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Home , Strix (bird), Tawny owl

j Raptor Res. 26(4):239-242 ¸ 1992 The Raptor ResearchFoundation, Inc.

DIET CHANGES IN BREEDING TAWNY ( aluco)

DAVID A. KIRK 1 Departmentof Zoology,2 TillydroneAvenue, University of Aberdeen, Aberdeen,, U.K. AB9 2TN

ABSTVO,CT.--I examinedthe contentsof Tawny (Strix alucosylvatica) pellets, between April 1977 and February 1978, in mixed woodlandand gardensin northeastSuffolk, England. Six , 14 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, , rather than , contributedsignificantly to 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 ( Oryctolaguscun- in preyselection or availability.Because Tawny Owls wulus), Cockchafers (Melolontha rnelolontha).and disgorgepellets before roosting (Gu•rin 1932), pel- (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 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 Clethrionomysglareolus 1 16 1.0 29 464 15.7 Oryctolaguscuniculus 1 100 6.1 0 0 0 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 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 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 (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 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 identifybirds were 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 than in autumn and winter when commonlytaken in the summer (Table 1). Cock- the conversewas true (G = 24.29, P < 0.001; Table chaferbeetles were found in pelletsfrom June and 2). Similar resultswere found when bird prey were July, the period of emergencefor this species. consideredseparately (G = 8.57, P < 0.005), al- A shift from small to larger (mammal) prey in though the total biomassintake of small and me- Tawny Owl diet during the breeding seasonwas dium-sized birds was similar betweenthe breeding alsofound in Derbyshire, England, but in contrast and non-breedingseason (Table 2). The weight of to my study bird prey was most important in the vertebrateprey was significantly higher in thebreed- 'winter' season(Yalden 1985, Table 2). Increased ing than the non-breedingseason (breeding season weight of prey taken by Tawny Owls during the • = 62.8 g, SE = 9.7, N = 26; non-breeding• = breedingseason was suggestedby Nilsson(1984) in 23.9 g, SE = 1.5, N = 122; Mann Whitney U test, Sweden, and by Southern (1954) in Oxfordshire, Z = 4.473, P < 0.001), but no differencewas found England. However, in the latter studyowls appar- when invertebrates were included. ently did not prey on abundantfledgling passerines. During the breedingseason, thrushes, European The increase in medium-sized (often fledgling) Starlings,Jay, and EurasianKestrel together con- bird prey in this studyduring the breedingseason tributed 69% of the diet by weight.Wood Mice and suggestedthat Tawny Owls might selectivelytake Bank Voles comprisedonly 5%. In winter, Wood largerprey when theyhave chicks, as noted by Mik- Mice, Bank Voles, Field Voles and Common Shrews kola (1983, Table 26) and as documentedin some accountedfor 47% of the diet by weight.Small birds CommonBarn Owl (Tyro alba) pairs (Buckley and (e.g.,House Sparrows) comprised 23% of the winter Goldsmith 1975). Southern (1969) alsorecorded diet diet by weight (Tables 1 and 2). Earthworms-erere changesin Wytham owls when their young were taken mostly in the non-breedingseason, particu- half-grownand no longerbrooded. Tawny Owls are larly in Octoberand December,probably due to the sexuallydimorphic, and the larger size of females wet conditionsin thesemonths resulting in increased (26% heavierthan males;Hardy et al. 1981) might availabilityof earthworms.The importanceof earth- allow them to selectlarger prey than the male (for wormswas probablygreatly underestimated since I other owl speciessee Earhart and Johnson 1970, did not weigh the granular contentof pelletsas rec- Mikkola 1983), especiallywhen they have limited ommendedby Yalden (1985). Two speciesof dung huntingtime dueto demandsfrom their chicks.Also, beetles (Geotrupes stercocariusand Typhaeus ty- individualTawny Owls can specializeon particular phoeus)were alsotaken in largenumbers. Geotrupes prey types,so perhapsthe femaleowl in this study occurredin pelletsmost frequently in autumn and selectivelytook large bird prey at this time. Con- winter, showing that Tawny Owls foraged over versely,more small birdswere foundin winter pel- 242 DAVID A. KIRK VOL. 26, NO. 4 lets probably as a result of owls feeding on com- GLUE,D.E. 1972. Bird preytaken by Britishowls. Bird munally roostingbirds as occurs in Long-caredOwls Study 19:91-95. (Asio otus;Glue and Hammond 1974). -- ANDG.J. HAMMOND. 1974. Feedingecology of Prey availability for Tawny Owls is determined the Long-caredOwl in Britain and . Br. Birds 67:361-369. by groundcover (Southern and Lowe 1968) which GUIgRIN,G. 1932. La Hulotte et sonr•gime. Encyclo- may accountfor seasonalchange in the diet of Taw- pedicOrnithologique. P. Lechevalier,Paris, France. ny Owls at Herringfleet.Dense vegetation cover (es- HARDY,A.R., G.J.M. HIRONS,P.I. STANLEYAND L.W. pecially BrackenPteridium aquilinum) in summer HUSON. 1981. Sexualdimorphism in sizeof Tawny couldprevent owls from locatingsmall mammal prey. Owls (Strixaluco): a methodfor sexingin field studies. Conversely,dieback of vegetationin autumn may Ardea 69:181-184. mean that small are more vulnerable to owl HARRISON,C.J.O. 1960. The foodof someurban Tawny . Rodentsmight make more noisemoving Owls. Bird Study7:236-240. through leaf litter on the ground in autumn and HICKLING,R. (ED.). 1983. Enjoyingornithology. T. and winter and thus be more easily locatedby foraging A.D. Poyser,Caltofi, U.K. HIRONS,G.J.M. 1985. The effectsof territorial behavior owls. The fact that a major prey species,the Wood on the stabilityand dispersionof Tawny Owl (Strzx Mouse, spendsless time foragingoutside the nestin aluco)populations. J. Zool.,Lond. (B) 1:21-48. winter on moonlit nights (Wolton 1983) also indi- LOWE,V.P.W. 1980. Variationin digestionof prey by cates that small mammals are more vulnerable to the Tawny Owl (Strix aluco).J. Zool.,Lond. 192:283- Tawny Owl predationin the winter season.Thus, 293. changesin vegetationcover could account for the MARTI, C.D. 1974. Feedingecology of four sympatric abundanceof small rodentsin Tawny Owl diet at owls. Condor 76:45-61. Herringfleet during the winter season. MIKKOLA,H. 1983. Owls of .T. and A.D. Poy- My resultssuggest one, or a combinationof factors ser, Calton, U.K. in the apparent diet shift; 1) small mammalswere NILSSON,I.N. 1978. Huntingin flight'byTawny Owls Strix aluco. Ibis 120:528-531. more vulnerableto owl predationoutside the breed- 1984. Prey weight, food overlap, and repro- ing season;2) owls switched,opportunistically, to ductiveoutput of potentiallycompeting Long-cared fledgling birds during the breedingseason because Owls and Tawny Owls. OrnisScan& 15:176-182. they were easierto catchor 3) medium-sizedbirds RACZYglSKI,j. AND A.L. RUPRECHT. 1974. The effect were taken selectivelyby owls becausethey were of digestionon the osteologicalcomposition of owl pel- more 'profitable'prey (with a highernutrient intake lets. Acta Ornithologica14:25-38. per handlingtime) than rodents,during the period SHORT, L.L. AND L.C. DREW. 1962. Observations con- when owls had dependentyoung. cerningbehavior, feeding and pelletsof Short-cared Owls. Amer. Midl. Nat. 67:424-433. SOUTHERN,H.N. 1954. Tawny Owls and their prey. ACKNOWLEDGMENTS Ibis 96:384-410. I thank A. Johnson,I. Kirk, T.C. Kirk, D. Knott, and 1969. Prey taken by Tawny Owls during the K. Robinsonfor their encouragementwith this work and breedingseason. Ibis 111:293-299. F Bulsara for inspiration. Earlier versionsof the manu- -- ANDV.P.W. LOWE. 1968. The pattern of dis- scriptwere improvedby commentsfrom P.J. Ewins, C.D. tributionof prey and predationin Tawny Owl terri- Marti, M. M6nkkSnen and D.G. Smith. tories. J. Anita. Ecol. 37:75-97. WOLTON,R.J. 1983. The activityof free-rangingWood Mice, Apodemussylvaticus. J. Anita. Ecol. 52:781-794. LITERATURE CITED YALDEN, D.W. 1977. The identification of remains in BEVEN,G. 1965. The food of Tawny Owls in . owl pellets.Occasional Publication. Mammal Society LondonBird Rept. 29:56-72. Reading,U.K. BUCKLEY,J. ANDJ.G. GOLDSMITH. 1975. The prey of 1985. Dietary separationof owls in the Peak the ( albaalba) in eastNorfolk. Mammal District. Bird Study32:122-131. Review 5:13-16. -- ^ND R. JONES. 1971. The food of suburban EARHART,C.M. AND N.K. JOHNSON. 1970. Size di- Tawny Owls. The Naturalist 914:87-89. morphism and food habits of North American owls. Condor 72:251-264. Received18 March 1992; accepted21 August1992