6
This article was downloaded by: [University of Strathclyde] On: 06 October 2014, At: 18:04 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Italian Journal of Zoology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tizo20 Differences in the dimensions of diurnal and nocturnal pellets of the barn owl, Tyto alba Franca Guidali a & Giorgio Pigozzi b a Dipartimento di Biologia, Sezione di Ecologia , Università di Milano , via Celoria 26, Milano, I20133, Italy b Dipartimento di Biologia, Sezione di ZoologiaSN , Universita di Milano , via Celoria 26, Milano, I20133, Italy Published online: 28 Jan 2009. To cite this article: Franca Guidali & Giorgio Pigozzi (1996) Differences in the dimensions of diurnal and nocturnal pellets of the barn owl, Tyto alba , Italian Journal of Zoology, 63:2, 157-161 To link to this article: http://dx.doi.org/10.1080/11250009609356124 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

Differences in the dimensions of diurnal and nocturnal pellets of the barn owl, Tyto alba

  • Upload
    giorgio

  • View
    218

  • Download
    1

Embed Size (px)

Citation preview

Page 1: Differences in the dimensions of diurnal and nocturnal pellets of the barn owl,               Tyto alba

This article was downloaded by: [University of Strathclyde]On: 06 October 2014, At: 18:04Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

Italian Journal of ZoologyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tizo20

Differences in the dimensions of diurnal andnocturnal pellets of the barn owl, Tyto albaFranca Guidali a & Giorgio Pigozzi ba Dipartimento di Biologia, Sezione di Ecologia , Università di Milano , via Celoria 26,Milano, I‐20133, Italyb Dipartimento di Biologia, Sezione di Zoologia‐SN , Universita di Milano , via Celoria26, Milano, I‐20133, ItalyPublished online: 28 Jan 2009.

To cite this article: Franca Guidali & Giorgio Pigozzi (1996) Differences in the dimensions of diurnal and nocturnalpellets of the barn owl, Tyto alba , Italian Journal of Zoology, 63:2, 157-161

To link to this article: http://dx.doi.org/10.1080/11250009609356124

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”)contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensorsmake no representations or warranties whatsoever as to the accuracy, completeness, or suitability for anypurpose of the Content. Any opinions and views expressed in this publication are the opinions and viewsof the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sources of information. Taylorand Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses,damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connectionwith, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Differences in the dimensions of diurnal and nocturnal pellets of the barn owl,               Tyto alba

. Ital. J. Zool., 63-. 157-161 (1996)

Differences in the dimensionsof diurnal and nocturnal pelletsof the barn owl, Tyto alba

FRANCA GUIDALIDipartimento di Biologia, Sezione di Ecologia,Universita di Milano,via Celoria 26, I-20133 Milano (Italy)

GIORGIO PIGOZZI*Dipartimento di Biologia, Sezione di Zoologia-SN,Universita di Milano,via Celoria 26, 1-20133 Milano (Italy)

ABSTRACT

Linear dimensions and weight of barn owl pellets collected at ahay-loft and at a roost-site were examined. Pellets ejected in the hay-loft during nocturnal hunting activity were significantly smaller andlighter than those ejected at the roost-site during resting periods indaylight hours. This observation is in agreement with the "pelotediurne et pelote nocturne" theory suggested by Guerin (1932).Linear dimensions and weight of pellets varied from one to anotherseason. Thus, it was suggested that the feeding tactics exhibited bybarn owls may affect the content and the linear dimensions andweight of pellets; moreover, it was pointed out that further studiesare necessary on pellet characteristics in habitats where barn owlfaces spatial and temporal variations in the availability of its most im-portant food resources.

KEY WORDS: Birds - Tyto alba - Pellets - Feeding behaviour.

ACKNOWLEDGEMENTS

We are grateful to Mr. G. Barbieri for hospitality and logistic sup-port, Dr. M. Salinetti for assistance during field-work, Dr. J.-M.Weber for valuable discussions and Dr. N. Saino for constructivecomments on the manuscript.

(Received 21 December 1995 - Accepted 20 February 1996)

* Author to whom correspondence and offprint requests should beaddressed

INTRODUCTION

Pellet analysis provides information on predatorpressure and gives indications on seasonal differences inthe owls' food habits (Errington, 1932; Glue, 1970). Awealth of information is available on the content of barnowl, Tyto alba (Scopoli, 1769), pellets (e.g., Bunn et al.,1982 and references therein); however, little attentionhas comparatively been paid to the pattern of temporaldeposition of pellets or to their external features. Mostly,barn owl pellets are tightly massed collections of fur witha central core of mammal and bird bones and are quiteconspicuous objects due to their large size, black colourand shiny appearance when fresh. These distinctivecharacteristics make them relatively easy to identify evenwhen dry (Bunn et al., 1982). Pellets have an ovoidalellipse shape with three distinct radii of variable size(Nos, 1961). According to Guerin (1928), barn owls mayproduce a maximum of four pellets in 24 hunder excep-tional conditions, the usual number being two. The firstof the two is small, "pelote nocturne", and is droppedduring hunting activity (hereafter referred to as nocturnalpellet), while the second is usually large, "pelote diurne",and is dropped at the day roost-site (hereafter referred toas diurnal pellet). A few years later Guerin (1932) collect-ed a small number of pellets from two adjacent siteswhich were both used by the same barn owl, one at nightand the other during the day. Of the 37 pellets found in25 days, 23 were diurnal pellets while the remaining 14were nocturnal ones; interestingly, on ten different daysthe barn owl produced both a diurnal and a nocturnalpellet and on two separate occasions even three pelletsper day (one diurnal and two nocturnal).

Most researchers have subsequently maintained thatbarn owls produce two pellets over a 24 h period (but seeMarti, 1973; Goszczynski, 1976; Schmidt, 1977), with thediurnal one being larger than the nocturnal one (e.g.,Geroudet, 1965; Glue, 1973 in Mikkola, 1983); however,no data have subsequently been presented to corro-borate the incidental observations reported by Guerin(1932). In this paper we present additional data on thepattern of temporal deposition of pellets in sites used bybarn owls at night or during the day; then, we testGuerin's theory by comparing the external features ofthese pellets and provide information on the dimensionalcharacteristics of the undigested material ejected by barnowls at different times over the 24 h period (day or night)and in different seasons.

MATERIALS AND METHODS

Barn owl pellets were collected every two weeks between March1992 and February 1994 in a farm located at Casalmoro (MantovaProvince) in the Po Valley, northern Italy. Pellets were searched forsystematically both under the roost-site and in a hay-loft where barnowls used to perch during their nocturnal hunting activities. In theformer place, pellets accumulated at the base of a disused silo whichwas used by barn owls both as a roosting site and as a hiding placeduring daylight hours. In the latter place, barn owls used to perch on

Dow

nloa

ded

by [

Uni

vers

ity o

f St

rath

clyd

e] a

t 18:

04 0

6 O

ctob

er 2

014

Page 3: Differences in the dimensions of diurnal and nocturnal pellets of the barn owl,               Tyto alba

158 F. GU1DALI, G. PIGOZZI

the wooden ridgepoles of the hay-loft that was located ap-proximately 50 m from the disused silo. Pellets accumulated at thesesites were all fresh and less than fifteen days old when collected.

We carried out thorough observations both at night and par-ticularly during the day to locate barn owls perching in the hay-loft.We were able to observe individuals at night leaving the hay-loftfollowing our arrival; however, on no occasion did we record thepresence of barn owls there in daylight. Similarly, no observation ofindividuals perching on the wooden ridgepoles of the hay-loft wasreported by local people who regularly lived and worked on thefarm. In addition, during our routine survey at two-weekly intervals,we never found fresh pellets when we returned to the hay-loft atdusk before the beginning of the barn owl's hunting activity. On thecontrary, when we returned on the following morning, we oc-casionally found fresh pellets which had been regurgitated duringthe previous night. On two mornings, individuals hidden in the up-per portion of the disused silo dropped a large fresh pellet while wewere carrying out our routine collection. Thus, it seems reasonableto assume that the pellets accumulated at the base of the disused silowere regurgitated primarily during the day, whereas those collectedin the hay-loft were regurgitated exclusively at night.

Only complete pellets were used for assessing their linear dimen-sions and weight (i.e., no fragments, no broken ends, no splitting orloosening of pellets). Fresh pellets were dried and then measured tothe nearest mm using a dial calliper. Three different linear dimen-sions were measured: length (maximum distance between the twoextreme ends of the major axis of the pellet), width (maximumdistance between the two extreme ends of the major transversal axisof the pellet) and height (maximum distance between the two ex-treme ends on the minor transversal axis of the pellet). Pellets wereweighed on a digital scale professional balance (Model 1479, TanitaCorporation, Japan) to the nearest 0.1 g (data available from Septem-ber 1992 onwards).

We defined seasons (i.e., winter: December-February, spring:March-May, summer: June-August, and autumn: September-November) on the basis of the monthly mean of minima tem-peratures recorded at a meteorological station (Istituto TecnicoAgrario Forestale «Bonsignori», Remedello) located at a distance ofapproximately 4 km from the farm. Accordingly, pellets collectedduring each month were then lumped in their respective season.

Differences in length, width, height and weight between pelletscollected from the disused silo and the hay-loft (hereafter diurnaland nocturnal station, respectively) were analysed by Student'st-test. Seasonal differences were tested using one-way ANOVA andcomparisons were made by the Scheffe method which is particularlyuseful in the case of multiple contrasts when sample sizes are notequal (Zar, 1984). In addition, we evaluated the statistical power ofthe test in all cases in which the null hypothesis was not rejected(Zar, 1984). Weight values were cube root-transformed beforeevaluating their relationship with the linear dimensions of pellets. Alltests were two-tailed unless otherwise stated and statisticalsignificance was accepted at the 0.05 probability level. Data areshown with mean and standard deviation (SD).

50 - ,

40 -

1 0 -

•**** + *• +4-+ 4-4• + + + + +4- + + + + +• • • + + + +• + + + 4- + +• + + + 4- +

• • • • + + ••4 + ++4-4

••++++• • • • * • + • *******

1Nocturnal

Diurnal

30-,

E

2 10-

T•Him

T

111 371 111 378

* • *

£

o 10-X

* * *

T

+ 4-

+ 4-

+ +

• • +

•>+•

4-+

++

+4

+ 4-4-*4-4-4

• •+•+++4••+>+*4-4*********************

T 4 -

n 111 376

a) 2 -

n

Tf++*•++••*•+****•********n**************

i

68 284

Fig. 1 - Linear dimensions and weight (X ± SD) of barn owl pelletscollected at the diurnal and the nocturnal station. Values in abscissadenote sample sizes, while asterisks show significant differences be-tween the two groups (***, P < 0.001).

TABLE I - Correlation coefficient (r) between linear dimensions andweight (cube-root transformed values) of pellets ejected by barnowls at a nocturnal station (values above the diagonal) and at adiurnal station (values below the diagonal) in northern Italy (sam-ple size in parentheses).

RESULTS Length Width Height Weight

Tests were carried out to evaluate whether lineardimensions of pellets (and particularly of the smaller sam-ple ejected exclusively during nocturnal hunting activity)differed significantly between samples collected in 1992and 1993. Pellets collected at the nocturnal stationshowed no significant between-year differences in length(t109 = 0.86, P > 0.30), width (t109 = 0.66, P > 0.50) andheight (t109 = 0.88, P > 0.30). There were insufficient datain 1992 to test between-year differences in weight. Wehave assumed that the homogeneity displayed by lineardimensions of pellets was repeated also for weight and

Length

Width

Height

Weight

0.49"*(370)

0.53"*(370)

0.81"*(284)

0.65"*(111)

0.59***(376)

0.71"*(284)

0.68*"(111)

0.81"*(111)

0.60"*(284)

0.77***(68)

0.76* **(68)

0.85***(68)

* " , P < 0.001

Dow

nloa

ded

by [

Uni

vers

ity o

f St

rath

clyd

e] a

t 18:

04 0

6 O

ctob

er 2

014

Page 4: Differences in the dimensions of diurnal and nocturnal pellets of the barn owl,               Tyto alba

BARN OWL PELLETS 159

6O-1

(D

lit

023 Nocturnal

I I Diurnal

X

Wi Sp Su Au

30-i

E20H

2 10-"

J1++++

++• +

++•••+++++++

I

T

T

- + + •

j

1T

+ 4 + 4

T

^

I+ + 4H+ 4-4'

+ + «-<+ + + 1+ + 4-I+ + ++ + •+ + +

T

Wi Sp Su Au

30 -,

f2,H.? 10 HX

TiI- + + + H

4 + + -I+ + + •<

:J«J+ + + H+ + + -I

« «

Tr

««•+++

f + + + H

* + + +f + + +

I!• + + +»• + + +*• + + + .• + + + •* + + + •• + + +

•• + + + ••• + + +

T T• + + +

I* + + + Hf + + + H

T

6-

'to

2-

Wi Sp Su Au

( d)

T JI

*•+++••+++•

| |

Wi

Th + + + -

liltSn

T

T

Su

TTI

•• + + + •

rh

Au

Fig. 2 - Average linear dimensions and weight (± SD) of barn owl pellets collected at the diurnal and the nocturnal station in different seasons:Wi, winter; Sp, sping; Su, summer; Au, autumn.

have pooled data for both years in subsequent analyses.We compared length, width, height and weight in the

two samples of pellets collected at the diurnal (n = 379)and the nocturnal station (n = 111), respectively (Fig. 1).Pellets-ejected at the diurnal station were significantlylonger (t480 = 8.39, P < 0.001), wider (t487= 11.15, P <0.001), higher (t485 = 10.79, P < 0.001) and heavier (t35O =9.04, P < 0.001) than those ejected at the nocturnal one.

Not surprisingly, the correlation between linear dimen-sions and the cube root of weight proved highlysignificant (always P < 0.001) in pellets found in both thediurnal and the nocturnal station (Table I).

Linear dimensions and weight of pellets showed sub-stantial seasonal differences. For example, the length ofnocturnal pellets were different among seasons (ANOVAF3107 = 4.05, P < 0.01), being significantly greater duringwinter (Scheffe method, one-tailed test, P < 0.001) (Fig.2a). On the contrary, the length of diurnal pellets provedsimilar in different seasons (ANOVA F33S7 = 0.44, P =0.72). However, since the power of the test was par-ticularly low (1 - p = 0.19), we suggest that no evidenceof a difference in length was found in this case (i.e. therewas an 81% chance of committing a type II error). Thewidth of diurnal pellets showed considerable seasonaldifferences (ANOVA F3374 = 4.51, P < 0.01), beingsignificantly smaller in spring (Scheffe method, one-tailed test, P < 0.05) (Fig. 2b). On the contrary the widthof nocturnal pellets proved similar in different seasons(ANOVA F31O7= 2.53, P = 0.06) but the power of the testwas low (1 - p = 0.33). Again we suggest that no evidence

of a seasonal difference in width was found due to a typeII error. The height of nocturnal pellets was significantlydifferent among seasons (ANOVA F3107 = 3-80, P < 0.05),being significantly greater in winter (Scheffe method,one-tailed test, P < 0.001) (Fig. 2c). On the other hand, noseasonal differences in height were found in diurnalpellets (ANOVA F3372 = 0.60, P = 0.61), although wesuspect that a type II error may be involved (1 - P =0.13). Finally, the weight of diurnal pellets was significant-ly different among seasons (ANOVA F323O= 5.09, P <0.01), being greater in spring and summer (Scheffemethod, one-tailed test, P < 0.05) (Fig. 2d). No suchseasonal difference was found in nocturnal pellets(ANOVA F367 = 1.69, P = 0.18), but the power of the testwas low (1- p = 0.24).

DISCUSSION

The linear dimensions and weight of pellets found inthis study proved significantly different between the twosamples collected at the nocturnal and diurnal stations.Table II reports the linear dimensions and weight of barnowl pellets that are available in the literature. It seemsreasonable to believe that previous studies referredprimarily to diurnal pellets collected at roost-sites andhiding places during daylight hours. In this respect, thelength and width of pellets collected in the diurnalstation in this study (n = 317) are consistent with thoseof other pellet samples collected in different countries

Dow

nloa

ded

by [

Uni

vers

ity o

f St

rath

clyd

e] a

t 18:

04 0

6 O

ctob

er 2

014

Page 5: Differences in the dimensions of diurnal and nocturnal pellets of the barn owl,               Tyto alba

160 F. GUIDALI, G. PIGOZZI

(Table II). In addition Ticehurst (1935) reported theminimum and maximum values of 165 barn owl pellets,comprised between a small size of 40x27 mm and a largesize of 83x23 mm or 72x28 mm. However, the greatestvalue of length reported in the literature is 102 mm andwas measured independently of each others by Webster(1973) and Walsh (1984), while the greatest value ofwidth is 51 mm (Tome, 1992). These values are con-siderably greater than those found in this study (greatestlength of 80 mm and greatest width of 38 mm).

Fewer data are available for the height and the weightof pellets. Both variables present average values in thisstudy that are consistent with data previously published,suggesting an average height of 19 mm and an averageweight of 4 g per pellet (Table II). The greatest value ofwidth reported in the literature is 28 mm (Mikkola, 1983).Not surprisingly, the variability of the length is greaterthan that of either the width or the height (Table II), thelatter dimensions being presumably related to the size ofthe digestive tract (Balgooyen, 1971). Little informationis available on the weight of barn owl pellets. Mostresearchers did not report any indication of the weight(see Table II) or alternatively they showed only the ex-treme values found in their respective pellet sample. Ac-cording to Ticehurst (1935), the weight of pellets wascomprised between 3.84 g and 9.60 g, while Cuisin &Cuisin (1979) reported a maximum weight of 11 g.However, the greatest value of weight measured so far is12.1 g (Sorgo, 1992), which is considerably larger thanthat of the present study (9.9 g).

Incidental observations and preliminary indications

TABLE II - Table II. Average linear dimensions (mm) and weight (g)(± SD) of barn owl pellets.

Nos (1961)Glue (1967)Webster (1973)Camacho (1975)Petretti(1977)Camacho &Pleguezelos (1980)

Mikkola (1983)Walsh (1984)Sorgo (1992)

Tome (1992)Guidali & Pigozzi

n

41248591394150

306274

8065756029

219383

99111371

Length

X

39.345453741

3537504536.337.836.438.137.640.530.438.4

SD

1.4

7.77.66.8

10.19.1

11.38.09.0

Width

X

23.926252425

2424272623.923.123.725.924.924.919.924.9

SD

0.4

4.03.34.75.85.23.84.14.1

Height

X SD

181822

19.9 3.115.7 2.719.1 3.6

Weight

X

3.03.6

3.64.14.03.93.9

2.24.0

SD

1.31.41.11.71.5

1.21.6

suggest the existence of a seasonal variation in the lineardimensions and weight of pellets. For example, Camacho& Pleguezuelos (1980) analysed two pellet samplescollected in winter {n = 306) and in summer (n = 274).The average dimension of pellets collected in winter wassmaller, but not significantly so, than that of the summersample (Table II). On the contrary, Petretti (1977) foundthat pellets collected in winter were smaller than thosecollected in summer. Similarly, preliminary indicationssuggest that the weight of pellets may vary seasonally andaccording to the age class of barn owls. For example,Saint-Girons (1964) found that pellets regurgitated byyoung barn owls were lighter (average 3-1 g, SD = 1.61g) than those of adult individuals (average 4.3 g, SD =1.50 g for fresh pellets, 4.2 g, SD = 1.23 gfor old pellets,respectively). Camacho & Pleguezelos (1980) reportedthat pellets collected in winter were significantly lighter(3.04 g) than summer ones (3.64 g). However, the formerstudy is based on a small sample size while the latter is in-complete lacking the data of the remaining seasons. Fewof the above-mentioned studies are based on a monthlycollection of pellets, and none give detailed informationon the dimensions of pellets found in the four seasons.Furthermore, there is a great variability in the number ofpellets analysed, which ranges from a minimum of 29(Sorgo, 1992) to a maximum of 591 (Webster, 1973). Inaddition, only the most recent studies report the stan-dard deviation of the measured variables, a lack whichseriously limits the usefulness of those pellet samplesanalysed up to the eighties (Table II). Thus, the com-parison of data available in the literature with those of thepresent study should be undertaken with caution.

Although the results of this study add supportingevidence to the existence of seasonal differences in thelinear dimensions and weight of pellets (e.g., Petretti,1977; Camacho & Pleguezelos, 1980), we stress the needfor further studies on this subject in different environ-ments, where principal food resources may exhibitspatial and temporal variations. Preliminary analyses(Pigozzi & Guidali, unpublished data) suggest that thefeeding tactics of barn owls may affect both the contentand the dimensions of pellets. The state of hunger is aninternal factor determining the size of pellets of short-eared owls, Asio flammeus, (Chitty, 1938) but only ex-perimental studies conducted in laboratory conditionsare likely to add comparable information for barn owls.

After several years of painstaking field observations,Guerin (1932) was able to collect only a very small sam-ple of nocturnal pellets to provide partial evidence insupport of his popular «pelote diurne et pelote nocturne*theory. The data presented in this paper undoubtedlycorroborate the preliminary indications given by Guerin(1928, 1932), which were nevertheless persistentlyquoted by barn owl researchers during the followingyears. Several reasons may be suggested to explain whyGuerin's theory has never been tested. Firstly, barn owlsare territorial and defend a territory the size of whichmay vary between 2.5 and 5-0 km2 depending on the

Dow

nloa

ded

by [

Uni

vers

ity o

f St

rath

clyd

e] a

t 18:

04 0

6 O

ctob

er 2

014

Page 6: Differences in the dimensions of diurnal and nocturnal pellets of the barn owl,               Tyto alba

BARN OWL PELLETS 161

food availability and vegetation of the area (Bunn et al.,1982). Barn owls are likely to have several nocturnalstations within their territory which are visited duringthe hunting activity; the chance of collecting nocturnalpellets depends on the knowledge of the location of noc-turnal stations and on their actual use by the owls. Unlessbarn owls are equipped with radio-transmitters, it isalmost impossible to know their movements and theiruse of the various nocturnal stations.

Secondly, in spite of a widespread interest in the foodof barn owls during the last decades (Bunn et al, 1982),very little attention has been paid to the pellet samplingmethod. Few studies involve the systematic collection ofpellets on a monthly basis and for periods of two or moreyears (see Korgschen, 1980), the vast majority being in-stead based on occasional or seasonal collections of allundigested remains found primarily under diurnalstations (i.e, roost-sites). In these circumstances, thechance of collecting nocturnal pellets may be significant-ly hampered by the action of moisture, rain, inver-tebrates, and fungi that rapidly break down pellets par-ticularly in exposed situations (Philips & Dindal, 1979).

Thirdly, barn owl pellets are formed by the muscularaction of the ventriculus during the process of digestion;the freshly formed pellet is subsequently stored in theproventriculus where it remains until the proper stimulusfor ejection is received (Smith & Richmond, 1972).Laboratory experiments conducted with captive short-eared owls, tawny owls, Strix aluco, great horned owls,Bubo virginianus, and barn owls have shown that theseraptors egest a pellet when presented with another fooditem (cf., Chitty, 1938; Smith & Richmond, 1972, but seeDuke et al, 1993). The minimum time elapsing before acaptive barn owl could be induced to egest a pellet by ad-ditional prey was about 6.5 hours (Smith & Richmond,1972). However, the presentation of a new prey at inter-vals of less than six hours acted to delay pellet egestionuntil the last prey item was digested (Smith & Richmond,1972). This may partly explain the small proportion ofnocturnal pellets found between December and February(e.g. 12% in 1993), a period in which barn owls would betheoretically expected to devote longer time to nocturnalhunting activity.

Finally, nocturnal pellets are relatively small and maybe overlooked sometimes even by experienced field-workers. We believe that the effect of a combination ofthe first, second and third factor may have caused con-siderable methodological difficulties, thus preventingbarn owl researchers from planning an experimentaldesign aimed at testing Guérin's theory successfully.

In conclusion, this study has shown that during theirnocturnal hunting activity barn owls eject pellets that aresignificantly smaller and lighter than those ejected duringtheir diurnal resting periods, thus confirming Guérin'stheory. In addition, we have found supporting evidenceindicating the existence of seasonal differences in thelinear dimensions and weight of both nocturnal and diur-nal pellets; we suggest that such differences may berelated to seasonal variations in the exploitation of the

most important food resources by barn owls, a relation-ship that will be analysed and discussed in a subsequentpaper (Pigozzi & Guidali, unpublished data).

REFERENCES

Balgooyen T. G., 1971 - Pellet regurgitation by captive SparrowHawks (Falco sparverius). Condor, 73: 382-385.

Bunn D. S., Warburton A. B., Wilson R. D. S., 1982 - The Barn Owl.Poyser, Calton, 264 pp.

Camacho I., Pleguezuelos J. M., 1980 - Variacion estacional de laalimentacion de Tyto alba (Scopoli, 1769) en la Vega de Granada.Trab. Monogr. Dep. Zool. Univ. Granada, 3: 29-44.

Camacho Munoz I., 1975 - La alimentacion de Tyto alba (Scop.) en laVega de Granada. Cuad. C. Biol., 4: 111-124.

Chitty D., 1938 - A laboratory study of pellet formation in the Short-eared Owl (Asio flammeus). Proc. zool. Soc. Lond., 108:267-287.

Cuisin J., Cuisin M., 1979 - Le regime alimentaire de la Chouette Ef-fraie (Tyto alba (Scopoli)) dans le canton des Riceys (Aube) et sesenvirons immediats. Oiseau et R.F.O, 49: 81-89.

Duke G. E., Jackson S., Evanson O. A., 1993 - Great Horned Owls donot egest pellets prematurely when presented with a new meal.J. Raptor Res., 27: 39-41.

Errington P. L., 1932 - Technique of raptor food habits studies. Con-dor, 34: 75-86.

Evans F. C., Emlen J. T. Jr., 1947 - Ecological notes on the prey selec-tion by a Barn Owl. Condor, 49: 3-9.

Géroudet P., 1965 - Les rapaces diurnes et nocturnes d'Europe.Delachaux & Niestlé, Neuchatel, 415 pp.

Glue D. E., 1967 - Prey taken by the Barn Owl in England and Wales.Bird Study, 14: 169-183.

Glue D. E., 1970 - Avian predator pellet analysis and the mam-malogist. Mammal Rev., 1: 53-62.

Goszczynski J., 1976 - Estimation of daily food ratio of Tyto alba un-der natural conditions. Pol. Ecol. Stud., 2: 95-102.

Guérin G., 1928 - La vie des chouettes. Régime et croissance de l'ef-fraye commune. Lechevalier, Paris.

Guérin G., 1932 - La vie des chouettes. II. La hulotte et son régime.Lassaud Fréres, Fontenay-Le-Comte, 242 pp.

Korgschen L. J., 1980 - Procedures for food-habits analyses. In: S. D.Schemnitz (ed.), Wildlife management techniques manual. TheWildlife Society, Washington, pp. 113-127.

Marti C. D., 1973 - Food consumption and pellet formation rates infour owl species. Wilson Bull., 85: 178-181.

Mikkola H., 1983 - Owls of Europe. Poyser, Calton, 397 pp.Nos M. R., 1961 - Estudio de los ovillos regurgitados por una pareja

de Tyto alba en la comarca de La Maresma (Prov. de Barcelona).Misc. Zool., 1: 139-146.

Petretti F., 1977 - Seasonal food habits of the Barn Owl (Tyto alba) inan area of central Italy. Gerfaut, 67: 225-233.

Philips J. R., Dindal D. L., 1979 - Decomposition of raptor pellets.Raptor Res., 13: 102-111.

Saint-Girons M. C., 1964 - Comparaison entre le regime du poussin etcelui de l'adulte chez Tyto alba. Oiseau et R.F.O., 34: 204-209.

Schmidt A., 1977 - Zur Enharungsokologie der Schleiereule, Tytoalba Scopoli. Beitr. Vogelkd., 23: 235-244.

Smith C. R., Richmond M. E., 1972 - Factors influencing pelletegestion and gastric pH in the barn owl. Condor, 84: 179-186.

Sorgo A., 1992 - Prehrana pegaste sove Tyto alba na Dravskem polju.Acrocephalus, 13: 166-173.

Ticehurst C. B., 1935 - On the food of the Barn-Owl and its bearingon Barn-Owl population. Ibis, 13: 329-335.

Tome D., 1992 - Prehrana pegaste sove Tyto alba Ljublianskembarju. Acrocephalus, 13: 33-38.

Walsh P. M., 1984 - Diet of Barn Owls at an urban Waterford roost.Irish Birds, 2: 437-444.

Webster J. A., 1973 - Seasonal variation in mammal contents of BarnOwl castings. Bird Study, 20: 185-196.

Zar J. H., 1984 - Biostatistical analysis. Prentice-Hall, EnglewoodCliffs, 718 pp.

Dow

nloa

ded

by [

Uni

vers

ity o

f St

rath

clyd

e] a

t 18:

04 0

6 O

ctob

er 2

014