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DIET CHANGES in BREEDING TAWNY OWLS &Lpar;<I>Strix Aluco</I>&Rpar;

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DIET CHANGES in BREEDING TAWNY OWLS &Lpar;<I>Strix Aluco</I>&Rpar; j Raptor Res. 26(4):239-242 ¸ 1992 The Raptor ResearchFoundation, Inc. DIET CHANGES IN BREEDING TAWNY OWLS (Strix aluco) DAVID A. KIRK 1 Departmentof Zoology,2 TillydroneAvenue, University of Aberdeen, Aberdeen,Scotland, U.K. AB9 2TN ABSTVO,CT.--I examinedthe contentsof Tawny Owl (Strix alucosylvatica) pellets, between April 1977 and February 1978, in mixed woodlandand gardensin northeastSuffolk, England. Six mammal, 14 bird and 5 invertebratespecies were recordedin a sample of 105 pellets. Overall, the Wood Mouse (Apodemussylvaticus) was the most frequently taken mammal prey and the House Sparrow (Passer domesticus)was the mostfrequently identified bird prey. Two typesof seasonaldiet changewere found; first, a shift from mammal prey in winter to bird prey in the breedingseason, and second,a shift from smallprey in the winter to medium-sized(> 30 g) prey in the breedingseason. Contrary to somefindings elsewherein England, birds, rather than mammals, contributedsignificantly to Tawny Owl diet during the breedingseason. Cambiosen la dieta de bfihosde la especieStrix alucodurante el periodode reproduccibn EXTRACTO.--Heexaminado el contenidode egagrbpilasdel bfiho de la especieStrix alucosylvatica, entre abril de 1977 y febrerode 1978, en florestasy huertosdel norestede Suffolk, Inglaterra. Seismamlferos, catorceaves y cincoespecies invertebradas fueron registradosen una muestrade 105 egagrbpilas.En el total, entre los mamlferos,el roedorApodemus sylvaticus fue el que conmils frecuenciafue presade estos bfihos;y entre las aves,la presaidentificada con mils frecuenciafue el gorribnPasser domesticus. Dos tipos de cambib en la dieta estacionalfueron observados:primero, un cambib de clase de presa: de mamlferosen invierno a la de avesen la estacibnreproductora; y segundo,un cambiben el tamafio de las presas:de pequefiasen el inviernoa medianas(>30 g) en la estacibnreproductora. En contrastecon hallazgosrealizados en otras partes de Inglaterra, las aves,en vez de los mamiferos,contribuyeron significativamentea la dieta del Strix alucosylvatica durante las estacibnreproductora. [Traducci6n de Eudoxio Paredes-Ruiz] The diet of many owl speciesis influenced by Few data exist with regard to Tawny Owl diet habitat and season(e.g., Marti 1974, Yalden 1985, in discontinuouswoodland habitats, where prey spe- Mikkola 1983). Amongsedentary "generalist" spe- ciesand hunting techniquesmay differ from that of cies,Tawny Owls (Strix alucosylvatica) inhabiting owls inhabiting larger foresttracts (Nilsson 1978). deciduouswoodland in England preyed on Bank In this paper, I report on seasonalvariation in the Voles (Clethrionornysglareolus) and Wood Mice diet of Tawny Owls from a site in south-eastern (Apodemussylvaticus) in winter, but switched to England in relation to breedingand possiblechanges Moles ( Talpidae),young Rabbits ( Oryctolaguscun- in preyselection or availability.Because Tawny Owls wulus), Cockchafers (Melolontha rnelolontha).and disgorgepellets before roosting (Gu•rin 1932), pel- earthworms(Lurabricina) in summer(Southern 1954, lets are scatteredthroughout territories, making them 1969). In urban or other open habitats,birds may difficult to find. However, in this study sufficient formimportant components of Tawny Owl diet(e.g., numbersof pelletswere foundby intensivesearching Harrison 1960, Beven1965, Yalden andJones 1971, and knowledgeof roost sitesof individual owls. Glue 1972), but these have generally been aggre- STUDY AV, EA AND METHODS gatedin analysesso that the seasonalimportance of different speciesor size classescannot be investi- This study was carried out betweenApril 1977 and February 1978 at Herringfleet, north-eastSuffolk, in a gated. 0.06 km2 woodland dominatedby ScotsPine (Pinus syl- vestris),with mixed woodsof birch (Betulapendula), oak (Quercusrobur), rowan (Sorbusaucuparia), maple (Acer • Present address:Canadian Wildlife Service, Ontario Re- platanoides)and ash(Fraxinus excelsior), interspersed with gion, 49 Camelot Drive, Nepean, Ontario, Canada K1A large gardens.Marshes used for grazing and reedbeds 0H3. (Phragmitesaustralis) occur along a river to the west and 239 240 D^VID A. KIRK VOL. 26, NO. 4 Table 1. Total numbersand percentagecontribution by weight of prey speciesrecovered in Tawny Owl pellets, during and outsidethe breedingseason (April-August; non-breedingseason September-February). BREEDING SEASON WINTER SEASON WEIGHT % WEIGHT % PREYSPECIES NO. (g) WEIGHT NO. (g) WEIGHT Common Shrew Sorex araneus 0 0 0 1 8 o.3 Wood Mouse Apodemussylvaticus 4 72 4.4 34 612 20.7 Field Vole Microtus agrestis 0 0 0 14 294 10.0 Bank Vole Clethrionomysglareolus 1 16 1.0 29 464 15.7 Rabbit Oryctolaguscuniculus 1 100 6.1 0 0 0 Norway Rat Rattus norvegicus 3 180 10.9 4 240 8.1 Kestrel Falco tinnunculus 1 220 13.3 0 0 0 Wren Troglodytestroglodytes 0 0 0 1 8 0.3 Dunnock/Robin Prunella modularis/Erithacus rubecula 1 20 1.2 1 20 0.7 Blackbird/Song Thrush Turdusmerula/T. phitomelos 7 599 36.3 3 257 8.7 Redwing/Starling Turdusiliacus /Sturnus vulgaris 0 0 0 5 368 12.5 Coal Tit/Blue Tit Parusater/P. caeruleus 0 0 0 4 48 1.6 Jay Garrulusglandarius 1 161 9.8 0 0 0 Starling Sturnusvulgaris 2 164 9.9 0 0 0 House Sparrow Passerdomesticus 0 0 0 12 294 10.0 Chaffinch Fringilla coelobs 0 0 0 2 49 1.7 Greenfinch Carduelis chloris 0 0 0 2 52 1.8 Small bird (unidentified) 5 100 6.1 10 200 6.8 Dor beetle Geotrupesstercocarius 0 0 0 30 30 1.0 Dung Beetle Typhaeustyphoeus 13 13 0.8 6 6 0.2 Cockchafer Melolontha melolontha 4 4 0.2 0 0 0 Beetles Carabidae 0 0 0 4 0.4 0.01 Earthworms Lumbricidae 1 0 0 34 0 0 Total 43 a 1649 162 a 2950 a Excluding earthworms. farmland to the east. Exotic shrubs such as rhododenron feet,pelvises, gizzard size and feathersin the pelletmatrix. (Rhododendronspp.) and laurel (Prunuslaurocerasus) pro- The number of individualsrepresented was determined vide roostsfor small birds during winter. by countsof skulls,jaws or pelvisesfor mammals,and I •ollectedpellets at weeklyintervals at roostsin 2-3 skulls,mandibles and longbones for birdsas suggestedby ha of mature ScotsPine in two large woodedgardens. Of experimentswith Tawny and otherowl species(Short and the 105 pellets,77% were collectedduring the first 5 mo, Drew 1962, Raczyfiskiand Ruprecht 1974). the remaining 23% were collectedbetween September and Coleopterawere identifiedby elytra striationsand chi- February. One pair of Tawny Owls nestedin a nestbox, tinous remains. Earthworms were identified by chaetae approximately300 m from the roostsites used for pellet and I estimatedearthworm numbersby the proportionof collection.However, few pelletswere found beneaththe fibrousmaterial and sandin pellets(Southern 1954). Es- nestbox.The principalsource of pelletswas from this pair timates of earthworms were excluded from table totals of owls but due to territorial infringementssome pellets becausethey were not comparablewith countsof other might have beenfrom other individuals(territories in dis- prey. Averageweights of bird specieswere calculatedby continuouswoodland in Wytham averaged22 ha; Hirons the length of humeri recoveredin pellets using the re- 1985). I collectedup to 16 pelletsper week from October- gressionequation; log weight = (2.706 x log humerus February (21% of pellets cast by owls, assuming 1.27 length) - 2.062 (Yalden 1977) or by usingaverage weights pellets/dayare producedin winter; Lowe 1980), but be- in Hickling (1983, Appendix 12). I useddata in Yalden tween April and Septemberrelatively few pellets(1-6 per (1977, 1985) for weightsof small mammalsand Cole- week) were found (6% of pelletscast, assuming 1.03 pel- optera. lets/dayare producedin summer)for the reasonsdescribed by Southernand Lowe (1968). RESULTS AND DISCUSSION Mammal remains were identified to speciesby dental and cranial features (Yalden 1977), while birds were iden- I recorded 6 mammal, 14 bird and 5 invertebrate tified by comparingskulls or bills with referenceskeletons speciesin the 105 pelletsexamined (Table 1). Of a collectedlocally. Other remainsused to identify birdswere total of 204 prey items recoveredfrom pellets(ex- DECEMBER 1992 TAWNY OWL DIET 241 eludingearthworms), 45% by number were mam- Table 2. The contribution of different-sized prey in mals.Wood Mice predominated(19%), followedby Tawny Owl diet during and outsidethe breedingseason Bank Voles (15%) and Field Voles (7%). Birds com- (percentagefigures refer to weight of prey itemsin grams). prised28% of total prey; birds smallerthan 30 g estimatedbody weight contributed 19%. Of the spe- BREEDING ciesidentified, House Sparrows(6%) and thrushes SEASON WINTER SEASON (Turdusspp. 5%) weremost important. Numerically, (24 PELLETS) (81 PELLETS) Coleopterarepresented 30% of total diet. By weight, % WEIGHT % WEIGHT mammalsformed 43% and birds 56%, respectively. No. (g) No. (g) EuropeanBlackbirds and SongThrushes were most Small mammals 5 5.3 78 46.7 importantby weight(19%), followedby Wood Mice Medium-sized 4 17.0 4 8.1 (15%), Bank Voles (10%) and Norway Rats (9%). mammals The contributionof Coleopteraby weight was neg- Small birds 6 7.3 32 22.7 ligible. Medium-sized birds 11 69.4 8 21.2 Significantlymore birds than mammals were taken Invertebrates a 18 1.0 44 1.2 between April-August than between September- Total number 43 166 February(G = 8.08, P < 0.005), suggestinga switch Total weight 1649 2950 from small rodentsto birds during the breedingpe- a Excluding earthworms. riod. Also, significantly more medium-sized than smallvertebrate prey were takenby owlsduring the marsheswhere there were cattle. Typhaeuswas most breedingseason
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