J RaptorRes. 28(4):253-258 ¸ 1994 The Raptor ResearchFoundation, Inc.

DIET COMPOSITION OF THE LONG-EARED OWL IN CENTRAL SLOVENIA: SEASONAL VARIATION IN PREY USE

DAVORIN TOME Instituteof Biology,University of Ljubljana, Karlovska19 - POB 141, 61000 Ljubljana, Slovenia

AI•STI•½T.--The diet of long-earedowls (Asiootus) was examinedin central Sloveniaduring 1989-91. Mamahalswere the mostcommon prey group (97% by number), with the commonvole (Microtusarvalis) as the most frequent prey species(46%); its proportionin the diet varied yearly and seasonally(range 11-90%) accordingto its population density. When this vole's densitieswere low, owls shifted their predationpressure to alternate prey. Mice from the genusApodemus were the most commonalternate prey. Their proportionin the diet increasedduring winter and during year-roundshortages of main prey as well. Other prey speciescontributed significantly only during winter (Microtusagrestis and Pityrays subterraneus)or only during year-roundshortages (Arvicola terrestris and birds). Body size of the three preyspecies with highestproportions in the pellets(M. arvalis,M. agrestis,and Apodemus sylvaticus) varied with the season,the largest being during summer and the smallestduring winter. KEY WORDS: Asio otus;diet composition;long-eared owl; prey size;seasonal variation; Slovenia.

Composici6nde la dieta de Asiootus en EsloveniaCentral: variaci6nestacional en el uso de presas RESUMEN.--Durante 1989 a 1991 se examin6 la dieta de Asio otus en Eslovenia Central. Los mamlferos constituyeronel grupode presasmils comfin(97% por nfimero)y cuya especie-presamis frecuentefue Microtusarvalis (46%). La proporci6nde M. arvalisen la dieta varla anual y estacionalmente(rango: 11 a 90%) de acuerdoa su densidadpoblacional. Cuando susdensidades fueron bajas, A. otusincrementaba la depredaci6nsobre presas alternativas; su proporci6nen la dieta se incrementabadurante el invierno yen los perlodosde baja densidadintra-anual de la presaprincipal. Otras especies-presacontribuyeron significativamentea la dieta solamenteen invierno(Microtus agrestis, Arvicola terrestris y aves).E1 tamafio corporalde tresespecies-presa, con altas proporciones en las egagr6pilas(M. arvalis,M. agrestisy Apodemus sylvaticus,variaba con la estaci6n,siendo mis grandesen veranoy mils pequefiasdurante el invierno. [Traducci6n de Ivan Lazo]

The dietof the long-earedowl (Asiootus) has been METHODS AND MATERIALS studiedextensively throughout Europe and North Long-earedowl pelletswere collectedsystematically from America (summarized in Schmidt 1974, Marti 1976, January 1989 to December 1991 at three localities on Mikkola 1984, Cramp 1985). Due to difficultiesin Ljubljana moor (southof the city of Ljubljana, Slovenia) findingpellets during the breedingseason, however, The study area (about 160 km2) was homogenousfarm- land. In localitieswhere pelletswere collectedforest patch- many studieshave a bias toward the winter diet. es of predominantlyPino sylvestris-Betulletumand Betulo- Some authorstried to overcomethis by presenting (•uercetumrobotis types (for a more detaileddescription resultsof pellet analysesseparately for winter and seeTome 1991). Samplingat each locality took place at summeror breedingand nonbreedingseason (Arm- least once a month. Small mammal remains in the pellets strong 1958, Sulkava 1965, Glue and Hammond were identifiedto speciesaccording to Kry•tufek (1985) Birds and insectswere not identifiedbeyond class because 1974, Goszczinsky1981). Only Nilsson(1981) and of their low numbers. Biomass was calculated using av- Wijnandts(1984) evaluatedlong-eared owl dietyear eragemass of prey species(see Tome 1991). round. For studyof year-roundvariation in the diet, prey (by I presentdata on long-earedowl diet derivedfrom number) were pooledfor eachyear into six 2-mo periods beginningwith January. Due to their small proportions pelletscollected throughout the year in central Slo- and low variability in the diet, speciesin the genera venia. My major goal was to evaluateseasonal dif- cromys,Rattus, and Apodemuswere pooledas "mice," and ferencesin prey speciescomposition and prey size. speciesin the generaSorex, Neomys, Crocidura as "shrews."

253 254 D^VORIN TOME VOl,. 28, NO. 4

Table 1. Diet compositionof the long-earedowl in centralSlovenia (N% -- dietary percentby number, B% -- dietary percentby biomass).

1989 1990 1991 1989-91

PREY N% N% N% N% B %

Microtus arvalis 66.6 52.3 21.2 44.2 45.6 Microtusagrestis 13.3 17.8 11.6 14.3 20.2 Pityrnyssubterraneus 7.1 8.3 10.2 8.7 6.0 Clethrionornysglareolus 2.5 1.2 3.8 2.5 2.4 Arvicola terrestris tr. a 0.4 5.7 2.3 5.0 Apodernussylvaticus 5.1 7.1 14.4 9.4 7.7 Apodernusflavicollis tr. 2.1 0.3 0.9 0.8 Apodernusspp. 2.2 5.6 15.5 8.5 6.9 Micrornysrninutus 0.9 2.9 7.5 4.1 1.2 Rattus norvegicus tr. tr. 0.1 tr. 0.1 Sorex araneus tr. 0.3 2.4 1.0 0.4 Neomysfodiens tr. tr. 0.1 tr. tr. Crocidura leucodon 0.9 0.3 0.1 0.4 0.2 Crocidura suaveolens tr. 0.1 0.2 0.1 tr. Crociduraspp. 0.1 tr. 0.2 0.1 tr. Muscardinus avellanarius tr. 0.2 0.4 0.2 0.3 Talpa europaea tr. 0.1 0.4 0.2 0.8 Total mammals 98.7 98.7 94.1 97.0 97.7

Birds 1.2 1.1 4.8 2.5 2.3 Insects 0.1 0.2 1.1 0.5 tr. Total number 691 921 999 2611 Total biomass(g) 60 726

<0.1%.

Dormice (Muscardinusavellanarius), moles (Talpa euro- from Ljubljana moor with the commonvole being paea), and insectswere groupedtogether as "other." themost frequent (> 40%by number).Among major Food-nichebreadth (FNB) was calculatedaccording to Levins (1968). In these calculationsunidentified individ- preygroups, voles (Microtus, Pityrnys, Clethrionornys, uals in the generaApodemus and Crocidurawere assigned and Arvicola)were dominant,constituting 72% of to speciesin the sameproportions as their identifiedcoun- prey itemsby number,followed by mice (Apodernus, terparts. Birds and insectswere regardedas only two tax- Microrays,Rattus; 23%), birds (Aves;3%) and shrews OhS. (Sorex,Neornys, Crocidura; 2%). The proportionof An index of the size of prey individuals of the three mostfrequent prey speciesin the diet (commonvole [Mi- insectswas negligible(<1%). Proportionsof prey crotusarvalis], field vole[Microtus agrestis], and woodmouse speciesby biomasswere similar to proportionsby [Apodernussylvaticus]) was obtained by measuringthe dis- number,because of similar averageweights of the tancebetween the upperincisor and the third molar (IM3) mostfrequent prey species(Table 1). on one side of each unbroken skull. Measurements were taken to the nearest 0.1 mm using a caliper. Year-to-yeardiet of the owls changedconsider- To assessrelative abundanceof dominant prey species ably (X2 = 564, P < 0.01). Speciesmost variable in in the field, snaptraps were set in spring and early sum- thediet were common voles and mice from the genus mer. Traps were placed5 m apart in lines of 30 and left Apodernus,followed by water voles(Arvicola terres- for one night. Altogether 1290 trap nights were accu- tris), harvestmice (Micrornysrninutus), and birds. mulatedon thegrasslands in 3 yr. A snap-trapindex (STI = number of animals caught per 100 traps) was used to Proportionsof field voles,common pine voles(Pz- determine density. tyrnyssubterraneus), and bank voles(Clethrionornys glareolus)were more stable (Table 1). RESULTS SeasonalVariation in Prey Use. Most promi- Diet Composition.Fifteen species of smallmam- nent in the seasonalvariation in prey usewere the mals were found in pelletsof the long-earedowl summer-autumn peaks of the commonvole, which DECEMBER 1994 LONG-EARED OWL PREY USE 255

ß Birds [] M. arvalis [] M. agrestis

[] P. subterraneus [] C. glareolus [] A. terrestris

[] Mice [] Shrews [] Other

1989 1990 1991 lOO%

80%

60%

40%

20%

0%

JF MA MJ JA SO ND JF MA MJ JA SO ND JF MA MJ JA SO ND 162 233 98 110 19 69 210 279 238 96 50 48 125 248 202 147 95 182 Figure 1. Seasonalvariation in the diet (proportionsby number) of the long-earedowl during 1989-91 in central Sloveniain 2-too intervals.Sample size is givenbelow x-axis intervals. constitutedup to 90% of food intake by number in revealedthat the commonvole was the only main that period. The winter diet shifted notably from prey for long-earedowls. Wood mice were the most commonvoles to mice, field voles,and commonpine important alternate prey, followed by water voles voles. In 1991, mice were taken more frequently and shrews (Table 2). throughoutthe year, but field and commonpine voles Population Density of Common Voles. Density did not surpassthe proportionsfound in pellets in of commonvoles, the main prey speciesand the most previousyears. During the summer of 1991, the proportionsof water volesand birds markedly in- creased(Fig. 1). Table 2. Spearmanrank correlationsbetween propor- The relationshipsbetween main and alternateprey tionsof speciesfound in pelletsof 1ong-earedowls and the in the diet were investigatedusing correlationsbe- food-niche breadth of the owls' diet. tween proportionsof the speciesin the diet and the FNB. Optimal foraging theory predictsthat FNB SPECIES r s shouldexpand when the densityof the main prey Microtus arvalis -0.97 a speciesdecreases and shrink when the main prey Microtusagrestis 0.21 increases(Pyke 1984). This means that the most Pityrayssubterraneus 0.48 important main prey specieshave the largestneg- Clethrionomysglareolus 0.52 ative correlationcoefficient and the most important Arvicola terrestris 0.68 a alternateprey have the highestpositive correlation. Apodemusspp. 0.84 a FNB was usually low during the summer and Microraysminutus 0.56 high during the winter, but in 1991 it was high Soricidae 0.66 a throughoutthe year (Fig. 2). Correlation between Birds 0.56 the proportionof speciesin the diet and the FNB ap < 0.01. 256 DAVORIN TOME VOL. 28, NO. 4

1989 1990 1991

JF lvL/k MJ •/• SO bid •F lvL/k MJ J/• SO bid •F • M• •/• SO bid

Figure 2. Seasonalvariation in the food-nichebreadth (FNB) in the 1ong-earedowl in central Sloveniain 2-mo •ntervals. variableone in the long-earedowls' diet, varied widely of alternateprey in the diet increasednotably. Mice in the field. It was highestin 1989 (STI = 21.03), were the mostimportant alternateprey becausetheir intermediate in 1990 (STI = 11.71), and the lowest proportionincreased during winter aswell as during in 1991 (STI = 0.26). year-roundshortages of commonvoles. Other species SeasonalVariation in Prey Size. Body size as contributedsignificantly only during winter (field estimatedby the averageIM3 distancevaried sig- vole and commonpine vole) or only during year- nificantly between 2-mo periodsin the three most round shortagesof commonvoles (water vole and commonprey species(ANOVA; commonvole, F = birds) clearly diminishingtheir importanceas al- 9.50, P < 0.01; field vole, F = 6.84, P < 0.01; and ternate prey. wood mouse, F = 5.95, P < 0.01). This measure- In Sweden,dense vegetation in summer presum- ment was greater during the summer than during ably reducedthe availability of volesin relation to the winter, beingshortest in the last third of the year other prey which resultedin increasedproportions (Septemberto December;Fig. 3). of alternateprey and alsoincreased FNB in the iong- eared owl (Nilsson 1981). Open habitats on Lju- DISGUSSION bljana moor (main hunting habitat of long-eared Diet of the long-earedowl in this studywas sim- owls;Tome 1991) regularly had low vegetativecover ilar to diets elsewherein Europe (summarizedin during winter. Snow cover during this study was Schmidt 1974, Marti 1976, Mikkola 1984, Gramp practicallynonexistent. Consequently heavy vege- 1985). Small mammals contributeda majority of tative or snow cover could not have been the reason prey itemsby number(97%) and by mass(98%). for increasedFNB on Ljubljana moorduring winter. The rest were birds (2%) and insects(1%). FNB It is well-known that populationsof small mam- index was, in contrastto Sweden(Nilsson 1981) and mals are lowest during winter (Petrusewicz1983, the Netherlands (Wijnandts 1984), higher during Tamarin 1985). On the other hand, the proportion winter than during the summer. of the commonvole in the diet of the long-earedowl In yearsof its abundance,the commonvole was is dependenton the abundanceof this speciesin the by far themost important species in thesummer and owls' hunting habitat (KorpimSki 1992). I suggest autumndiet. During the winter and spring,as well that low densityof commonvoles in open habitats as during the summerand autumn in 1991 when during winter, as well as during most of 1991, was densitiesof the commonvole were low, proportions the main reason for decreasedproportion of this DECEMBER 1994 LONG-EARED OWL PREY USE 257

Microtus arvalis (N = 781•

14,5

14 13,5

13

12,5 JF MA MJ JA SO ND

Microtus agrestis (N = 299• 16

15,5

15

14,5

14

13,5 JF MA MJ JA SO ND

A. s¾1vaticus (N = 191• 13

12,5

E E 12

11.5

11' JF MA MJ JA SO ND

Figure 3. Averagedistance between the upperincisor and the third uppermolar in smallmammals found in pellets of long-earedowls during 1989-91. Results are pooledaccording to dateof origininto 2omoperiods. Vertical lines show standard deviation. 258 DAVORIN TOME VOL. 28, NO. 4 speciesin the diet and in consequencefor increased MARTI, C.D. 1976. A review of prey selectionby the FNB. How the variable body sizesof prey species long-earedowl. Condor78:331-336. influencesthe diet of the long-earedowl is still to MIKKOLA,H. 1984. Owls of Europe.T. & A.D. Poyser, be evaluated. Staffordshire, U.K. NILSSON,I.N. 1981. Seasonalchanges in food of the LITERATURE CITED long-earedowl in southern Sweden. Ornis Scand.12. ARMSTRONG,W.H. 1958. Nesting and food habits of 216-223. the long-earedowl in Michigan. Mich. State Univ. PETRUSEWICZ,K. [ED.] 1983. Ecologyof the bank vole. Publ. Mus. Biol. Ser. No. 1. Acta TherioI. 28:1-242. CRAMP, S. [ED.] 1985. The birds of the western Pale- PYKE,G.H. 1984. Optimal foragingtheory: a critical arctic. Vol. 4. Oxford Univ. Press, New York, NY review. Annu. Rev. EcoI. Syst.15:523-575. U.S.A. SCHMIDT, E. 1974. Die ern•ihrung der Waldohreule GLUE,D.E. ANDG.J. HAMMOND. 1974. Feedingecology (Asiootus) in Europa.Aquila 81:221-238. of the long-earedOwl in Britain and Ireland. Br. Birds SULr•VA, P. 1965. Vorkommen und Nahrung der Wal- 67:361-369. dohreule,Asio otus (L.) in Ilmajoki (EP) in denJahren GOSZCZINSKV,J. 1981. Comparativeanalysis of foodof 1955-1963. AquiloSet Zool. 2:41-47. owls in agrocenoses.EkoI. PoI. 29:431-439. TAMARIN,R.H. [ED.] 1985. Biologyof new world Mi- KORPIM•KI,E. 1992. Diet composition,prey choice, and crotus.Am. Soc. Mammal., Spec. Publ. 8. Stillwater, breeding successof longoearedowls: effectsof mul- OK U.S.A. tiannual fluctuations in food abundance. Can. J. Zool. TOME, D. 1991. Diet of the long-earedowl (Asiootus) 70:2373-2381. in Yugoslavia.Ornis Fenn. 68:114-118. KRY•TUFEK,B. 1985. Mali sesalci.NaSa rodna gruda. WIJNANDTS,H. 1984. Ecologicalenergetics of the Iong- Prirodosl.druõtvo Slovenije. Ljubljana, Slovenia. eared owl (Asiootus). Ardea 72:1-92. LEVINS, R. 1968. Evolution in changing environments. PrincetonUniv. Press,Princeton, NJ U.S.A. Received7 March 1994; accepted6 July 1994