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 studytook 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%). martins (Delichonurbica) to a greater extent than 248 ANDRZEJZALEWSKI VOL. 28, NO. 4

Table 1. Tawny owl diet compositionin the breedingseason (1 Februaryto 30 June) in 1988-90. (Cf = conversion factorin 'preyunits' [see text], N -- numberof individuals,%N = percentof preyspecimens, %B = percentof prey biomass.) Remains of insectswere not included in biomasscalculations.

URBAN SITE SUBURBAN SITE TOTAL

PREY SPECIES Cf N %N %B N %N %B N %N %B

Rana spp. 1.6 10 2.6 2.3 67 30.1 33.6 77 12.8 12.0 Pelobatesfuscus 0.9 1 0.3 0.1 9 4.0 2.5 10 1.7 0.9 Anura subtotal 11 2.9 2.4 76 34.1 36.1 87 14.5 12.9 Streptopeliadecaocto 10.0 12 3.2 16.9 ------12 2.0 11.7 Delichon urbica 0.8 14 3.7 1.6 ------14 2.3 1.1 Sturnusvulgaris 4.0 12 3.2 6.9 ------12 2.0 4.7 Turdus rnerula 4.0 2 0.5 1.1 2 0.9 2.5 4 0.7 1.6 Phoenicurusphoenicurus 0.8 2 0.5 0.2 ------2 0.3 0.1 P. ochruros 0.8 2 0.5 0.2 ------2 0.3 0.1 Erithacus rubecula 0.8 ------1 0.5 0.3 1 0.2 0.1 Parus major 1.0 13 3.5 1.8 4 1.8 1.3 17 2.8 1.7 P caeruleus 0.5 3 0.8 0.2 ------3 0.5 0.1 P palustris/ater 0.5 ------1 0.5 0.2 1 0.2 0.1 Pzcapica 9.5 1 0.3 1.3 ------1 0.2 0.9 Garrulusglandarius 8.5 ------1 0.5 2.7 1 0.2 0.8 Fringilla coelebs 1.0 ------2 0.9 0.6 2 0.3 0.2 Passer dornesticus 1.5 146 38.7 30.9 9 3.9 4.2 155 25.9 22.6 P rnontanus 1.0 27 7.2 3.8 2 0.9 0.6 29 4.8 2.8 Carduelis chloris 1.5 5 1.3 1.1 ------5 0.8 0.7 Serinus serinus 0.5 2 0.5 0.1 ------2 0.3 0.1 Ernberiza citrinella 1.2 1 0.3 0.2 ------1 0.2 0.1 Unidentified passerines 1.5 9 2.4 1.9 1 0.5 0.5 10 1.7 1.5 Aves subtotal 251 66.6 68.2 23 10.4 12.9 274 45.7 51.0 Talpa europaea 5.0 1 0.3 0.7 2 0.9 3.1 3 0.5 1.5 Sorex araneus 0.5 1 0.3 0.1 9 3.9 1.4 10 1.7 0.5 Eptesicusserotinus 1.2 1 0.3 0.2 ------1 0.2 0.1 Oryctolaguscuniculus 20.0 4 1.0 11.4 ------4 0.7 7.8 Mus rnusculus 0.8 34 9.0 3.8 4 1.8 1.0 38 6.3 3.0 Rattus norvegicus 5.0 1 0.3 0.7 1 0.5 1.6 2 0.3 1.0 Microrays rninutus 0.4 7 1.8 0.4 1 0.5 0.1 8 1.3 0.3 Apodernusagraflus 1.2 19 5.0 3.2 4 1.8 1.5 23 3.8 2.7 A sylvaticus 1.2 2 0.5 0.3 12 5.3 4.5 14 2.3 1.6 A fiavicollis 1.2 1 0.3 0.2 ------1 0.2 0.1 Apodernusspp. 1.2 14 3.7 2.4 21 9.4 7.9 35 5.8 4.1 Arvicola terrestris 4.5 1 0.3 0.6 ------1 0.2 0.4 Clethrionornysglareolus 1.2 9 2.4 1.5 13 5.8 4.9 22 3.7 2.5 Mzcrotus oeconornus 1.4 ------4 1.8 1.8 4 0.7 0.5 M. arvalis 1.4 19 5.0 3.7 52 23.3 22.8 71 11.8 9.7 M•crotusspp. 1.4 1 0.3 0.2 1 0.5 0.4 2 0.3 0.3 Mammals subtotal 115 30.5 29.4 124 55.5 51.0 239 39.8 36.1 Total vertebrates 377 223 600 Melolonthaspp. 28 -- 28 Carabidae 6 2 8 Geotrupesspp. -- 13 13 Coleoptera 3 6 9 Total invertebrates 37 21 58 DECEMBER 1994 DIET OF TAWNY OWLS 249

Table 2. Variation in tawny owl diet betweenthe three stagesof the breedingperiod in the urban and suburban sitesin 1988-90. (N = numberof individuals,%N = percentoccurrence of prey.)

16 MARCH tO 1 FEB to 15 MARCH 30 APRIL 1 MAY to 30 JUNE G-TEST PREY SPECIES N %N N %N N %N G P

Urban site Rana spp. 5 4.2 4 2.5 1 1.0 2.05 ns Pelobatesfuscus .... 1 1.0 -- Anuran subtotal 5 4.2 4 2.5 2 2.0 0.83 ns Parusmajor 7 5.8 6 3.8 -- -- 5.00 ns Passer domesticus 47 39.2 65 40.9 34 34.7 0.54 ns Other birds 19 15.8 34 21.4 39 39.8 11.79 b Birds subtotal 73 60.8 105 66.1 73 74.5 1.41 ns

Insectivora I 0.8 1 0.6 -- -- 0.27 ns Mus musculus 5 4.2 17 10.7 12 12.2 4.60 ns Apodemusspp. 21 17.5 11 6.9 4 4.1 10.09 b Microtusspp. 7 5.8 8 5.0 4 4.1 0.31 ns Other mammals 8 6.7 12 8.2 3 3.1 2.54 ns Mammal subtotal 42 35.0 50 31.4 23 23.5 2.40 ns Total prey 120 100.0 159 100.0 98 100.0 Number of pellets 61 86 76 Suburban site Rana spp. 50 35.0 14 28.0 3 10.0 15.49 c Pelobatesfuscus 4 2.8 1 2.0 4 13.3 12.37 c Anuran subtotal 54 37.8 15 30.0 7 23.3 3.44 ns Parusmajor 2 1.4 1 2.0 1 3.3 0.85 ns Passer domesticus 3 2.1 3 6.0 3 10.0 5.61 ns Other birds 3 2.1 5 10.0 2 6.7 7.69 ns Birds subtotal 8 5.6 9 18.0 6 20.0 9.79 b

Insectivora 4 2.8 3 6.0 4 13.3 7.87 a Mus musculus 2 1.4 1 2.0 1 3.3 0.85 ns Apodemusspp. 18 12.6 13 26.0 6 20.0 4.78 ns Microtusspp. 50 34.9 3 6.0 4 13.3 24.66 • Other mammals 7 4.9 6 12.0 2 6.7 3.36 ns Mammal subtotal 81 56.6 26 52.0 17 56.7 0.26 ns Total prey 143 100.0 50 100.0 30 100.0 Number of pellets 45 21 12

0.05. 0.01. 0.001. predictedfrom this species'abundance in the Chrob- to the diet. Among forest bird species,tawny owls ry residentialdistrict (D = -0.40; Table 3). The preferredthe Eurasian tree sparrow (D = 0.56) and yellowhammer(Emberiza citrinella), a speciesabsent tits (D = 0.53) which nested only in the forest. from the city of Torufi, was alsoa prey item, sug- Althoughthe chaffinch(Fringilla coelebs) was a dom- gestingthese owls alsohunted in nonurbanareas. inant speciesin the suburbanforest, it was not taken In the suburban site, house sparrows breeding in proportion to its availability by tawny owls (D closeto the forestcontributed the largestproportion = -0.41). The owlsdid not prey on chiffchaff(Phyl- 250 ANDRZEJZALEWSKI VOL. 28, NO. 4

Table 3. Proportionof birds in tawny owl diet (%N = percentby number) and their relative abundancein the bird communityin varioushabitats. (UGA = urban green areas [Zalewski and Przystalski1993], OCh • the Chrobry residentaldistrict [O•ga 1990], SF = suburbanforest [Zalewski 1994], + • occurringin the forestand breedingin the surroundingareas.)

URBAN SITE SUBURBAN SITE

SPECIES %N UGA OCH %N SF

Passer domesticus 58.2 + 37.7 39.1 + Passer montanus 10.7 2.9 1.6 8.7 2.4 Parus spp. 6.4 9.2 1.8 21.6 7.8 Fringilla codebs -- 10.3 1.2 8.7 19.1 Delichon urbica 5.6 -- 12.7 -- -- Streptopeliadecaocto 4.8 2.9 28.4 -- -- Sturnusvulgaris 4.8 19.5 2.8 -- 3.4 Turdus merula 0.8 0.6 0.4 8.7 9.0 Erithacus rubecula ------4.4 4.3 Carduelis chloris 2.0 1.7 0.4 -- 0.8 Phoenicurusphoenicurus 0.8 2.9 -- -- 0.8 P ochruros 0.8 -- 0.6 -- -- Pzcapica 0.4 2.3 0.6 -- -- Garrulusglandarius ------4.4 0.8 Serinus serinus 0.8 5.7 -- -- 3.5 Eraberiza citrinella 0.4 .... Phylloscopuscollybita .... 9.8 Sylviaatricapilla .... 6.2 Other passerines 3.5 42.0 11.8 4.4 32.1 Density (pairs/10 ha) 150.0 183.4 75.4 loscopuscollybita) and the blackcap(Sylvia atricap- al. 1991). Often, the alternative food source in the zlla), despitethe fact that they were the most abun- years of low rodent number was birds. This adapt- dant species(D = -1; Table 3). ability to alternativefood may enabletawny owls to penetrateareas with few rodentsbut many birds. DISCUSSION In Torufi, as in many other Europeancities, the In rural areas the diet of the tawny owl is com- major foodof tawny owls was birds (Manganaro et posedmainly of smallmammals (Bochefiski 1990, al. 1990, Galeotti et al. 1991, Goszczyfiskiet al. Goszczyfiskiet al. 1993). In towns,mammals are 1993), particularlythe housesparrow and other syn- lessimportant dietary components (Manganaro et al. anthropic species(collared dove and the housemar- 1990, Galeotti et al. 1991, Goszczyfiskiet al. 1993). tin). In urban areas house sparrowsare also im- However, in both these environmentsthe propor- portant prey for sparrowhawks (Accipiter nisus; tional contributionof mammals is lower in spring Frimer 1989) and long-eared owls (Asio otus; von and summerwhile higher in autumn and in winter Dathe 1988). A considerableincrease in avian prey (Bochefiski1990, Manganaro et al. 1990, Galeotti in tawny owl dietswas recordedin May and June, et al. 1991). In Torufi, betweenFebruary and June which coincidedwith arrival of migrants on their the proportionof Apodernusspp. (especiallyof A. breeding territories in spring. Similar increasesin agraflus)decreased. Similarly the proportionof Apo- the relative contribution of birds in the diet of these dernusspp. decreasedin the diet in other studiesof owls have been noted in other habitats (Gruzdev and tawnyowl diet in nonurbanenvironments (Southern Likhachev1960, Glutz and Bauer 1980, Manganaro 1954, Yalden 1985, Kirk 1992). Apart from seasonal et al. 1990, Kirk 1992). In Torufi, the population changes,annual variationin diet was recordedde- densityof housesparrows, a residentspecies, did not pendingon rodent availability (Gruzdev and Lik- changeduring this study. The densityof tits did hachev 1960, G/3rner and Kramer 1973, Galeotti et changebecause these birds were more frequent in DECEMBER 1994 DIET OF TAWNY OWLS 251

Y= 0.122X+ 29.2 to the owls' diet, as were larger birds suchas pigeons r=0.545 P=0.003 N=28 and ducks (Harrison 1960, Bogucki 1967, Galeotti 50- et al. 1991). Thus, becausethe tawny owl can adaptto preying on alternative food sources and can tolerate the close 40- presenceof humans, the speciesmay becomein- creasinglyabundant in towns.

ACKNOWLEDGMENTS 30- I am indebtedto Z. Pucek and B. J•drzejewska for their commentson an earlier draft. Reviews by S.J. Petty and D. Kirk are gratefully acknowledged.

ß LITERATURE CITED 20't t t t t t 0 20 40 60 80 100 BOCHEIqSKI,JUN. Z. 1990. The foodof suburbantawny Percent of birds in diet owls on the backgroundof birds and mammalsoccur- ring in the hunting territory. Acta Zool. Cracov.33: Figure 1. Relationshipbetween the meanprey sizeand 149-171. proportionof birdsin tawnyowl dietsin Europe(Schnurre B/3HME,G. 1977. Zur BestimmunEquartSrer Anuren 1961, Gbrner and Kramer 1973, Glutz and Bauer 1980, Europasan Hand von Skelettelmenten.Wiss. Z. Hum- Wendland 1980, Yalden 1985, Cervenyand Obuch 1988, boldt-Univ. Berl. Math.-Naturwiss. Reihe 26:283-300. Bochefiski1990, Manganaro et al. 1990,Wasilewski 1990, BOGUCKI,Z. 1967. On the foodof the tawny owls (Strzx Gramsz 1991). Only studieswith at least 200 prey items alucoL.) nestingin the city of Poznafi. Przegl. Zool were included. 11:71-74. CERVENY,J. AND J. OBUCH. 1988. Small mammals in Torufi in Februaryand March than in later stages a diet of tawny owls in Sumavapiedmont. Lynx 24:5- 14. of the breedingseason (D. Czeszczewikpets. comm.). FRIMER, O. 1989. Food and predationsin suburban However, in the suburbanforest their importance sparrowhawksAccipiter nisus during the breedingsea- increasedin the diet from April to June. son. Dan. Ornitol. Foren. Tidsskr. 83:35-44. In suburbanforests, Eurasian tree sparrowsand GALEOTTI, P., F. MORIMANDO AND C. VIOLANI. 1991 tits were preferredby the owls, and were frequent Feeding ecologyof the tawny owls (Strix aluco) in •n this owl's diet in other Europeanareas (Schnurre urbanhabitat (northernItaly). Boll.Zool. 58:143-150 1961, Wendland1980, Bochefiski1990, Manganaro GLUTZ VON BLOTZHEIM, U.N. AND K. BAUER. 1980. et al. 1990). Interestingly,tawny owlsdid not prey Strix alucoLinnaeus 1758--Waldkauze. Pages579- on chaffinchesalthough this speciesis dominant in 610 in Handbuchder Vfgel Mitteleuropas.Akadem- many forest areas (e.g., in the suburbanforest it ischeVerlagsgesellschaft, Wiesbaden, Germany. GORNER,M. ANDV. KRAMER. 1973. Ergebnisseeiner constitutes19.2% of the bird community).The chaf- elfjShrigenNahrungskontrolle des Waldkauzes (Strix finchhas beenthe predominantprey speciesof the alucoL.) im Gebiet desKottmar bei Ebersbach(Lau- tawny owl in only a few places(Southern 1954). sitzer Bergland). Abh. Bet. Naturkundemus.Gd•litz 48 Becausetawny owlsrely on hearingto locateprey 1-8. (Southern1954, Mikkola 1985), highernoise levels GOSZCZYiqSKI,J., P. JABIdOiqSKI,G. LESIiqSKIAND J in urban habitatsmay interfere with prey capture. ROMANOWSKI.1993. Variation in diet of tawny owls Also, becausethere are fewer rodents in town, the Strix aluco L. along an urbanization gradient. Acta owlsfocus mainly on birds.Birds are probablymore Ornithol. 27:113-123. difficultto catchthan the majorityof nocturnal,ter- GRAMSZ,B. 1991. Tawny owls Strix alucodiet in oak horbean forest near Ol•awa (south-westernPoland) restrial, and vocally noisy small mammals. Other Acta Ornithol. 26:3-13. dietary studiesof tawny owls in Europe showthat GRUZDEV, L.V. AND G.N. LIKHACHEV. 1960. Contri- increasingproportions of birds were correlatedwith bution to feedinghabits of Strix alucoin the Tula Za- an increasein mean size of prey items (r = 0.545, seki. Zool. Zh. 39:624-627. P -- 0.00•, Fig. 1). Hunting large prey may be an HARRISON,C.J.O. 1960. The foodof someurban tawny energeticcompensation for diminishedhunting time. owls. Bird Study 7:236-240. In Torufi, wild rabbitswere an importantaddition JACOBS,J. 1974. Quantitative measurementsof food 252 ANDRZEJZALEWSKI VOL. 28, No. 4

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