Andersson 2007) or

by concentrating their foraging effort on substrates where caterpillars remain abundant

(Smith and Dawkins 1971, Smith and Sweatman 1974, Eeva et al. 1997). If the scarcity of

large caterpillars limits the ability of adults to provide nestlings with adequate food,

adults may respond by feeding nestlings a greater proportion of non-caterpillar prey

items (Cooper et al. 1990, Rodenhouse and Holmes 1992, Eeva et al. 2005) and

experience no reduction in productivity (Marshall et al. 2002). However, nestlings fed

diets depauperate of caterpillars may still experience reduced growth, lower survival

41

after fledging (Perrins 1975, Hõrak et al. 2000, Eeva et al. 2009), and suffer the effects of

reduced carotenoid intake, including lower antioxidant defense (reviewed in Edge et al.

1997), reduced immune response (Blount et al. 2003, Freeman-Gallant 2011, Henschen et

al. 2016), and duller carotenoid-based plumage (Slagsvold and Lifjeld 1985, Eeva et al.

1998, Hõrak et al. 2000).

This study is the first to quantify how reduced caterpillar abundance associated with

alien plan invasion affects the reproductive success of a breeding passerine. The study

was conducted in three shrublands where plant composition in bird territories spanned

a gradient from a near monoculture of alien shrubs to territories composed only of

native shrubs. Common yellowthroats were selected as the focal species for this study

because they were the most abundant shrubland-dependent passerine at all study sites

and they were the only one to occur in abundance at my study site where alien shrubs

formed near monocultures. Yellowthroats are territorial and socially monogamous

during the breeding season; both parents feed nestlings a variety of arthropods,

including caterpillars and grasshoppers (Hofslund 1959, Guzy and Ritchison 1999), and

they forage primarily within their territory while feeding nestlings (personal

observation). Nestling yellowthroats fledge 8-9 days after hatching and most (> 75%)

adult pairs produce a single brood each year, but some complete two successful broods

(Peterson et al. 2001, Whittingham and Dunn 2005, Garvin et al. 2006).

42

After quantifying differences in caterpillar abundance in common yellowthroat

territories based on their composition of native and alien shrubs (Chapter 1), my first

objective was to determine if differences in shrub composition in territories resulted in

differences in yellowthroat territory size or in the types of prey that adults delivered to

nestlings. I expected territory size would increase as the proportion of alien shrubs

comprising the territory increased, and that nestling diets in territories dominated by

alien shrubs would be composed by a lower proportion of caterpillars and a greater

proportion of non-caterpillar arthropods than those of nestlings in territories dominated

by native shrubs. I then quantified if differences in the native/alien shrub composition

in territories were associated with differences in nestling growth or size at fledgling,

nestling plasma carotenoid concentrations, and nestling carotenoid-based plumage

color. I suspected that all variables would be lower in territories dominated by alien

shrubs than in those composed primarily by native shrubs

Methods

Study system

This study was conducted May-August 2013 on three shrub-dominated transmission

line rights-of-way (ROW) located in Rockingham and Strafford Counties, New

Hampshire. Because site types could not be replicated due to the intensive nest

monitoring effort required for this study, three sites were selected that were as similar

as possible to one another (e.g., in their growing conditions and surrounding landscape)

43

except for their level of invasion by autumn olive and glossy buckthorn. A complete

description of study sites and the vegetation composition in common yellowthroat

territories at each site is provided in Chapter 1. Territories at all sites were composed of

> 50% young trees and shrub cover < 3.5 m tall (hereafter “shrubs”), with the remainder

composed predominately of ferns, forbs and grasses. The alien shrub composition

(percent total shrub foliage volume, TSFV) in territories differed (F2, 33 = 105.0, P < 0.001)

among sites and was lowest in territories at the 6.4 ha “NATIVE” site (0.3 – 7.6% TSFV),

intermediate at the 13.9 ha “MIXED” site (5.7-43.3% TSFV), and greatest at the 6.9 ha

“ALIEN” site (55.3 – 85.0% TSFV). Shrub diversity and native shrub abundance were

nearly identical in territories between the NATIVE and MIXED sites, and greater than

those at the ALIEN site where autumn olive and glossy buckthorn formed near-

monocultures (Chapter 1). Of the 13 most abundant shrub species comprising

yellowthroat territories, the lowest caterpillar biomass (mg dry matter/m3 foliage)

occurred on autumn olive and glossy buckthorn, and on native red maple (Acer rubrum)

and northern arrowwood (Viburnum dentatum); the greatest caterpillar biomass occurred

on the native shrubs birch (Betula spp.), speckled alder (Alnus incana spp. rugosa),

willow (Salix spp.) meadowsweet (Spiraea latifolia), and sweetfern (Comptonia peregrina;

Fig 1.3, Chapter 1). Differences in the relative abundance of the specific native and alien

shrub species comprising territories resulted in territories at the MIXED site supporting

the greatest total estimated caterpillar abundance, and those at the ALIEN site

44

supporting the lowest that was 62% and 75% lower than that at the NATIVE and MIXED

sites, respectively. (Table 1.1, Chapter 1).

Nestling growth and annual productivity

At each site, all male common yellowthroats were marked with a unique combination

of three colored leg bands and their territory boundaries were mapped by generating

minimum convex polygons from ≥ 30 locations collected from each bird (Chapter 1).

Territory size was estimated to the nearest m2. The yellowthroat nest within each

territory was found by searching vegetation, following females carrying nesting

material, inadvertently flushing females off nests, or by following adults carrying food

to nestlings (Martin and Geupel 1993). Nests were visited every 2-3 days until the

expected hatching date, then daily until eggs hatched. Nestlings were weighed with a

portable electronic balance (± 0.005 g) and their tarsus length was measured every 1-2

days between 0630 and 1030 hr. when nestlings were 2-7 days old. Each nestling was

marked by clipping the tip of a different claw and growth rates were estimated for

every nestling with ≥ 3 days of measurements. Growth rates based on weight and tarsus

were estimated using a logistic growth model (Ricklefs 1967) with a fixed asymptote

(Austin et al. 2011) set at the mean body weight (10.5 g) and tarsus length (19.8 mm) of

214 adult common yellowthroats (161 males, 53 females) captured at study sites

between 2012 and 2013 (unpublished data). Growth rates (K) of each nestling were

estimated by first recalculating each data point as a percentage of adult body weight or

45

tarsus length (A) (Austin et al. 2011). Following Ricklefs (1967), each data point was

again recalculated using the conversion factor 0.25 · ln (w/1 –w) where w = %A; each

resulting point was plotted against age in days and growth rate was estimated directly

from the slope of the line (K = 4 · slope). Growth rates were averaged for each nest to

estimate two variables of nestling growth: NESTGROWTHRATE_WEIGHT (g/d) and

NESTGROWTHRATE_TARSUS (mm/d). Only 1st broods were used to estimate nestling

growth rates and I excluded nests parasitized by brown-headed cowbirds (Molothrus

ater) and those partially depredated if the predation event occurred before three

measurements were obtained from all nestlings. Nests were considered partially

depredated if individual nestlings disappeared without first demonstrating slow

growth relative to siblings, based on nest videos documenting garter snakes

(Thamnophis sirtalis) and eastern chipmunks (Tamias striatus) removing individual

nestlings from broods (unpublished data).

Nestlings were last measured when they were 7-days old to minimize the chance of

force-fledging older nestlings, and three variables of nestling condition were estimated

just prior to fledging. Specifically, tarsus length of 7 day-old nestlings was averaged for

each nest to estimate tarsus length at fledging (MEANTARSUS). Many nestlings from

broods at all sites lost weight between 6 to 7-days old, so the average maximum weight

attained by nestlings in each nest (MAXMEANWEIGHT) was used to estimate fledging

weight for each nest. Following Isaksson et al. (2005), the body condition of each

46

nestling at the time of fledging was estimated as: ln maximum weight/ 3 · ln day 7

tarsus, and averaged for each nest (FLEDGINGCONDITION).

The annual productivity of each territory was estimated as the total number of

fledglings produced from all successful nests within the territory. Nests producing ≥ 1

fledgling were considered successful and the number of 7 day old nestlings in each nest

was used to estimate the number of fledglings, following confirmation of banded adults

feeding fledglings.

Nestling plasma carotenoids

Brachial venipuncture was used to collect 15-35 µl of blood from each 7 day old

nestling, using a microcapillary tube. All samples were collected in the morning to

avoid temporal variation in plasma carotenoid levels (Eeva et al. 2008). Blood samples

were preserved on ice in the field, centrifuged the same day at 4000 rpm for 5 min to

separate plasma from red blood cells (Sillanpää et al. 2009), and plasma was frozen at

-80 o C. Plasma carotenoid concentration of samples was analyzed using a Hewlett

Packard/Agilent Technologies 1100 series High Performance Liquid Chromatography

(HPLC) systems with photodiode array detector (Agilent Technologies, Palo Alto, CA).

Carotenoids were detected at 452 nm and identified by comparing retention times and

spectral analysis with those of pure standards (>95%). Concentrations of compounds

were calculated using an external standard curve and adjusted by percentage recovery

of the added internal standard. Lutein, zeaxanthin, and β-carotene were the most

47

abundant carotenoids in plasma samples, followed by lower concentrations of α-

carotene, retinol, and β-cryptoxanthin. Therefore, four plasma carotenoid variables

were estimated for each nest, based on mean plasma concentrations (ng/ µl) of lutein

(MEANLUTEIN), zeaxanthin (MEANZEAXANTHIN), β-carotene (MEANβ-CAROTENE), and all

carotenoids (ALLCAROTENOIDS) that included lutein, zeaxanthin, β-carotene, α-carotene,

retinol, and β-cryptoxanthin.

Nestling plumage color

Ten feathers were collected from the center breast of each 7 day old nestling to

determine nestling plumage color as an indicator of nestling carotenoid intake and

health. Feathers from each nestling were stored in the dark (McGraw et al. 2003) and

then taped on a black index card to imitate the plumage surface (Quesada and Senar

2006). Index cards with feathers were scanned with a digital scanner and the images

were imported into Adobe ® PhotoshopC26 (www.adobe.com) using the same scanner

and computer for all samples. Following Jones et al. (2010), the color-picker tool (3 x 3

pixel average eye dropper sample) in Photoshop was used to quantify the average hue,

saturation, and brightness (HSB) from five random points within the largest area of

feather overlap on each sample. Hue is the actual tone of the color, with 60° being the

yellowest on a 0-360° color spectrum; saturation describes the purity of the color on a

scale of 0-100% with 100% representing pure color; brightness describes the amount of

black (0%) or white (100%) in the color. Following Hill et al. (1994), a HSB score was

48

calculated for each nestling by summing its HSB values; nestling plumage with the

purest and brightest yellow has an HSB score = 260 (hue = 60, saturation = 100, and

brightness = 100). Four nestling plumage variables were calculated based on averages

for each nest: hue, saturation, brightness, and HSB score.

Adult provisioning and nestling diet composition

Immediately after measuring 7-day old nestlings, a digital video camera (Sony

Handycam DCR-SR68) mounted on a flexible tripod was placed 1 m from the nest to

determine adult provisioning patterns and nestling diet composition. Cameras recorded

continuous video for the life of the camera battery (≥ 4 hr.) and the first hour of each

video was discarded to allow adults to acclimate to the camera. Three adult

provisioning variables were estimated from the remaining video at each nest:

provisioning rate (number of feeding visits/hr./nestling), duration of time between each

food delivery (TIME AWAY), and length of time spent at the nest during each food

delivery (TIME AT NEST), based on the individual feeding patterns of each parent. The

number of each of the following prey types delivered to nestlings were also tallied: ants

(Hymenoptera), bees/flies (Hymenoptera/Diptera), butterflies/moths (Lepidoptera),

caterpillars, grasshoppers (Orthoptera), odonates (dragonflies/damselflies (Odonata)],

Hemiptera (true bugs), spiders (Araneae, Opiliones), and unknown. The length of each

prey item was estimated relative to the average length of the exposed culmen of adult

yellowthroats captured at study sites (unpublished data). Following Adler and

49

Ritchison (2011), prey items were classified into one of six size classes and recorded as

the midpoint of each size class. The total weight of caterpillars delivered to each nest

(WEIGHTCATERPILLARS_DELIVERED, mg dry matter/nestling/hr.) was estimated using a

mass-length regression developed from caterpillars collected from shrubs at study sites

(Appendix C). The total biomass of all prey items delivered to each nest was estimated

using an index modified from Adler and Ritchison (2011), where prey biomass =

number of adult food deliveries/hr. · size of prey items · total number of prey items

delivered. The proportion of each prey type in nestling diets was estimated as a

proportion of total estimated prey biomass delivered.

Data analysis

All statistical analyses were performed using JMP Pro (Ver. 12, SAS, www.JMP.com)

with a P < 0.05 level of significance.

Adult provisioning and nestling diet composition

To determine if territory size, adult provisioning variables,

WEIGHTCATERPILLARS_DELIVERED, or the proportion of each prey type in nestling diets

differed among study sites, ANOVA and Tukey’s HSD were used to compare average

among sites. Pearson’s r was used to examine correlations between total caterpillar

abundance in territories with each of the following variables: territory size, adult

provisioning variables, WEIGHTCATERPILLARS_DELIVERED and proportion of each prey

50

type in nestling diets. Pearson’s r was used to examine correlations between the date

videos were recorded and all variables except territory size.

Nestling growth, condition, plasma carotenoids and plumage color

To determine if annual productivity, nestling growth, nestling condition, nestling

plasma carotenoids, and nestling plumage variables differed among sites, ANOVA and

Tukey’s HSD were used to compare average values among sites. Spearman’s rank

correlation was used to examine correlations between annual productivity and total

caterpillar abundance, WEIGHTCATERPILLARS_DELIVERED, and proportion of each prey

type delivered to nestlings. Pearson’s r was used to examine correlations between each

nestling variable and total caterpillar abundance, WEIGHTCATERPILLARS_DELIVERED, the

proportion of each prey type delivered to nestlings, hatching date, and mean

precipitation and temperature during the nestling period. Precipitation and

temperature data were obtained from the NOAA weather station located central to all

study sites, in Durham, NH. Using the entire dataset of nestlings, Pearson’s r was used

to examine correlations between each individual nestling’s plasma carotenoid values,

and its growth rates, maximum weight, tarsus length when 7 days old, and plumage

color variables.

51

Results

Adult provisioning and nestling diet composition

Average territory size was similar between the NATIVE and MIXED sites and ≥ 30% larger

than that at the ALIEN site, but these differences among sites were not significant (Table

2.1).

Video cameras were placed at 26 nests, but videos from seven nests were excluded from

analyses because adults never acclimated to the camera, vegetation fell and completely

obscured the camera, or nestlings either fledged or were depredated during the

acclimation period. An average of 4.7 hours/nest of post-acclimation video yielding

complete or partial data was collected from the remaining 19 nests (ALIEN n = 2, MIXED n

= 11, NATIVE n = 6). One additional nest was excluded only from calculations of TIME

AWAY and TIME AT NEST because the camera became partially obstructed by vegetation,

making it impossible to distinguish individual parents. Three of the 19 nests were

excluded only from estimates of nestling diet composition because adults fed nestlings

with their back to the camera, precluding the identification of prey items delivered.

There were no differences in adult provisioning variables among sites (P > 0.05).

However, at the ALIEN site, the provisioning rate was slightly greater and the average

TIME AWAY was > 4 min shorter than that at the other sites (Table 2.1, Fig. 2.1A & 2.1B).

Table 2.1. Mean [± SE (range)] territory size, adult provisioning variables, and nestling diet composition measured in common yellowthroat territories among three transmission line rights-of-way in Rockingham and Strafford County, New Hampshire, 2013.

1 n = Number of territories at site 2 Percent of total estimated prey biomass (see text). Difference % Caterpillar and Grasshoppers and % All Alternative Prey represents the proportion of nestling diets categorized as “unknown” prey items.

Figure 2.1. Comparing average adult common yellowthroat provisioning variables and the proportion of nestling yellowthroat diets (percent of total estimated prey biomass, see text) comprised by caterpillars estimated at the ALIEN, MIXED, and NATIVE study sites. Rockingham and Strafford County, New Hampshire, 2013.

A. B.

C. D

54

The TIME AT NEST was slightly greater at the MIXED site than at the other sites (Table 2.1,

Fig 2.1C).

Caterpillars and grasshoppers accounted for the greatest proportion of prey items in

nestling diets at all sites (Table 2.1). The average proportion of caterpillars in nestling

diets at the NATIVE site (48.0 %, SE = 3.8) and MIXED site (47.2 %, SE = 2.9) was similar

and 10% greater than that at the ALIEN site (37.5%, SE = 6.1, Fig. 2.1D). The proportion of

grasshoppers in nestling diets did not differ among sites but tended to be lower at the

ALIEN site. Combined, the proportion of caterpillars and grasshoppers in nestling diets

was similar between the NATIVE site (84.0 %, SE = 2.4) and MIXED site (85.8 %, SE = 1.8)

and greater (F2, 13 = 8.45, P = 0.005) than that at the ALIEN site (68.5 %, SE = 3.8, Table 2.1,

Fig. 2.2). The average WEIGHTCATERPILLARS_DELIVERED did not differ among sites (Table

2.1), but tended to be greater at the MIXED site where adults fed caterpillars averaging

slightly longer (14.8 mm, SE = 0.7) than those fed by adults at either the NATIVE site (13.4

mm, SE = 1.0) or the ALIEN site (13.3 mm, SE = 1.6). All caterpillars fed to nestlings were

smooth bodied (i.e., without hairs or spines) and they appeared to be either

Hymenoptera larvae or Lepidoptera in the families Geometridae, Lycaenidae,

Notodontidae, or Sphingidae, but I was unable to estimate accurately the proportion of

different caterpillar types fed to nestlings because adults frequently delivered a bolus of

multiple prey and moved too quickly to capture non-blurry still images from videos.

Overall, adults at the ALIEN site delivered slightly fewer prey items/visit (1.3, SE = 0.2),

Figure 2.2. The proportion of the most common prey items composing diets of nestling common yellowthroats raised in three transmission line rights-of-way in Rockingham and Strafford County, New Hampshire, 2013. Prey composition is percent total estimated prey biomass delivered to nestlings (see text). Bars with different letters differ significantly (P < 0.05).

56

than adults at the MIXED site (1.7, SE = 0.1) or NATIVE site (1.8, SE = 0.1), but the

differences were not significant (F2, 13 = 2.48, P = 0.12).

The proportion of alternative prey types (i.e., prey other than caterpillars or

grasshoppers) in nestling diets was greater (F2, 13 = 10.6, P = 0.002) at the ALIEN site (22%,

SE = 3.2) than at either the MIXED site (6.3%, SE = 1.5%) or the NATIVE site (11.8%, SE =

2.0, Table 2.1, Fig. 2.2). The proportion of bees/flies, butterflies/moths, odonates, and

spiders were all greatest at the ALIEN site where the proportion of spiders and the

proportion of odonates differed only from those at only the MIXED site (F2, 13 = 5.22, P =

.022 and F2, 13 = 4.59, P = 0.03, respectively).

Total caterpillar abundance estimated in territories was correlated only with the

proportion of odonates (r = -0.65, R2 = 0.42, P = 0.01) and with the proportion of all

alternative prey (r = -0.54, R2 = 0.29, P = 0.03) in nestling diets.

Annual productivity, nestling growth, and nestling condition

Common yellowthroats in a total of 35 territories completed one successful nest.

Specifically, six (100%) territories at the ALIEN site, 19 (73.1%) territories at the MIXED

site, and 10 (83.3%) territories at the NATIVE site completed successful nests. Only one

(2.9%) territory (at the ALIEN site) produced two successful nests. There was no

difference in the likelihood of a territory producing a successful nest among sites (P >

0.05, Fischer’s Exact test each pair) and there was no difference in the average annual

57

productivity among sites (Table 2.2). All nestling growth and nestling condition

variables were greatest at the NATIVE site, but only NESTGROWTHRATE_TARSUS differed

significantly among sites (Table 2.2, Fig. 2.3); it was lowest at the MIXED site (0.56 mm,

SE = 0.06), intermediate at the ALIEN site (0.74 mm, SE = 0.1), greatest at the NATIVE site

(0.82 mm, 0.07), and differed only between the NATIVE and MIXED sites (P = 0.02).

Average MAXMEANWEIGHT, MEANTARSUS, and FLEDGINGCONDITION followed a similar

pattern with lowest values at the MIXED site, intermediate values at the ALIEN site, and

greatest values at the NATIVE site (Table 2.1, Fig 2.3).

Annual productivity was correlated negatively with the combined proportion of

caterpillars and grasshoppers in nestling diets (ρ = -0.63, P = 0.01).

NESTGROWTHRATE_WEIGHT was correlated positively with the proportion of caterpillars

in nestling diets (r = 0.6, R2 = 0.36, P = 0.04). MEANTARSUS was correlated positively with

the proportion of spiders in nestling diets (r = 0.56, R2 = 0.31, P = 0.03).

Nestling plasma carotenoids and carotenoid-based plumage color

Plasma carotenoid concentrations were analyzed for 85 nestlings occurring in 30

territories (Table 2.3). All plasma carotenoid variables were greatest at the NATIVE site

and there were two trends in the values among sites. Specifically, MEANβ-CAROTENE at

the MIXED site was 28% and 19% lower than that at the NATIVE and ALIEN site,

respectively, and the difference between the NATIVE and MIXED site was significant (F2, 83

= 4.13, P = 0.02, Fig. 2.4). All other plasma carotenoid variables were lowest at the ALIEN

Table 2.2. Comparing the average [± SE (range)] annual productivity, nestling growth, and nestling condition estimated in common yellowthroat territories in three transmission line rights-of-way (ALIEN, MIXED, NATIVE) in Rockingham and Strafford County, New Hampshire, 2013.

1 n = Number of territories at site 2 Columns with different letters are significantly different

Site

ALIEN MIXED NATIVE Annual Productivity n¹ n n

Total fledglings 3.2 ± 0.4 (2 to 5.6) 6 3.5 ± 0.2

(2 to 4) 19 3.3 ± 0.3 (2 to 4) 9

Nestling Growth

NESTGROWTHRATE_WEIGHT (g/d) 0.40 ± 0.05 (0.16 to 0.55) 4 0.42 ± 0.03

(0.26 to 0.56) 12 0.46 ± 0.4 (0.29 to 0.58) 7

NESTGROWTHRATE_TARSUS (mm/d)² 0.74 ± 0.1 (0.4 to 0.94) ab 4 0.56 ± 0.06

(0.4 to 0.81) b 11 0.82 ± 0.07 (0.51 to 1.32) a 7

Nestling Condition

MAXMEANWEIGHT (g) 8.52 ± 0.26 (8.36 to 8.61) 4 8.34 ± 0.15

(6.83 to 9.37) 13 8.75 ± 0.19 (8.04 to 9.65) 8

MEANTARSUS (mm) 18.9 ± 0.5 (17.8 to 19.4) 4 18.4 ± 0.2

(16.8 to 20.1) 15 19.2 ± 0.3 (17.6 to 20.5) 9

FLEDGINGCONDITION³ 0.243 ± 0.004 (0.242 to 0.246) 4 0.242 ± 0.002

(0.225 to 0.256) 15 0.245 ± 0.003 (0.234 to 0.256) 8

Figure 2.3. Comparing average growth and size variables of nestling common yellowthroats raised in territories located at the ALIEN, MIXED, and NATIVE study sites. Rockingham and Strafford County, New Hampshire 2013. Within each sub-figure, bars with different letters differ significantly (P < 0.05).

C.

A. B.

D.

b

ab a

Table 2.3. Comparing mean [± SE (range)] nestling common yellowthroat plasma carotenoid and carotenoid-based plumage variables estimated within common yellowthroat territories among three transmission line rights-of-way (ALIEN, MIXED, NATIVE) in Rockingham and Strafford County, New Hampshire, 2013.

Site¹

ALIEN MIXED NATIVE Plasma Carotenoids n² n

MEANZEAXANTHIN (ng/μl) 3.3 ± 0.5 (1.6 to 6.2) b 6 4.2 ± 0.3 (1.1 to 9.2) b 15 6.0 ± 0.3 (2.0 to 9.1) a MEANβ-CAROTENE (ng/μl) 0.72 ± 0.1 (0.21 to 1.14) ab 6 0.58 ± 0.1 (0.0 to 1.96) b 15 0.81 ± 0.06 (0.19 to 2.02) a ALLCAROTENOIDS (ng/μl) 41.3 ± 3.9 (223.0 to 71.0) b 6 47.0 ± 2.1 (18.3 to 75.6) ab 15 53.4 ± 2.6 (18.0 to 78.5) a

Carotenoid-based Plumage

HUE (degrees) 45.9 ± 1.0 (38.4 to 51.2) 5 43.8 ± 0.5 (31.6 to 50.6) 17 44.7 ± 0.6 (40.6 to 49.0) SATURATION (%) 46.9 ± 2.4 (28.2 to 54.8) ab 5 44.1 ± 0.8 (24.2 to 53.4) b 17 48.7 ± 1.1 (35.6 to 54.2) a BRIGHTNESS (%) 77.9 ± 1.8 (73.0 to 83.0) a 5 72.2 ± 0.8 (54.2 to 82.4) b 17 77.8 ± 1.1 (69.6 to 81.6) a

HSB Score (Max = 260) 170.6 ± 3.5 (153.2 to 180.0) a 5 160.1 ± 1.6 (114.4 to 179.6) b 17 171.2 ± 2.1 (155.4 to 179.0) a

1 columns with different letters differ significantly (P < 0.05) 2 n = number of territories at site

C.

A. B.

D.

ab

b

a a

b

a

a

a

b

Figure 2.4. Comparing average plasma carotenoid concentrations and HSB (Hue, Saturation, Brightness) plumage scores of nestling common yellowthroats raised in territories located at the ALIEN, MIXED, and NATIVE study sites. Rockingham and Strafford, County New Hampshire, 2013. Within each sub-figure, bars with different letters differ significantly (P < 0.05).

62

site. MEANZEAXANTHIN at the NATIVE site was greater than that at the other sites (F2, 83 =

11.3, P < 0.0001, Fig 2.4). ALLCAROTENOIDS at the NATIVE site was also greater than that

at the ALIEN site (F2, 82 = 3.8, P = 0.03).

Plumage color was analyzed for 94 nestlings from 31 territories (Table 2.3). All

carotenoid-based plumage variables were lowest at the MIXED site. Average hue was

greatest at the ALIEN site but did not differ among sites. Average saturation was greatest

at the NATIVE site and differed from that at the MIXED site (F2, 91 = 5.9, P = 0.004). Average

brightness and HSB score were similar between the NATIVE and ALIEN sites and differed

from those at the MIXED site (F2, 91 = 10.26, P < 0.0001 and F2, 91 = 10.26. P < 0.0001,

respectively, Fig. 2.4D).

Estimated caterpillar abundance in territories (Chapter 1) was negatively correlated

with plumage saturation (r = -0.39, R2 = 0.15, P = 0.04). Plumage saturation was also

positively correlated with the proportion of caterpillars delivered that were ≥ 18 mm

long (r = 0.53, R2 = 0.28, P = 0.04). Nestling plasma carotenoids were not correlated with

any nestling diet components and all correlations with nestling growth, condition, or

plumage color variables were weak (i.e., r2 ≤ 0.10, Table 2.4). Specifically, plasma lutein

was correlated positively with nestling tarsus length and plumage hue; plasma

zeaxanthin was correlated positively with plumage saturation and HSB score; plasma β-

carotene was correlated positively with nestling tarsus growth rate, tarsus length,

plumage saturation, plumage brightness, and HSB score; plasma all-carotenoids was

r r² P r r² P r r² P r r² PNestling growth & condition

tarsus growth rate - - - - - - 0.28 0.08 0.03 - - -tarus length 0.29 0.08 0.01 - - - 0.26 0.07 0.02 0.28 0.08 0.01

Nestling plumage colorhue 0.27 0.07 0.01 - - - - - - 0.26 0.07 0.02

saturation - - - 0.23 0.05 0.04 0.31 0.1 0.004 - - -brightness - - - - - - 0.23 0.05 0.03 - - -HSB score - - - 0.25 0.06 0.02 0.31 0.1 0.03 0.24 0.06 0.03

Nestling plasma carotenoidslutein β-carotene all-carotenoidszeaxanthin

Table 2.4. Pairwise correlations (Peterson’s r) between plasma carotenoids and growth, condition, and plumage color variables of 85 common yellowthroat nestlings raised in three transmission line rights-of-way in Rockingham and Strafford Co. New Hampshire 2013.

64

correlated positively with nestling tarsus length, plumage hue, and HSB score (Table

2.4).

Discussion

Nestling diet composition and adult provisioning

Similarities and differences in nestling diet composition among study sites (Table 2.1,

Fig. 2.2) indicated that alien shrubs did not reduce caterpillar availability to birds until

these shrubs were abundant enough to reduce the diversity and abundance of native

shrubs in bird territories (Chapter 1). Where this occurred (at the ALIEN site), adult

yellowthroats altered their provisioning behavior and incorporated a greater proportion

of non-caterpillar prey items in nestling diets. Specifically, at the MIXED and NATIVE sites

where the alien shrub composition ranged from 0 - 43% of the total shrub foliage

volume in territories (Table 1.1, Chapter 1), adult provisioning rate, TIME AWAY, and the

proportion of caterpillars in nestling diets were similar (Table 2.1, Fig 2.1; Table 1.1

Chapter 1). Comparatively, at the ALIEN site, where alien shrub composition in

territories ranged from 80 - 85% of the total shrub foliage volume (Table 1.1 Chapter 1),

adults fed nestlings fewer caterpillars and a greater proportion of alternative prey

items, they made 15% more provisioning trips, and their average time away from the

nest between provisioning visits was > 4 min shorter (Table 2.1, Fig 2.1).

65

The actual abundance of alien shrubs that limits caterpillar availability to birds should

be expected to vary depending on the relative abundance of the specific native and alien

shrub species comprising each territory. Specifically, low caterpillar abundance on

glossy buckthorn and autumn olive can be offset by greater caterpillar abundance on

certain native shrubs (e.g., birches, Chapter 1), particularly if birds maximize their

foraging efficiency by avoiding plant substrates that support predictably low caterpillar

abundance and forage primarily on high-profit substrates supporting an abundance of

large caterpillars (e.g., Smith and Sweatman 1974, Singer et al. 2012, Newell et al. 2014).

My study suggests that bird foraging efficiency may not be reduced by a moderate level

of alien shrub invasion (e.g., at the MIXED site), but in habitats where unprofitable alien

shrubs reduce the diversity and abundance of profitable native shrubs (e.g., at the

ALIEN site), bird options for avoiding alien shrubs can be limited and low caterpillar

availability can cause adults to feed nestlings a greater proportion of non-caterpillar

prey (sensu Rodenhouse and Holmes 1992, Sample et al. 1993, Eeva et al. 2005).

Compared to adults at the MIXED and NATIVE sites, adults at the ALIEN site fed nestlings

more frequently and they spent less time away from the nest during each foraging trip

(Fig. 2.1). Frequent provisioning visits by adults can occur when large caterpillars are

abundant (Isaksson and Anderson 2007), but are more indicative of adults delivering

many small alternative prey items to meet nestling food demands when caterpillar

abundance is low (Smith and Sweatman 1974, Schwagmeyer and Mock 2008, Naef-

66

Daenzer et al. 2000). Provisioning rate should be related inversely to the average weight

of each meal, with adults delivering either frequent light meals, or less frequent heavy

meals (Gibb and Betts 1963, Nour et al. 1998). This expectation was confirmed by

camera data that indicated adults at the ALIEN site delivered ≥ 29% fewer prey items per

provisioning visit than adults at the other sites. Therefore, adults at the ALIEN site made

more trips with fewer prey items than adults at the other sites that spent more time

collecting more caterpillars that they delivered in fewer trips.

Longer TIME AWAY at the NATIVE and MIXED sites could indicate that adults increased

their search effort to find large caterpillars that occurred in low abundance (Naef-

Daenzer and Keller 1999, Apigian and Allen-Diaz 2006). Typical of adults provisioning

nestlings (Brodmann and Reyer 1999, Schwagmeyer and Mock 2008), adults at all sites

delivered caterpillars that were slightly longer than the average length of those

occurring on shrubs at each study site (unpublished data). However, lower

provisioning rates combined with high caterpillar delivery rates at the NATIVE and

MIXED sites indicate that large caterpillars were relatively available at these sites, and

the greater TIME AWAY here may indicate that adults were engaged with other activities

(e.g., preening, resting, foraging for themselves, territory defense) while foraging for

nestlings. Compared to the NATIVE and ALIEN sites, adult TIME AT NEST was greatest at

the MIXED site (Fig 2.1) and this was most likely because adults there delivered slightly

67

larger caterpillars that required longer handling time by nestlings and, therefore, longer

tending by adults (Stewart 1953, Royama 1970, personal observation).

Productivity, nestling growth, plasma carotenoids, and plumage color

Differences in caterpillar abundance among sites seemed to have little or no effect on

common yellowthroat productivity or on nestling health during my study. Specifically,

the lower proportion of caterpillars in nestling diets at the ALIEN site (Table 2.1) resulted

in little or no reductions in annual productivity or in nestling growth, condition, plasma

carotenoids, or carotenoid-based plumage color (Tables 2.2 and 2.3). Instead, nestling

growth rate based on tarsus, nestling maximum weight, tarsus length of 7-day old

nestlings, FLEDGINGCONDITION, and all nestling plumage variables were all lowest at the

MIXED site (Figs 2.3 and 2.4) where estimated caterpillar abundance in territories

(Chapter 1) and the total weight of caterpillars in nestling diets were the greatest. It is

not entirely clear why these values were lowest at the MIXED site, but they may be

related to the low plasma β-carotene levels of nestlings here. Specifically, plasma β-

carotene was correlated positively with tarsus growth rate and tarsus length of 7-day

old nestlings, and its values mirrored the pattern of those and nestling maximum

weight among sites (Tables 2.2 and 2.3). Plasma β-carotene was also correlated

positively with plumage saturation, brightness, and HSB score, all of which were lowest

at the MIXED site. In birds, β-carotene functions in immune response (reviewed by von

Schantz et al. 1999) and as a precursor to vitamin A (Surai et al. 2001, Blount 2004)

68

essential for promoting growth (Stahl and Sies 2005). Therefore, the low plasma β-

carotene of nestlings at the MIXED site may be responsible for their slower tarsus growth

and smaller 7-day old size, compared to nestlings at the other sites where plasma β-

carotene was greater.

Regarding nestling plumage color, lutein is the sole carotenoid used by common

yellowthroats to color their yellow plumage (McGraw et al. 2003). Previous studies

have considered caterpillars to be the best source of lutein for the birds being studied

(e.g., Slagsvold and Lifjeld 1985, Hõrak et al. 2000, Sillanpää et al. 2008), but for

common yellowthroat nestlings, grasshoppers should also be a good source of lutein

(Olson 2006). Because caterpillars and grasshoppers accounted for

≥ 67% of nestling diets (Table 2.1), nestlings at all sites were fed diets comprised

primarily of lutein-rich food. Although plasma lutein levels were lowest at the ALIEN

site where estimated caterpillar abundance was lowest both in territories (Table 1.1,

Chapter 1) and in nestling diets (Table 2.1), the high plumage color values of nestlings

at the ALIEN site indicate that nestlings at all sites had access to an absolute amount of

lutein adequate for coloring their feathers similar to those at the NATIVE site. This result

suggests that the low plumage values at the MIXED site were not the result of inadequate

lutein in nestling diets, but rather, that nestlings at the MIXED site were allocating less

lutein into their feathers than nestlings at the other sites.

69

Reduced plumage color values in common yellowthroats can occur when birds

experiencing oxidative stress allocate less lutein toward feather pigmentation (Freeman-

Gallant et al. 2011, Henschen et al. 2016), and depressed expression of carotenoid-based

ornaments can indicate an overall low availability of carotenoids, vitamins, and/or

minerals available for antioxidant and immune defense (Pike et al. 2007). Because β-

carotene may be more biologically active at lower doses than lutein for antioxidant and

immune defense (Stahl and Sies 2005, Sillanpää et al. 2008), as well as for conversion to

vitamin A (Surai et al. 2001, Surai and Speak 1998), low plumage color values of

nestlings at the MIXED site may be indicative of their low plasma β-carotene

concentrations. There were no obvious differences in habitat conditions among sites to

indicate that nestlings at the MIXED site experienced greater oxidative stress or were in

poorer health compared to those at the other sites. However, greater plasma β-carotene

concentrations of nestlings at the NATIVE and ALIEN site may have provided them with

elevated antioxidant, immune, or provitamin function that had a sparring effect on

lutein, allowing nestlings at these sites to allocate a greater proportion of lutein to

feathers than those at the MIXED site (sensu Pike et al. 2007).

Nestling plumage color can be influenced by maternal effects of carotenoids deposited

in egg yolks and to genetic inheritance of plumage characteristics (Blount et al. 2000,

Tschirren et al. 2003, Evans and Sheldon 2012), but the plumage HSB values of nestlings

in my study suggest that the environment nestlings were raised in (i.e., their diet and/or

70

environmental stressors) was more important than either maternal effects or genetic

inheritance for influencing nestling plumage color. Specifically, hue is the color variable

under the greatest genetic control, while saturation and brightness are most influenced

by environmental conditions experienced by birds when their feathers are growing

(Slagsvold and Lifjeld 1985, Johnsen et al. 2003, Eeva et al. 2008). Among sites, hue was

the least variable HSB value, differing by ≤ 2.2% among sites. Comparatively, saturation

and brightness differed among sites by as much as 11% and 7%, respectively, and both

differed significantly between the NATIVE and MIXED sites (Table 2.3). Further, among

all 96 nestlings from which I collected feather samples, the variance in hue (10.3) was

> 3x less variable than that of saturation (35.2) and brightness (37.4). Differences in

nestling plumage color among sites are therefore, more likely the result of differences in

plasma β-carotene and the amount of lutein allocated into nestling feathers, rather than

genetic or maternal effects.

Two possible dietary differences among sites might explain the low nestling plasma β-

carotene, growth rates and size, and plumage values at the MIXED site. The first is the

lower proportion of alternative prey in nestling diets at the MIXED site (Table 2.3).

Specifically, the proportion of spiders in nestling diets explained 31% of the variability

in nestling 7 day old tarsus length and both variables were lowest at the MIXED site.

Spiders are a regular dietary component of nestling yellowthroats (Shaver 1918,

Hofslund 1959, this study) and nestling coal tits (Periparus ater; Gibb and Betts 1963),

71

and adult great tits (Parus major) feed spiders preferentially to their nestlings (Royama

1970, Eeva et al. 1997, Naef-Daenzer et al. 2000). Royama (1970) proposed that

consistent trends of spiders reported in nestling diets independent of study area,

season, or habitat, indicate that spiders may contain nutrients important for nestling

growth that are not found in other types of prey. Gosler (1993) noted that both spiders

and bird feathers are rich in the amino acid cysteine and proposed that spiders may be

important for supporting feather development in nestlings. Ramsay and Houston (2003)

reported that spiders have greater protein content than caterpillars and they contain

particularly high levels of taurine that could buffer the use of endogenous cysteine

during bile formation, freeing cysteine for feather development in nestlings. Although

spiders represented ≤ 6.5% of prey items in nestling diets at all sites (Table 2.1), this

value equates to an average of 9 (SE = 2.0, range 5 to 13), 1.9 (SE = 0.9, range = 0 to 5),

and 6.6 (SE = 1.3, range = 2 to 11) individual spiders delivered to nestlings over an

average video period of only 4.7 hours at the ALIEN, MIXED, and NATIVE sites,

respectively. If spiders contain macronutrients important for nestling growth and

development (Royama 1970, Gosler 1993, Ramsay and Houston 2003), a comparatively

low number of spiders in nestling diets at the MIXED site may explain, in part, the lower

tarsus growth rate, 7-day old tarsus length, and maximum weight of nestlings there.

Lower plumage color values of nestlings at the MIXED site may also reflect the lack of

these nutrients in nestling diets (Pike et al. 2007, Costantini and Møller 2008).

72

Differences in β-carotene concentration among alternative prey items and caterpillars

comprising nestling diets at each site may provide a second explanation for the lower

plasma β-carotene for nestlings at the MIXED site. Specifically, the proportion of

butterflies/moths, odonates, and spiders were all lowest in nestling diets at the MIXED

site (Table 2.1). Compared to caterpillars, adult butterflies and moths contain less lutein

and more β-carotene (Eeva et al. 2010). I could find no reports of the carotenoid profile

for Odonates, but those fed to nestlings were mostly teneral juveniles that would not

have ingested carotenoids as adults before being fed to nestlings. Regarding spiders,

Eeva et al. (2010) determined that spiders contain low concentrations of lutein and β-

carotene, but the sample of spiders they analyzed did not include harvestmen

(Opiliones; T. Eeva, personal communication) that were the most common spiders fed

to nestlings in my study. Unlike most other spiders, harvestmen may include fruit and

plant material in their diet (Sankey and Savory 1974, Halaji and Cady 2000, Hvam and

Toft 2008) and therefore, may contain more carotenoids than the spiders assessed by

Eeva et al. (2010). Compared to lepidopteran caterpillars, sawfly caterpillars may

contain greater concentrations of β-carotene (Sillanpää et al. 2008), but this is not always

the case (Eeva et al. 2010). Although I do not know the specific proportion of

lepidopteran: sawfly caterpillars in nestling diets at each site, the lower absolute

amounts of butterflies/moths and possibly odonates and spiders in nestling diets at the

MIXED site may have contributed to their lower β-carotene levels. Overall, nestlings at

73

the NATIVE site that were fed a large proportion of caterpillars and an intermediate

proportion of alternative prey, had the greatest growth rates, condition, plasma

carotenoid concentrations, and plumage HSB score of all nestlings in this study.

This study was the first to quantify the effects of reduced caterpillar abundance

associated with alien plant invasion on the reproductive success of a breeding bird. This

study focused on bird reproductive success up to the point in the breeding season that

nestlings fledged, so I cannot conclude how low caterpillar abundance at the ALIEN site

may have influenced the ability of adult yellowthroats to raise fledglings to

independence. Energy demands of young are maximal between the time they fledge

and become independent, and energy costs of adults are high during this period when

they feed fledglings for twice as long as nestlings (Martin 1987). Low caterpillar

availability during the post-fledging period can therefore, increase the length of time

that fledglings depend on their parents for care (Nolan 1978, Murphy 2000). The fact

that the only pair of yellowthroats to produce two complete broods of young in my

study occurred at the ALIEN site, suggests the lower caterpillar abundance there did not

limit the ability of adults to raise fledglings. Ideally, future efforts to quantify the effects

of alien plant invasion on passerine productivity should be designed to monitor adult

health over the nesting season and to track fledgling survival through independence.

74

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APPENDICIES

88

APPENDIX A

A MODIFICATION TO JOHNSON’S (2000) “BRANCH-CLIPPING” METHOD FOR ESTIMATING CATERPILLAR

ABUNDANCE ON LIVE TREE/SHRUB FOLIAGE

Introduction

In temperate forests, passerine productivity is associated with the seasonal availability

of large caterpillars that serve as primary prey items fed to nestlings (e.g., Rodenhouse

and Holmes 1992, Newell et al. 2014). There can be marked differences in caterpillar

abundance among plants resulting from caterpillar host specialization (e.g., Rhoades

and Cates 1976, Chapter 1), leaf phenology (Crawley and Akhteruzzaman 1988, van

Asch and Visser 2007) and caterpillar phenology (Futuyma and Gould 1979, Graber and

Graber 1983). Differences in caterpillar abundance among plants can explain bird

preferences for particular foraging substrates (Holmes et al. 1979, Singer et al. 2012,

Newell et al. 2014).

Sampling methods that enable researchers to quickly and accurately estimate caterpillar

abundance on live tree and shrub foliage are therefore, valuable for assessing food

availability for birds at multiple scales. Branch-clipping (Johnson 2000) is a common

method used to quantify arthropod abundance on live foliage (Gibb and Betts 1963,

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Morse 1976, Naef-Daenzer et al. 2000). It is essentially a three-step process involving

enclosing a branch within a net, cutting the enclosed branch from the tree/shrub, and

counting arthropods captured. The primary benefit of branch-clipping is that by using a

net with a known volume, caterpillars can be sampled from an approximate known

volume of foliage, and total caterpillar abundance within a habitat can be estimated

based on the total foliage volume of each plant species comprising the habitat (Chapter

1). Disadvantages of branch-clipping include logistic constraints of handling a large

number of branch samples in the field and the potential for multiple sampling efforts

conducted during the same nesting season to be overly destructive to the habitat (M.

Tarr, personal observation). Therefore, I modified the branch-clipping method

described by Johnson (2000; hereafter “BRANCH-CLIPPING”) and instead of cutting

branches, I simply dislodged caterpillars into the net by beating the enclosed foliage

with a stick and by shaking the branch vigorously as I removed it from the net

(hereafter “NET&SHAKE”). I immediately transferred arthropods collected in the net to

plastic baggies that could be transported easily in the field.

As part of a larger study assessing differences in caterpillar abundance on native and

non-native shrubs comprising shrubland bird habitats (Chapter 1), I wanted to test

whether my NET&SHAKE method was as effective at collecting caterpillars from live

shrub foliage as the BRANCH-CLIPPING method. I also wanted to determine if there was a

significant difference in the amount of time required to count caterpillars collected in

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shrub samples between the two sampling methods. I chose to sample caterpillars from

red maple (Acer rubrum) stump sprouts (i.e., “shrubs”) for this test, because during my

larger study (Chapter 1) I collected an unexpected low number of caterpillars from red

maple shrubs. Because red maple foliage filled the sampling net more than that of the

other 14 shrub species I sampled (Chapter 1), I questioned if the density of red maple

foliage reduce the efficacy of the NET&SHAKE method to dislodge caterpillars into the

net.

Methods

During the first week of July 2015, I compared my NET&SHAKE method to the BRANCH-

CLIPPING method by sampling caterpillars from 2-yr old red maple shrubs growing

within a shrub-dominated transmission line right-of-way (ROW) in Strafford Co. New

Hampshire. Vegetation within this ROW had been maintained with an industrial

forestry mower once every 4-5 years for ≥ 15 yr. prior to 2015 and it was last treated in

this manner in winter 2014, such that all red maple foliage was growing 1-3 m above the

ground by the time of this study. I collected all samples using the same ± 0.04 m3 heavy-

duty sweep net and frame (Fig A1), and I collected all samples between 1100 hr. and

1400 hr. only when the foliage was dry to the touch.

To collect samples, I selected a 200 m area of the ROW that was dominated by red

maple shrubs. I moved down and across the ROW, collecting paired samples from

different shrubs growing < 4 m away from, but not touching, each other. For the first

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Figure A1. A 0.04 m3 net used to sample caterpillars from shrub foliage. This is a standard heavy-duty sweep net adapted to be held in an open position with a wire frame and then to fall free of the frame once a shrub branch is enclosed within the net.

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sample of each pair, I used my NET&SHAKE sampling method. I stored baggies containing

NET&SHAKE samples in a cooler in the field until the end of the sampling day. For the

second sample of each pair, I used the BRANCH-CLIPPING method and I transferred each

branch from the net into its own white plastic, kitchen-sized garbage bag. I knotted the

end of the bag and stored bags containing BRANCH-CLIPPPING samples in multiple piles in

the shade until I left the site for the day. Thus, each pair of samples included one

NET&SHAKE sample (n = 101) and one BRANCH-CLIPPING samples (n = 101). I switched order in

which I either shook or cut branches into the net after every 25 pairs of samples. At the

end of each sampling day I stored all samples in a walk-in freezer at approximately - 3°

C until I could sort samples beginning 12 hr. after the end of the last sampling day. I

collected all samples over three consecutive days.

In the lab, I processed all BRANCH-CLIPPING samples, followed by all NET&SHAKE

samples. I counted caterpillars collected from each BRANCH-CLIPPING sample by

spreading the contents of each sample on a large stainless steel necropsy table and I

examined each leaf and all branches for caterpillars. I also counted the number of red

maple leaves contained within each BRANCH-CLIPPING sample. I counted caterpillars

from each NET&SHAKE sample by emptying the contents of each baggy into a metal

roasting pan and I examined any leaves or branches contained within the sample,

though very few of either were collected with the NET&SHAKE method. I recorded the

length of time required to count caterpillars from each sample, from the time I opened

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the bag through the time required to clean either the table or the pan to begin

processing the next sample. I used Chi-square to determine if there was a difference in

the frequency that caterpillars were collected in either NET&SHAKE or BRANCH-CLIPPING

samples. I used a pooled t-test to determine if there was a difference in the number of

caterpillars collected between sampling methods and a t-test with unequal variances to

determine if there was a difference in the average amount of time required to process

samples from each caterpillar sampling method.

Results

There was no difference (χ2 = 1.086, df = 1, p = .297) in the number of NET&SHAKE or

BRANCH-CLIPPING samples that contained caterpillars (6 out of 101 and 10 out of 101,

respectively). All samples with caterpillars contained only one caterpillar and there was

no difference (t = 1.04, df = 200, p = .3) in the mean number of caterpillars collected per

sample between the NET&SHAKE (0.06, SE = 0.03) and BRANCH-CLIPPING (0.1, SE = 0.03)

methods. The average number of leaves contained within each BRANCH-CLIPPING

sample was 70 leaves (SE = 2.3, range = 20 to 153) and the total number of leaves in all

101 samples was 7070. The average time required to process each BRANCH-CLIPPING

sample (4.4 min, SE = 0.2) was significantly (t = 21.01, df = 117.46, p < .0001) longer than

required for each NET&SHAKE sample [0.7 min (i.e., 42 sec.), SE = 0.06). The total amount

of lab time required to process all BRANCH-CLIPPING samples was 7.5 hr and that to

process all NET&SHAKE samples was 1.2 hr.

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Using the BRANCH-CLIPPING method I estimated about 1.4 caterpillars/1000 leaves

sampled. The NET&SHAKE method estimated 7.7 caterpillars/5 m3 of foliage and the

BRANCH-CLIPPING method estimated 12.8 caterpillars/5 m3 of foliage. When the two

methods were combined, caterpillars were collected in only 16 out of 202 (7.9%) red

maple samples.

Discussion

My NET&SHAKE method of sampling caterpillars was equally as effective as the

BRANCH-CLIPPING method at extracting caterpillars from red maple foliage. Although

the BRANCH-CLIPPING method captured slightly more caterpillars than the NET&SHAKE

method, the difference was not significant and likely reflects the natural variability in

caterpillar distribution that is common among individual plants (Morse 1976, Crawley

and Akhteruzzaman 1988, Holmes and Shultz 1988), rather than reflecting differences

in the efficacy of the two sampling methods at extracting caterpillars from foliage.

Based on these results, I conclude that if caterpillars are present on red maple foliage,

the NET&SHAKE method is effective at extracting them.

The NET&SHAKE method estimated only 7.7 caterpillars/ 5 m3 of red maple foliage,

which is slightly higher but not different (t = 0.99, df = 450, p = .32) than my average

estimate of 5.1 caterpillars/ 5 m3 from 351 NET&SHAKE samples I collected from red

maple shrubs during my study in 2013 (Chapter 1). These results suggest that the low

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abundance of caterpillars I estimated on red maple in 2013 reflect the actual abundance

of caterpillars occurring on this shrub species and are not the result of my NET&SHAKE

sampling method being ineffective at extracting caterpillars from red maple foliage.

This conclusion is further supported by the fact that only 10 of 101 (9.9%) BRANCH-

CLIPPING samples contained caterpillars and this method estimated only 1.4

caterpillars/1000 leaves. Similarly low or lower caterpillar abundance has been reported

on other native tree species in NH. For example, Robinson and Holmes (1982) reported

an average of 2.2 caterpillars/1000 leaves on sugar maple (Acer saccharum), beech (Fagus

grandifolia) and yellow birch (Betula alleghaniensis), and Holmes and Schultz (1988)

reported caterpillar abundance ranging from 0.5 – 7.3 /1000 leaves of sugar maples, 2.3 –

8.8/1000 leaves of yellow birch, and 4.3 – 8.3/1000 leaves of beech.

Compared to the BRANCH-CLIPPING method, my NET&SHAKE method is less destructive

to the habitat, requires less time in the field and in the lab per sample, and is logistically

easier because the samples stored in baggies are easier to transport than the BRANCH-

CLIPPING samples stored in garbage bags. Further, the NET&SHAKE samples are small

enough to store easily in a cooler in the field if arthropod samples need to be preserved

for identification, weighing, or for other analysis such as carotenoid extraction.

Comparatively, the BRANCH-CLIPPING samples were large and difficult to keep cool in

the field; even though I stored BRANCH-CLIPPING samples in the shade, heat generated

within the garbage bags caused much of the foliage to lose its color before I could

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transport samples to the lab, and by the time I was able to separate caterpillars from the

samples they were too degraded to make accurate measurements of their length and

weight. Further, the amount of space and time required to process BRANCH-CLIPPING

samples was substantially greater than that for NET&SHAKE samples which required

very little lab space to store and process, and they had very little plant material

requiring disposal.

The NET&SHAKE method of sampling provides a simple method for extracting

caterpillars from an approximate known volume of shrub foliage. However, similar to

the BRANCH-CUTTING method, each sample collects caterpillars from a small volume of

foliage, so a large number of samples will be required to account for the typically large

variability in caterpillar distribution among individual plants (Gibb and Betts 1963,

Crawley and Akhteruzzaman 1988, Holmes 1990). As a result, the number of samples

required to collect a statistically viable sample may be prohibitively large if the goal is

to accurately compare caterpillar numbers on more than a few plant species within a

habitat (Chapter 1). Further work is needed to develop a quick and effective method for

conducting a statistically viable sample of caterpillar abundance within diverse habitats

such as shrublands composed by a diverse community of caterpillar host species.

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List of References

Crawley, M. J., and M. Akhteruzzaman. 1988. Individual variation in the phenology of oak trees and its consequences for herbivorous insects. Functional Ecology 2: 409-415.

Futuyma, D. J., and F. Gould. 1979. Associations of plants and insects in deciduous forest. Ecological Monographs 49:33-50.

Gibb, J. A., and M. M. Betts. 1963. Food and Food supply of nestling tits (Paridae) in Breckland Pine. Journal of Animal Ecology 32:489-533.

Graber, J. W., and R. R. Graber. 1983. Feeding rates of warblers in spring. Condor 85:139-150.

Holmes, R. T., J. C. Shultz, and P. Nothnagle. 1979. Bird predation on forest insects: an Exclosure Experiment. Science, New Series 206:462-463.

Johnson, M. D. 2000. Evaluation of an arthropod sampling technique for measuring food availability for forest insectivorous birds. Journal of Field Ornithology 71:88-109.

Morse, D. H. 1976. Variables affecting the density and territory size of breeding spruce-woods warblers. Ecology 57:290-301.

Naef-Daenzer, L., B. Naef-Daenzer, and R. G. Nager. 2000. Prey selection and foraging performance of breeding great tits Parus major in relation to food availability. Journal of Avian Biology 31:206-214.

Newell, F. L., T-A. Beachy, A. D. Rodewald, C. G. Rengifo, I. J. Ausprey, and P. G. Rodewald. 2014. Foraging behavior of cerulean warblers during the breeding and non-breeding seasons: evidence for the breeding currency hypothesis. Journal of Field Ornithology 85:310-320.

Rhoades, D. F. and R. G. Cates. 1976. Toward a general theory of plant antiherbivore chemistry. Recent Advances in Phytochemistry 10:168-213.

Rodenhouse, N. L. and R. T. Holmes. 1992. Results of experimental and natural food reductions for breeding black-throated blue warblers. The Auk 109:321-333.

Singer, M. S., T. E. Farkas, C. M. Skorik, K. A. Mooney. 2012. Tritrophic interactions at a community level: effects of host plant species quality on bird predation of caterpillars. The American Naturalist 179:363-374.

van Asch, M., and M. E. Visser. 2007. Phenology of forest caterpillars and their host trees: the importance of synchrony. The Annual Review of Entomology. 52:37-55.

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Study site1

ALIEN MIXED NATIVE Alien shrubs Autumn olive 107 - - Glossy buckthorn 107 228 99

Native shrubs Aspen - 194 98 Arrowwood - 70 - Birch - 243 104 Blueberry 107 200 104 Dogwood - 67 - Maleberry - 138 - Meadowsweet 99 234 107 Red maple - 240 99 Oak - 235 103 Speckled alder 103 239 103 Sweet fern - 151 107 Winterberry holly - 175 99 Willow - 247 108

APPENDIX B

THE TOTAL NUMBER OF 0.04 M3 NET SAMPLES COLLECTED TO ESTIMATE CATERPILLAR ABUNDANCE ON EACH SHRUB SPECIES AT EACH

TRANSMISSION LINE RIGHT-OF-WAY STUDY SITE. ROCKINGHAM AND STRAFFORD COUNTY, NH. 2013.

1 Twenty-two to 84 individual shrubs of each species were sampled during each sampling date depending on the distribution and abundance of each particular species at each study site. For example, a total of 107 individual autumn olive shrubs were sampled at the ALIEN site where autumn olive was abundant, but this species was absent at the NATIVE site and too uncommon to warrant sampling at the MIXED site where the same < 10 individual shrubs would have had to have been sampled each sampling period. Shrubs such as arrowwood and dogwood that were sampled less than the other shrub species at the MIXED site, were abundant enough in some areas of the site (and therefore, in some common yellowthroat territories) to warrant sampling, but were absent throughout the remainder of the site.

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APPENDIX C

A MASS-LENGTH REGRESSION FOR ESTIMATING CATERPILLAR BIOMASS

Introduction

I developed a mass-length regression model from caterpillars collected from native and

alien shrubs growing within three transmission line right-of-way study sites in

southeastern New Hampshire in 2013. This model was used to estimate the biomass of

caterpillars delivered by adult common yellowthroats to nestlings at these same study

sites during the 2013 nesting season.

Methods

Caterpillars [Lepidoptera larvae and sawfly larvae (Hymenoptera: Symphyta)] were

collected from the dominant native and alien shrub species growing within three shrub-

dominated transmission line rights-of-way in southeastern NH (Rockingham and

Strafford Counties) between late-Jun and late-Jul 2013. Caterpillars were collected from

shrub foliage growing 0.5 to 3.5 m above the ground using a sweep net design and

technique adapted from Johnson (2000), in which caterpillars were dislodged from live

foliage that was first enclosed within a ± 0.039 m3 sweep net. At each shrub, a branch

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with enough live foliage to fill the net was selected and quickly enclosed within the net.

The enclosed foliage was beaten forcefully with a stick and shaken vigorously as it was

pulled from the net. All arthropods collected in the net were immediately transferred to

a plastic baggy containing ethyl acetate, and all baggies were stored in a cooler in the

field and then in a cooler indoors until caterpillars could be sorted from samples 3 to 48

hours later. Caterpillars were sorted to order and Lepidoptera caterpillars were sorted

to family. The length of each caterpillar was measured to the nearest mm, then

caterpillars were dried at 75 ⁰ C for 72 hours and then weighed to the nearest 0.1 mg

with an analytic balance to determine their weight (mg dry matter). I log-transformed

caterpillar weight and used least squares regression to analyze log-weight as a function

of caterpillar length. A matched pairs analysis was used to test how well weights

predicted by the model matched the actual weights of caterpillars used to develop the

equation.

Results

A total of 587 caterpillars were collected, including 389 Lepidoptera larvae representing

11 families and 198 Hymenoptera larvae (Table A2). Caterpillars range from 1 - 40 mm

in length and 0.1 – 243.3 mg in weight. The resulting regression model was log-weight =

0.186 Length – 7.983 (R2adj = 0.95, P < 0.0001, n = 587, Fig. A2). The matched pairs

analysis indicated there was no difference (t = 1.057, df = 47, P = 0.30, n = 48, correlation

= 0.88) between weights predicted by the model and the actual weights of caterpillars

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used to develop the model. To estimate the biomass of caterpillars delivered by adults

to nestlings, I entered the lengths of caterpillars estimated from videos into the

predictive equation and back-transformed the resulting weight predictions (log-weight)

as 2.62(log-weight) to estimate caterpillar biomass in mg.

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Table A2. Order and family of 587 caterpillars collected in Rockingham and Strafford County New Hampshire, used to develop a regression of caterpillar weight as a function of caterpillar length. 2013.

Order Family n Lepidoptera 389

Arctiidae 1 Geometridae 101 Hesperiidae 1 Limacodidae 1 Lycaenidae 27 Lymantriidae 1 Noctuidae 181 Notodontidae 60 Nymphalidae 3 Pantheidae 2 Sphingidae 4 unknown 7

Hymenoptera 198

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Figure A2. Linear regression of log weight (mg dry matter) as a function of length (mm) for 587 caterpillars collected from native and alien shrubs in Rockingham and Strafford County, New Hampshire, Jun-Jul 2013.

Log-weight = 0.186 Length – 7.983 (R2adj = 0.95, P < .0001, n = 557)

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APPENDIX D

APPROVAL FOR UNIVERSITY OF NEW HAMPSHIRE INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE (IACUC) PROTOCOL # 110902.