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<I>STRIX ALUCO</I> j Raptor Res. 28(4):246-252 ¸ 1994 The Raptor ResearchFoundation, Inc. DIET OF URBAN AND SUBURBAN TAWNY OWLS (STRIX ALUCO) IN THE BREEDING SEASON ANDRZEJ ZALEWSKI Mammal ResearchInstitute, Polish Academy of Sciences,77-230 Biatowieœa, Poland ABSTI•CT.--The diet of tawny owls (Strix aluco)was studiedduring the breedingseasons 1988-90 in an urbanand a suburbanarea in Torufi, Poland.Two amphibian,18 bird,and 14 mammalspecies were recordedas prey in a sampleof 312 pellets.From 600 prey itemsfound in bothsites, the housesparrow (Passerdomesticus) was the mostfrequently taken bird prey and the commonvole (Microtus arvalis) the mostfrequently taken mammal prey. Significantlymore mammals than birdswere taken at the suburban sitethan at the urbansite (P < 0.001). At the urbansite, the proportionof birds(except for tits [Parus spp.]and housesparrows) increased over the courseof the breedingseason, while the proportionof Apodemusspp. decreased.Similarly, at the suburbansite the proportionof all birds increasedand the proportionof Microtusspp. decreased. House sparrows at the urban site and Eurasiantree sparrows (Passermontanus) and tits at the suburbansite were takenin higherproportion than their availability. An examinationof dietarystudies from elsewhere in Europeindicated that therewas a positivecorrelation betweenmean prey sizeand increasingproportion of birdsin tawny owl diets(P = 0.003). KEY WORDS: tawnyowl; diet;urban and rural area;bird selection;prey-size selection. Dieta de Strixaluco urbano y suburbanoen la estaci6nreproductiva RESUMEN.--Seestudi6 la dietade Strixaluco durante las estacionesreproductivas de 1988 a 1990 en un fireaurbana y suburbanaen Torun, Polonia.Dos antibios, 18 avesy 14 especiesde mam•ferosfueron registradoscomo presa en una muestrade 312 egagr6pilas.De 600 categorlasde presasencontradas en ambossitios, Passer domesticus rue el ave-presamils frecuentejunto al mfimfferoMicrotus arvalis. Se consumieronsignificativamente m•ts mamfferos que avesen el sitio suburbanoqueen el urbano (P < 0.001). En el sitiourbano, la proporci6nde aves(excepto para Parus y P. domesticus)increment6 a medida que transcurrlala estaci6nreproductiva, mientras que la proporci6nde Apodemusspp. disminuy6. Si- milarmente,en el sitiosuburbano la proporci6nde todaslas avesincrement6 y la proporci6nde Microtus spp.disminuy6. Tanto P. domesticusen el sitiourbano como Passer montanus y Parusspp. en el sitio suburbanoœueron consumidos en una proporci6nmaor a su disponibilidad.Un examende estudios dietariosen Europaindic6 que hubouna correlaci6npositiva entre le tamafiopromedio de presasy el incrementode la proporci6nde avesen la dieta de bfihos(P = 0.003). [Traducci6n de Ivan Lazo] Increasingnumbers of animal speciesare adapting breedingperiod, and diet changesin relation to bird to urban environments;among birds, omnivorous prey availability in both study areas. and granivorous speciesmost frequently inhabit towns(Tomialoj6 and Profus 1977, Zalewski ! 994). STUDY AREA AND METHODS Predatory birds are also increasinglycolonizing ur- The study took placein and around the city of Torufi, ban areas, and thus becomeimportant links in the centralPoland (53ø01'N, 18ø35'E). The humanpopulation urban food webs. Such birds have to either find a of Torufi is 200000 and the city coversan area of 115.8 habitat containingnatural food resourcesor change km2. Forests of pine(Pinus sylvestris) predominate on sandy soil around the city. Oaks (mainly Quercusrobur) and their feeding ecology.The tawny owl (Strix aluco) birches(Betula verrucosa)form admixturesin the pine is an example of a polyphagousspecies (Mikkola forest. 1983), that caninhabit many environmentsand adapt The urban site consistedof a park (2.1 ha) and a cem- to preying on the most abundant species. etery (3.7 ha) locatednear the city center. The eastern sideof the urban area joined a small villa district,and on In this paper I comparetawny owl diets during the northern side it was borderedby an open grassland the breeding seasonbetween an urban and a sub- area with residentialestates behind it (the Chrobry resi- urban area. I focuson diet changesthroughout the dential district). The suburban site (17 ha) was located3 246 DECEMBER 1994 DIET OF TAWNY OWLS 247 km from the city center.It was a 60-70-yr-old pine forest Significantly more mammals than birds were taken with deciduoustrees (black alder [Alnusglutinosa], maple at the suburban site than at the urban site (G = [Acersp.], and horsechestnut[Aesculus hippocastanum]) and a rich shrub layer. A small stream flowed through this 125.64, df = 1, P < 0.001). These were mainly site which resembled the forest surrounding Torufi. commonvoles (Microtus arvalis) and Apodemusspp. On the easternside it was flanked by buildings,and on which together comprised39.8% prey by numbers the westernside by a large open grassarea. and 36.7% of the prey biomass.At the urban site, Tawny owl pellets were collectedbetween February mammalscomposed only 30.5% of tawny owl prey, and June in 1988, 1989, and 1990 in the urban site, and in 1989 and 1990 in the suburbanone. At bothsites, pellets and rabbits were also important food items (11.4% were collectedfrom one or two breedingpairs of owls in of the prey biomass).Amphibians were not a sig- the sameterritories in eachyear. Thesepellets were gath- nificant componentof the owls' diet, especiallyin ered regularly from roostsof adults and young owls and town. Invertebrateswere recordedin the diet during also near nests.The pellets were analyzed by standard methods.Prey remains were identified using keys for the studyat both sites(Table 1). The mean weight mammals (Pucek 1981), birds (Moreno 1985, 1986), and of prey in the urban siteswas 37.6 g, and 28.6 g in amphibians(BShme 1977). The numberof vertebrateprey the suburban site. were calculatedseparately for eachsample of pelletsfound Little changeoccurred in the diet compositionof at the sametime on the basisof skulls,jaws, or other bone elements. Insects were identified and counted based on tawny owls at the urban site through the breeding exoskeletonremains. For each site the frequencyof oc- season(Table 2). However, the proportion of Apo- currenceand percentagebiomass were calculated.Con- demus spp. decreasedfrom the first stage of the versionfactors were applied (Table 1), using20 g as the breedingseason (1 February to 15 March) to the standardweight for smallmammals (Southern 1954). Thus, next two stages.Likewise, the proportionof tits and total weightof prey was convertedto "prey units." Pellets mammalsdecreased. By contrast,except for tits and were collectedduring three periods:(1) egg laying and hatching (from 1 February to 15 March); (2) nestling sparrows, the proportion of birds and house mice (from 16 March to 30 April); and (3) fledgling--when (Mus musculus)increased, although in the latter case youngowls were fledgedbut still dependenton their par- to a minor degree. ents (from 1 May to 30 June). This divisionwas based Amphibians from the genusRana were important on observationsof breedingand youngtawny owls around torufi. The G-testwas usedto testthe proportionsof prey in the diet of the suburbanowls in March and April items in the owls' diet (Sokal and Rohlf 1981). For both when they migratedfrom the placesof hibernation. studysites, the proportionof eachbird speciesin the diet In later months,their frequencydecreased (Table was then comparedwith its relativeabundance in the bird 2). In contrast,the contributionof spadefoottoads community(O•ga 1990, Zalewski and Przystalski1993, increasedas the breeding seasonprogressed, asso- Zalewski 1994) closeto where the pellets had been col- lected. In all the sites,breeding birds were censusedby ciated with this species'shift to terrestrial life •n the territorial mappingmethod. In urban greenareas cen- May and June (Juszczyk1987). The proportionof suseswere carried out in 1988-89, in the Ghrobry district birds increasedgreatly in the later stagesof the in 1989, and in the suburban forest in 1989-90. breedingseason, although the increasein housespar- rows was minor. The proportion of insectivorous RESULTS mammals,especially the commonshrew (Sorexara- In the urban site 223 pelletswere found (30, 96, neus),also increased. The frequencyof Microtusspp., and 97 in consecutiveyears). In the suburbanforest however,dropped noticeably from February to June. 89 pelletswere collected(11 and 78 in consecutive Nevertheless,throughout the study the proportion years).In all, 18 speciesof birds, 14 speciesof mam- of mammals in the diet was invariably above 50%. mals, and two speciesof amphibians(common frog Bird Prey. Comparisonof the proportionsof bird [Rana temporaria],and spadefoottoad [Pelobatesfus- speciesin the tawny owl diet with their relative cus]) were recorded. availability indicatedthat the most abundant bird Comparisonof Owl Diets Betweenthe Urban specieswere taken (Table 3). Ivlev's electivityindex and Suburban Areas. At the urban site,tawny owls (modifiedby Jacobs1974) was calculatedto quantify preyedmainly on birds (66.6% by numbersand 68.2% tawnyowls' selectiveness for a few bird species.House by biomass).The mostimportant prey were the house sparrowswere clearly preferred by the owls in the sparrowand the Eurasiantree sparrow(Table 1). urban site (D = 0.39). Eurasian tree sparrowscom- Largerbirds such as the collareddove (Streptopelia prised a large proportionof the diet, althoughthey decaocto)and Europeanstarling (Sturnusvulgaris) were not commonbirds. Tawny owlspreyed on house contributeda high percentageby biomass(23.8%).
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