Breeding Distribution and Diet Composition of the Barn Owl Tyto alba (SCOPOLI, 1769), (Aves:...

1 University of Forestry - Wildlife Management Department, 10, Kl. Ohridski Blvd., 1756 Sofia,Bulgaria, E-mails: boyan.m@abv.bg, ndkodjak@abv.bg2 National Museum of Natural History, 1, Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria,E-mail: boev@nmnh.bas.bg

ACTA ZOOLOGICA BULGARICA

Acta zool. bulg., 58 (1), 2006: 83-92

Breeding Distribution and Diet Composition of theBarn Owl Tyto alba (SCOPOLI, 1769), (Aves:Strigiformes) in the North-Western Upper ThracianPlain (Bulgaria)

Boyan Milchev1 , Zlatozar Boev2 , Nikolay Kodjabashev1

Abstract: The Barn Owl (Tyto alba gutatta) is a relatively widespread breeding bird in thesettlements of the north-western part of the Upper Thracian Plain (91% of visited UTMsquares of 10-km grid, n=11). The small mammals predominate in the diet, having 100%occurrence and forming 97% by the number and biomass. Birds, amphibians and reptilesare a negligible part of the food. The voles (Microtus spp.) are the most frequent prey (32%of number) and form the larger contribution to the biomass (54%). These, the white-toothed shrews (Crocidura suaveolens and C. leucodon) and the mice (Mus spp.) reach up to86% by number and 88% by the biomass. The average food-niche breadth in the studiedlocalities is 3.31± 0.57 (n=5). It correlates positively with the wetland area within thehunting territory and with the proportions in the diet of Miller’s Water Shrew (Neomysanomalus), Harvest Mouse (Micromys minutus) and Water Vole (Arvicola terrestris), inhabit-ing these areas. The importance of the voles (Microtus spp.) in the diet increases with theelevation of localities above the sea level and causes a narrowing of the food-niche.

Key words: Barn Owl, Tyto alba, breeding distribution, diet composition, Bulgaria

Introduction

The first purposeful research on the breeding distribution of the Barn Owl in Bulgariaproved that the species is unexpectedly more widely spread in the south-eastern parts ofthe country, than formerly estimated (MILTSCHEV et al. 2002). The main reason for thedata scarcity in the remaining parts of the country may result from the difficulties in itsrecording during routine ornithological field studies due to its secret daytime mode oflife. The examination of its diet is extremely useful for the elucidating of the dependen-cies among the preferred preys and the geographical location, the hunting territory’speculiarities and the local assemblages of small mammals, as well as for obtaining ofmore detailed data on the spatial distribution of these preys (MIKKOLA 1983, GLUTZ VON

BLOTZHEIM, BAUER 1991, TAYLOR 1994, BRUCE 1999, ALASDAIR LOVE et al. 2000,BONVICINO, BEZERRA 2003, GOUTNER, ALIVIZATOS 2003, ROULIN 2004, POPOV et al.2004). The Barn Owl’s diet has been examined so far only in the eastern part of the

country, having more complete data only for South-East Bulgaria, where it was foundthat the mice (Mus spp.) and the white-toothed shrews (Crocidura spp.) dominate theprey spectra (SIMEONOV 1978, SIMEONOV et al. 1981, MILTSCHEV et al. 2004).

The present study aims to examine the breeding distribution and the diet composi-tion of the Barn Owl in the north-western part of the Upper Thracian Plain, a regionwith no data for this species until now. On the other hand, analysis of its diet will enrichour knowledge for the distribution of many species of small mammals in this region.

Study Area

The study has been carried out in the north-western part of the Upper Thracian Plainand the neighbour foothills of the Sredna Gora Mountains, covering an area of ca. 500sq. km of 11 squares of 10-km grid of the UTM system. The study area limits reach N42° 14' E 24° 11' southwards, N 42° 19' E 24° 02' westwards, N 42° 21' E 24° 13'northwards, and N 42° 12' E 24° 30' eastwards. The altitude varies from 150 m a. s. l.in the southern parts to the 370 m a. s. l. in the northern, and to the 410 m a. s. l. in thewestern parts. The region lies in the transitional climatic zone between the moderate-continental (northern and western parts) and the continental-Mediterranean (southernand eastern parts) zones (YORDANOVA, DONTCHEV 1997). The plain and hilly landscapehas agricultural and arable lands and low-exploited pastures. The formerly well-devel-oped ameliorative system is now abandoned and is out of use during the last 14 years.The farmlands are cropped mainly with wheat and barley. The vineyards have largercover in the western and the norten parts of the region. The woods are broad-leavedand have insignificant area, predominantly preserved along the river beds. The urbanterritories are represented by villages and only larger plant is the copper-mining complexin the norten part of the stydy area.

Material and Metods

The purposeful search of the Bran Owl has been carried out only in the farm buildings,but it covers 20 villages and a remote stock-breeding complex visited on 17-18.09.2004.The species’ breeding has been categorised “certain” after the numerous pellets, depos-ited in suitable nesting grounds, where some eggshells and feathers of adult moultingbirds have been collected. “Probable” breeding has been defined after the fresh prima-ries and rectrices of the adult moulting birds among the roosting and loafing places.“Possible” breeding has been accepted for the localities, where a small number of freshand old pellets with no any feathers of moulting birds have been found in the roostingand loafing sites, which obviously are visited seldom and lie far from the nest.

The food remains in and around the nests and the roosting places in twelve nestinglocalities include both well-preserved and destroyed pellets possibly deposited duringseveral breeding seasons. The separately analyzed five localities contain over 300 preys,forming 94% of the total number of preys (n=4842 individuals). The samples havingsmaller number of preys were not examined separately in ordеr to avoid the influence ofthe fact that they came of one season and possibly a single bird had rejected the pellets.

The mammal species determination is after POPOV, SEDEFCHEV (2003) and ourcomparative osteological collection. Because of the difficulties in species determinationof Sylvaemus sylvaticus - Sylvaemus flavicollis and Microtus arvalis - Microtus

rossiaemeridionalis, these species pairs have been presented as Sylvaemus spp. andMicrotus spp., respectively. Both forms (semispecies) of house mouse, Mus musculusdomesticus (SCHWARZ, SCHWARZ, 1943) and M. m. musculus (LINNAEUS, 1758), weredetermined according to morphological keys proposed by MACHOLAN (1996). Bothforms probably occur in the material and their data have been considered at specieslevel. The birds have been identified by their bone remains, using the comparativeosteological collection of Fossil and Recent Birds Department of the National Museumof Natural History, BAS. The amphibian remains have been determined after ENGEL-MANN et al. (1985) and MДRZ (1987). The feathers have been determined by J. Menzel,and grasshoppers (Ortoptera) by D. Chobanov. The minimum number of individuals(MNI) of vertebrates has been estimated mainly on the basis of the remains of craniaand cranial fragments, mandibles, and pelvic bones, while the MNI of invertebrates isgiven after the representations of the elytra, the head-capsules and mandibles. The bodymass of the prey species is given according to GLUTZ VON BLOTZHEIM, BAUER (1991)and POPOV, SEDEFCHEV (2003). In calculating the mass of Water Vole (Arvicola terrestris),Black Rat (Rattus rattus) and Common Rat (R. norvegicus) we take into account theresults of GARDE, ESCALA (1993) and ZAMORANO et al. (1986) showing that the BarnOwl prefers to catch younger individuals with smaller weight. Undetermined passerinebirds have been divided into five weight-groups: size of Warbler (Phylloscopus sp.), BlueTit (Parus caeruleus LINNAEUS, 1758), Great Tit (Parus major LINNAEUS, 1758), HouseSparrow (Passer domesticus) and Red-backed Shrike (Lanius collurio LINNAEUS, 1758).

In defining the percentage cover of the habitats within the breeding pairs’ huntingterritory we accepted 1 km radius around the nest after TAYLOR (1994). Four types ofhabitats have been recorded: woodlands and scrublands (5.32% ± 6.21 of the huntingterritory, n=5), open land areas (pastures, agricultural and arable lands) (72.29% ± 7.64),wetlands (open water area and vegetated water lots) (1.19% ± 0.74) and urban settledterritories (settlements, separate buildings and main road network) (21.20% ± 11.32). Allhave been estimated through the maps of the local area of 1:25000 scales.

The food niche breadth (FNB) has been calculated as follows:

where pi is the proportion of prey category i in the Barn Owl diet (LEVINS 1968).

In order to obtain results, comparable to these of MARTI (1988), mammals have beenclassified up to genera, while birds, amphibians and insects have been classified up toclass level.

Overlapping (O) of the food spectra in the various localities was calculated throughthe formula:

∑ ∑∑=

where pi is the proportion of prey type i in one dietary sample, and q

i is the

proportion of the same type in the other dietary sample (PIANKA 1973). The bird

specimens which remained undetermined to species level in the calculation were notincluded in the lists. Undetermined to species level mice (Mus sp.) have been dividedbetween both forms according to the established proportion in the examined locality.Vertebrates were not categorized in broader systematic categories, while insect speci-mens were grouped into orders. The values were multiplied by 100 and the overlapvalues were given in percentages.

The dependences between the habitat characteristics of the hunting territory, thefood niche breadth and the proportions of the preys in the diet were tested by thePearson’s correlation, based on log-transformed data. All means are arithmetic mean ±standard deviation.

Results

Breeding Distribution and Location of the Nests

Barn Owl breeding was recorded in eleven villages and in a stockbreeding complex(57 % of the visited sites, n=21). These localities lie within ten UTM-grid squares of the(91 % of the visited, n=11): confirmed breeding - five localities (squares KG 67. 68. 88.89. 99); probable breeding - five localities (squares KG 69. 78. 79. 98. 99); and possiblebreeding - two localities (squares KG 58. 79). The subspecies Tyto alba guttata BREHM,1931 was found to breed in the region. Its breeding has been proved after the sixobserved individuals and the collected primaries from the confirmed breeding andpossible breeding localities. Only once an individual of T. a. alba (SCOPOLI, 1769) wasobserved resting at the entrance of the nesting place in square KG 99 on 17.09.2004.

All the five nests were situated in abandoned storage or stock-breeding buildings,very seldom visited by people. Two of these nests were on the ceiling under the coverconstruction, other two - on the ceiling of the intermediate floor in a one-store building,and one nest was located on a half-meter wide concrete beam at 4 m height. Themissing doors or window cases or parts of the walls and the building covers supplied theaccess to the nests and the roosting sites.

Diet Composition

General Diet Characteristics

The small mammals constituted the main prey of the Barn Owl. They have 100 %occurrence and form 97% by number and by biomass (Table 1). The white-toothedshrews (Crocidura suaveolens and C. leucodon), voles (Microtus spp.) and mice (Musspp.) form 86% by number and 88% by biomass. The voles (Microtus spp.) representthe most numerous prey (32% by number) and have the largest contribution to thefood biomass (54%). Despite of the large species diversity of birds in the diet (16 generawith more than 17 species), their share is only 2% by number, 3% by biomass, and 42% by occurrence. Only the House Sparrow has over 0.5% by number and biomass.This species, together with the Starling (Sturnus vulgaris), the Tree Sparrow (Passermontanus) and the Corn Bunting (Miliaria calandra), has a local importance in onesample only. The amphibians and the insects form a negligible part in the diet (< 1%).Among the beetles the dung-beetles (Scarabaeidae - 33 % by number) predominate andamong the orthopterans - the mole-cricket (Grylotalpa sp. – 54% by number). Themean prey weight is 20.1 g.

Within-Population Comparisons of the Diet

Five small mammal species form 80% by number (Table 1), and the white-toothedshrews (Crocidura spp.) are the most numerous victims in three localities, while the voles(Microtus spp.) are the most numerous in two localities. Five small mammal species andthe House Sparrow make up about 80% by biomass and the voles (Microtus spp.) formthe bulk of the biomass of the five localities (56% ± 13). There are no significantcorrelations between the proportions of the most numerous preys across the preyspectra. Only the correlation between the voles (Microtus spp.) and the white-toothedshrews (Crocidura spp.) is close to significance (n=5; Pearson r=-0.851; P>0.05).Significant correlations between the hunting territory characteristics and the prey-pro-portions in the diet have been established between the percentage cover of the wetlandsand the share of some wet-loving species such as Miller’s Water Shrew (Neomysanomalus), Harvest Mouse (Micromys minutus) and Water Vole (n=5; Pearson r=0.886;P<0.05). A significant negative correlation was found between the share of the Mice(Mus spp.) and the area of the built-up territories (n=5; Pearson r=-0.960; P<0.01).The altitude of the localities correlates positively with the proportion of the voles(Microtus spp.) (n=5; Pearson r=0.941; P<0.05) and negatively with the proportion ofthe wetland dwellers (n=5; Pearson r=-0.881; P<0.05).

The food-niche overlap ranges from 59% between the localities 1 and 4 to 99%between the localities 2 and 5 (87% ± 11; n= 5). The food-niche breadth varies from2.43 to 3.83 (3.31 ± 0.57; n=5). It correlates positively with the wetland area in thehunting territory (n=5; Pearson r=0.979; P<0.01) and with the share of the above-mentioned three wet-loving species, (n=5; Pearson r=0.934; P<0.05). The negativecorrelation of food-niche breadth with the share of the voles (Microtus spp.) in the foodspectra is nearly significant (n=5; Pearson r=-0.843; P>0.05).

Discussion

Breeding distribution and location of the nests

The Barn Owl is a relatively widespread breeding bird in the settlements of the north-western parts of the Upper Thracian Plain. We do not evaluate this fact as a result ofexpanded species range or increased population density, since no special investigations inthese respects have been carried out so far. This result is similar to the obtained forSouth-East Bulgaria using the same method of special searching for the species in thesettlements in open cultural landscape (MILTSCHEV et al. 2002). The characteristics ofthe breeding sites correspond to those in other parts of Europe and the country (GLUTZ

VON BLOTZHEIM, BAUER 1991, TAYLOR 1994, MILTSCHEV et al. 2002). Tyto alba guttatabreeds in the study territory. The observation of one bird, belonging to the subspecies T.a. alba refers to the period of the earliest autumn passage of migrating Barn Owls inBulgaria (SIMEONOV et al. 1991). That is why we do not consider its breeding in theregion proved, although the breeding of this subspecies was established in a locality,situated at some 200 km eastwards (GEORGIEW 1998, MILTSCHEV et al. 2002).

Diet Composition

General Diet Characteristics

The voles (Microtus spp.), the white-toothed shrews (Crocidura spp.) and the mice (Musspp.) are the basic food of the Barn Owl in the present study. This coincides with the

results, obtained for South-East Bulgaria (MILTSCHEV et al. 2004). The food-spectrumoverlap between both regions is 96%. The proportion in the prey-representation in thediet follows their accessibility and the preferences of the Barn Owl. This owl catchesvoles with a great appetence, and this preference is particularly characteristic of thesubspecies Tyto alba guttata (MIKKOLA 1983, GLUTZ VON BLOTZHEIM, BAUER 1991,TAYLOR 1994, ROULIN 2004). Thus, in the conditions of the more humid climate in theexplored region, compared to that of South-East Bulgaria (YORDANOVA, DONTCHEV

1997), where Microtus spp. shows higher population density (STRAKA, GERASIMOV 1977),the voles replace the mice (Mus spp.) by the number in the diet. The white-toothedshrews (Crocidura spp.) represent a significant part of the Barn Owl’s diet in thesouthern parts of Europe (SCHMIDT 1973, ROULIN 2004). Both species (Crocidurasuaveolens and C. leucodon) have been more often hunted than voles, and their share inthe food (33% by number) is close to that in South-East Bulgaria (35% by number,MILTSCHEV et al. 2004). Birds, amphibians and insects represent insignificant compo-nents of the diet, another similar result to that for South-East Bulgaria (MILTSCHEV etal. 2004). The House Sparrow is also represented by a much smaller share in the food,compared to the earliest studies in Bulgaria (SIMEONOV 1978, SIMEONOV et al. 1981). Itbecomes a less-profitable victim for the Barn Owl. This situation probably correspondsto the species’ population decline as a result of the changes of the agricultural activitiesand the crop-storage after the restoration of the private property of land after 1991.

Within-Population Comparisons of the Diet

The composition and structure of the small mammals assemblages determines the BarnOwl diet to a great extent, having in mind that the bird is an opportunistic predator,catching the most profitable preys. The pairs could have different proportions of theprey in the diet, according to the food supply, even if their hunting territories are notapart from each other, but including different microhabitats (MIKKOLA 1983, MARTI

1988, GLUTZ VON BLOTZHEIM, BAUER 1991, TAYLOR 1994, BRUCE 1999, GOUTNER,ALIVIZATOS 2003). The present study confirms the importance of the habitat diversitywithin the hunting territory. The shares of the Miller’s Water Shrew, Harvest Mouse andWater Vole increase with the increasing of the wetland area. As a result a generalwidening of the food-niche is recorded (MILTSCHEV et al. 2004). On the contrary, themore uniform landscape determining the stronger domination of voles (Microtus spp.)causes a narrowing of the niche. While mice (Mus spp.) and white-toothed shrews(Crocidura spp.) are alternative victims in South-East Bulgaria (MILTSCHEV et al. 2004),in the present study this category is formed by voles (Microtus spp.) and white-toothedshrews (Crocidura spp.). Diversified topography at higher altitudes causes an increasingof humidity, which in turn affects positively the abundance of voles (Microtus spp.). Theonly specimen of the Pygmy Shrew (Sorex minutis) was caught in the highest locality(KG 79 - 370 m a. s. l.), while in one of the lowest localities a specimen of the PygmyWhite-toothed Shrew was caught. The proportion of the urban settled territories in theBarn Owl hunting area has not significant correlations to the shares of the forms ofmice (Mus spp.) separately, but it shows a highly significant negative correlation to theirtotal number in food. It is difficult to interpret this unexpected result because of theabsence of data on characteristics of the small mammals’ assemblages.

Acknowledgements: We thank Mr. J. Menzel and Mr. D. Chobanov for the identification ofsome of the food remains and Dr. V. Popov for the useful remarks, which improved the formerversion of the manuscript. Mr. J. Menzel is thanked also for the financial support for this study.

Received: 22.03.2005Accepted: 27.10.2005

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Гнездово разпространение и храна на забуленатасова Tyto alba (SCOPOLI, 1769), (Aves: Strigiformes) всеверозападната част на Горнотракийскатанизина (България)

Б. Милчев, З. Боев, Н. Коджабашев

(Резюме)

Забулената сова (Tyto alba gutatta) e сравнително широко разпространенагнездяща птица в селищата в северозападната част на Горнотракийскатанизина (91 % от посетените UTM квадрата при 10-км грид, n = 11). Нейнитеосновни жертви са дребните бозайници, които са с 100 % срещаемост иформират 97 % от броя и биомасата на храната. Птиците, земноводните инасекомите са незначителна част от храната. Полевките (Microtus spp.) санай-често улавяната жертва (32 %) и са с най-голям принос за биомасата нахраната (54 %). Те заедно с белозъбките (Crocidura suaveolens и C. leucodon) имишките (Mus spp.) са 86 % от броя и 88 % от биомасата. Средната ширинана хранителната ниша в отделните находища е 3.31 ± 0.57 (n= 5). Тя корелираположително с площта на влажните зони в ловната територия и пропорциитев храната на обитаващите ги малка водна земеровка (Neomys anomalus),оризищна мишка (Micromys minutus) и воден плъх (Arvicola terrestris). Значениетона полевките (Microtus spp.) в диетата нараства с надморската височина нанаходищата и това води до стесняване на хранителната ниша.

Breeding Distribution and Diet Composition of the Barn Owl Tyto alba (SCOPOLI, 1769), (Aves:...

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