j. RaptorRes. 37(1):55-62 ¸ 2003 The Raptor ResearchFoundation, Inc.

BREEDING DENSITY AND ALTITUDINAL DISTRIBUTION OF THE URAL, TAWNY, AND BOREAL IN NORTH DINARIC (CENTRAL )

AL VREZEC 1 National Instituteof Biology,Vecna pot 111, SI-1000 , Slovenia

ABSTRACT.--Ural( uralensis),Tawny (Strix aluco), and Boreal (Aegoliusfunereus) density and altitudinal distribution were determined using playback to censusowls on Mt. Krim (North Dinaric Alps, central Slovenia). Survey points were selected proportionally by altitude according to the relief of the area (320-1060 masl). Densityof Ural Owlswas estimated to be 2.2 territories/10 km2;high relativeto publisheddata from Europe, while densitiesof Tawny (4.0 territories/10 km2) and Boreal owls (2.8 territories/10 km•) were in the range or lower than other European data. The TawnyOwls were found at significantlylower altitudes (320-850 masl), while Boreal Owls were at higher altitudes (700-940 masl) than expected.I suggestthat territorieswere located in suboptimalhabitat for Tawny Owls. The segregationof these owlsby altitude in temperate-zone,continuous-montane is either a consequenceof competitiveexclusion or . The similarityin altitudinal distributionbetween Tawny and Boreal owlswas low, suggestingthat territories are not suitable habitat for Boreal Owls. At high altitudes, harsh conditions prevent the Tawny Owl from competing with the Ural Owl; an advantagefor the , which was capable of surviving harsh conditions within Ural Owl territories. Further studies are needed to determine competitive exclusion or predation interactions among these owls. KEYWORDS: BorealOwl; funereus; TawnyOwl; Strix aluco; Ural Owl; Strix uralensis;altitudinal distribution;density; Dinaric Alps; Slovenia.

DENSIDAD DE ANIDAMIENTO Y DISTRIBUCI(SN ALTITUDINAL DE LOS BUHOS URAL, LEONA- DO Y BOREAL EN LOS ALPES DINARICOS DEL NORTE (ESLOVENIA CENTRAL) RESUMEN.--Ladensidad y distribucitn altitudinal de los bfihos ural (Strixuralensis), leonado (Strixaluco), y boreal (Aegoliusfunereuso bfho de Tengmalm) se determint usando play back para censarbfhos en Mt. Krim (alpes dinaricos del norte, Esloveniacentral). Los puntos de conteo fueron seleccionados proporcionalmentede acuerdo con el relieve del firea (320-1060 msnm). La densidadde los bfihos uralesse estimoen 2.2 territorios/10 km2;relativamente alta con respectoa datospublicados en Europa, mientrasque la densidadde bfhos leonados(4.0 territorios/10 km•) y bfhos boreales(2.8 territorios/ 10 km2) estuvieronen el rango o por debajo de otros datos tomadosen Europa. Los bfhos leonados se encontraron en alturas significativamentemas bajas (320-850 msnm), mientras que los bfhos bore- alesse encontraronaltitudinalmente mas arriba (700-940 msnm) de lo esperado.Sugiero que los ter- ritorios del bfiho ural estaban localizadosen hfibitat suboptimo para bfihos leonados. La segregacitn de estosbfihos por altitud en un bosquemontano continuo de zona templada estanto una consecuencia de exclusitn competitivacomo de depredacitn. La similitud en la distribucitn altitudinal entre bfihos leonados y boreales fue baja, sugiriendo que los territorios de bfho leonado no proveen de hfibitat adecuado a los bfhos boreales. En elevadasaltitudes, las duras condicionesimpiden al bfiho leonado competir con el bfho ural; una ventaja para el bfho boreal, el cual fue capaz de sobrevivira dificiles condicionesdentro de los territorios del bfiho ural. Se requiere de mayoresestudios para determinar la exclusitn competitivao las interaccionesde depredacitn entre estosbfhos. [Traduccitn de Ctsar Mfirquez]

Bird densities and their altitudinal distributions are mechanisms,food supply,and availabilityof suitable influencedby habitatquality, competitive behavioral nest sites (Gill 1995, Zabel et al. 1995, Newton 1998). For owls,defense of an exclusivehunting E-mail address:[email protected] playsan important role; the size of the territory is

55 56 VP,•ZEC Vote. 37, No. 1

often governedby owl massand prey scarcity(Schoe- (Roch• and Mebs 1989). Surveyswere conductedfrom ner 1968, Carbone and Gittleman 2002). dusk to approximatelymidnight during the springand summer, up to three times per month. Playbackswere In the Dinaric Alps (western Balkan Peninsula) broadcasttbr 10 rain, followedby a 5-min listeningpe- the ecology of Ural (Strix uralensis),Tawny (Strix riod, at eachsurvey point; a samplinginterval suggested aluco),and Boreal owls(Aegoliusfunereus) is poorly to be adequatefor detectingmost owls that are occupy- known. Density and some breeding habits of Ural ing a territory (Zuberogoitia and Campos 1998). On a Owls are documented fbr Slovenia (Miheli• et al. specificsurvey night, only one species'call wasused dur- ing broadcastsampling. 2000), but only anecdotal data are available fi:om I estimatedhuman detectionof the playbackin other partsof the region (Kralj 1997). For the Taw- habitat with the equipmentused in the surveyat a dis- ny and Boreal owls,only distribution is known. Pri- tance of ca. 500 m. This distance was used to define the or to this paper, the altitudinal distribution of owls effectivesurvey area (0.78 km2) around each survey point. I assumedthat each responseat a point repre- fi:om Dinaric Alps were based on severalincidental sented an occupied territory. The presenceof two owl observations(Tome 1996, Miheli• et al. 2000). territories at one point count was recorded only if two In this paper, I present data on density and al- maleswere detectedat the sametime. Spontaneouscall- titudinal distribution of the Ural, Tawny,and Bo- ing owls, that were estimated to be outside (>500 m from point) the surveyarea, were excluded from further anal- real owls fi:om the north part of Dinaric Alps. Of ysis. particular value are data on the Ural and Boreal Crude densitywas estimated as the sum of all survey owls,because the studyarea is at the southernlimit areas at all altitudesdivided by the number of detected of their distributionand is disjunctfi:om the main owl territories. Becauseterritory-size data were not ob- European population; both speciesare glacial rel- tained, only approximationsof ecologicaldensity were ics. possible.Ecological densities (the number per unit of habitat space;Odum 1971) were calculatedfrom survey STUDY AP•^ areaswithin the lowestand the highestrecorded altitude for species;only forest-covered areas were used in the The field work was done on Mt. I•im (14ø25'55"E, analysis.Similar approaches to approximateowl densities 45ø58'15"N)in a studyarea covering140 km9, 10 km were employed by Penteriani and Pinchera (1990) and south of Ljubljana (central Slovenia), between 1997 and Diller and Thome (1999). 2000. Mr. Krim is a medium-high (290-1107 The owl altitudinal distributionwas presented as a rel- masl)with a widelyextended plateau. Most of the study ative abundance index. This standardized relative abun- area is north thcing and is within the Dinaric zoogeo- dance was calculatedas number of owl territoriesper graphical region (Mr•ifi 1997), part of the north Dinaric surveypoint in a 100 m altitudinal interval per year. I Alps. defined altitudinal range with 50% of all detected owl Clearings or nonforest areas, both natural and man- territories as the center of altitudinal distribution for made, represent25% of the studyarea. They are small each species.Disproportionate use of a particularaltitude and dispersed,mostly around the settlements.The mixed by each owl specieswas tested with Mann-WhitneyU-test forest is widespread(75%), belongingto the association (Sokal and Rohlf 1995) comparingthe altitudinaldistri- of Dinaric beechforest with (Omphal0d0-Fagetums.lat.) bution of all surveyareas with the altitudinal distribution appearing in different subassociations.The most fre- of surveyareas with occupiedowl territories.A similarity quent subassociationis Omphalodo-Fagetumasaretosum eu- index of altitudinaldistribution between three owl spe- ropaei(syn. Abieti-Fagetumdinaricum clematidetosum; for de- cieswas calculatedusing the MacNaughton-Wolfsimilar- scriptionssee Puncer 1980). Dominant tree spcciesare ity index as suggestedby Mikkola (1983) and Korpimfiki sdver fir (Abiesalba), (Piceaabies), and (1986). (Fagussylvatica). Most of the trees in the forest have trunk diameter >30 cm. (Furlan 1988, Slovenian RESULTS Forest Service unpubl.) In 1998, 343 surveypoints were completedin 25 METI IOI)S nights. Seven Ural Owl, 13 Tawny Owl, and nine To estimateowl densityand altitudinal distribution,41 Boreal Owl territorieswere ibund on 41 surveyar- surveypoints were selectedfrom the baseto the top of eas. Crude densitiesof Ural, Tawny,and Boreal Mt. Krim. Surveypoints were selectedproportionally by owls were estimated at 2.2, 4.0, and 2.8 territories/ altitude. Densitywas estimatedin the breeding season 1998 only, but the data for altitudinal distribution were 10 km• respectively.Estimated ecological density collectedbetween 1997 and 2000. Surveypoints were lo- wasmeasurably higher only in the Boreal Owl (Ta- catedabout 1000 m apart, a distancerecommended by ble 1). Holmberg(1979) and Zuberogoitiaand Campos(1998). In years1997-2000, 582 surveypoints were done The detectionof owlswas enhancedby using call play- back (Forsman 1983, Redpath 1994, Zuberogoitiaand during 50 nights. Twenty occupied territoriesof Campos1998, Appleby et al. 1999). Recordingsof male Ural, 23 of Tawny,and 17 of Boreal owlswere re- territorial callsof Ural, Tawny,and Boreal owlswere used corded. The Ural Owl occurred over the greatest MARCH 2003 URAL, TAWNY, AND BOREAL OWL DENSITY 57

Table 1. Estimated crude and ecological densities of ilarity regarding altitudinal distribution was found three owl speciesin 1998 on Mt. Krim. Crude density is between Ural and Boreal owls, and the least simi- the sum of all survey areas divided by the number of larity was found between the distribution of the detected owl territories (see Methods). Ecologicaldensity Tawny and Boreal owls (Table 3). rs the number of owl territories per unit of habitat space

(calculated on the basisof sample area). DISCUSSION

Ural Tawny Boreal Ural Owl Density. Density of Ural Owls in North Owl Owl Owl Dinaric Alps (including Mt. Krim) is high relative to other parts of Europe (Table 4). I suggestthree Crude density 2.2 4.0 2.8 (territory/10 km2) possible explanations for these differences. First, Ecologicaldensity 2.8 5.6 6.9 different censusmethods may be responsible;we (territory/10 km2) counted singing males, wlfile active nests were Samplearea (km'2 25.1 23.0 13.0 counted in some other studies.With the playback technique, it is impossible to distinguish between breeding and nonbreeding pairs or even nonmat- span in altitudinal distribution (410-1060 masl; ed, but territorial individuals. Pairs can occupy a Fig. 1). The Tawny Owl was found at significant territory even if they do not breed (Saurola 1989). lower elevations, wlfile Boreal Owls occurred at The proportion of Ural Owl pairs that actuallylay higher altitudes (Table 2, Fig. 1). The greatestsire- eggsvaries between 12 and 87% (PietiSinen1988).

1000-1090 Aegoliusfunereus 900-990

800-890

700-790

600-690

500-590

400-490

300-390

1000-1090 Stfix aluco

900-990

800-890 I 700-790

600-690

500-590

400-490

300-390

Stdx uralensis 1000-1090

900-990

800-890

700-790

600-690 I 500-590

400-490

300-390

0.4 0.6 0.8 1 1.2 1.4 1.6 Relative Abundance[territories per survey point] Figure 1. Altitudinal distributionof Ural (N = 20), Tawny (N = 23), and Boreal owl (N = 17) on Mt. Krim. Relative abundancewas calculatedas the number of owl territories per surveypoint in each 100 m altitudinal interval per year. 58 VP•ZF,C VOL. 37, No. 1

Table 2. Altitudinal distribution of all survey areas compared to survey areas, where Ural Owls, Tawny Owls, and BorealOwls were detectedin years1997-2000 on Mt. Krim. Data were comparedwith Mann-WhitneyU-test.

ALTITUDE (m) CENTER OF MINIMUM-- ALTITUDINAL MEDIAN MAXIMUM DISTRIBUTION a U P

All surveyareas (N = 4) 710 320-1060 520-820 Survey areas with Ural Owls 800 410-1060 650-840 271.0 >0.05 (N = 20) Survey areas with Tawny OMs 490 320-850 410-610 223.5 0.033 (N = 23) Survey areaswith Boreal Owls 800 700-940 770-850 163.5 0.037 (N = 17) Center of altitudinal distribution contains 50% of all detected owl territories.

For that reason density is herein presented as oc- high density of the Ural Owl. However, I have no cupied territories and not as breeding pairs. Sec- data on fat density on Mt. Krim to sup- ondly, density in is a function of the size of port this suggestion. the studyarea (Bezzel 1982). Areas with low den- Tawny Owl Density. According to data from Eu- sities of owls were considerably larger than my rope (Table 4), densityof Tawny Owls on Mt. Krim study area (Table 4). Finally, the amount of avail- was low; perhaps, becauseof interspecific compe- able food may be responsible.In Slovenia,a large tition with the larger Ural Owls (Mikkola 1983, part of Ural Owl's diet consistsof fat dormouse Ktnig et al. 1999, Vrezec 2000a). The Tawny Owl (Glisglis) (Vrezec 2000b), while in other countries, is lowland speciesin Slovenia (Tome 1996), and (Microtusspp.) are the predominant prey spe- that was confirmed also on Mt. Krim (Fig. 1). cies (Sladek 1961/62, Mikkola 1972, 1983,fitder- Boreal Owl Density. The Boreal Owl has a rela- holm 1987,Korpim//tki and Sulkava1987, Glutz von tively small territory (Ktnig et al. 1999). Neigh- Blotzheim and Bauer 1994, Czuchnowski 1997, boring malescan sometimessing very closeto each Stfirzer 1998, 1999). Fat dormouse is an abundant other without any aggressiveinteractions (Ktnig et small in Slovenian forests (Kry•tufek al. 1999), so ecological densities can exceed 10 1991) and its massis approximatelyfour times as pairs/10 km2 (Table 4). Three important factors much as voles, that is 245 g compared to 64 g that limit Boreal Owl density in forests were ad- (Kryitufek 1991, Glutz von Blotzheim and Bauer vanced by Locker and Flfigge (1998): (1) presence 1994). Prey availabilityand prey body massare im- of suitable nest tree holes of Black portant factors that inversely affect the size of a (Dryocopusmartius); (2) optimal foraging areas, predator's territory (Schoener 1968, Zabel et al. large clearingsor windfall areas;and (3) absence 1995). One consequenceof large territories is a of the TawnyOwl, an important predator of Boreal relatively lower density of owls (LaHaye et al. Owls throughout Europe. In the area of Mt. Krim 1997). Prey availabilityon Mt. Krim could resultin all factors are optimal, so high ecological densities small territories and may explain the observed are no surprise.Low crude density (Table 2) is the consequence of altitudinal limitations of the spe- cies' distribution, which was probably caused by Table 3. Similarity in altitudinal distribution between the presence of Tawny Owls at lower altitudes. three owl specieson Mt. Krim (MacNaughton-Wolfsim- fiarity index following Mikkola [1983] and KorpimSki Altitudinal Partitioning. The present study has [1986]). shown that in North Dinaric Alps, competing owl species are segregatedby altitude, an important

URAL OWL TAWNY OWL factor in habitat selectionfor some speciessuch as for Tawny and Boreal owls. The Ural Owl is the Boreal Owl 0.65 0.17 dominant speciesin the owl guild living in my Tawny Owl 0.42 study area, and its distribution is not restrictedby MARCH 2003 URAL, TAWNY, AND BOREAL OWL DENSITY 59

o 60 VP,EZEC VOL. 37, No. 1 altitude (Mihelia et al. 2000). Studies from Scan- ACKNOWLEDGMENTS dinavia indicate keen interspecificcompetition be- I would like to thank to Dr. Davorin Tome and to Dr. tween both Strixowls (Lundberg 1980, Korpim/iki Boris Kryitufek for being my mentors and for their valu- 1986). However,work from the more temperate cli- able comments.Special thanks goes to both referees,Drs Jim Duncan and Harri Hakkarainen, who helped me sub- mates of , where the Ural Owl was stantiallyin revisingthe manuscript.I also thank all the reintroduced, did not report competition between individualswho accompanied me during my nocturnal Ural and Tawny owls (Stfirzer 1998). It is possible, fieldwork on Mt. Krim. For valuable data on the forest of that in mild weather conditions, especiallyin low- Mt. Krim, I am very grateful to the SlovenianForest Ser- lands, the Tawny Owl can coexist with the Ural vice (Zavod za gozdove Slovenije, OE Ljubljana), espe- cially to Mr. Saio Skledar,Mr. Andrej Jekla•; and Mr. Bo- Owl, while in more extreme conditions (northern •tjan Jmjev•i& limit of distribution or high altitudes), the Ural Owl out-competesthe smaller Tawny Owl. Low LrrE•,ATU•E CrrE• breeding densityof Tawny Owls at Mt. Krim, com- APPLEBY,B.M., N. YAMAGUCHI,P.J. JOHNSON, AND D.W pared to other European countries (Table 4), MACDONALD.1999. Sex-specificterritorial responsesin could indicate, that the Ural Owl is a limiting fac- Tawny Owls Strix aluco.Ibis 141:91-99. tor. Evidence of regular breeding of the Tawny Owl AUGST,U. 2000. Der RauhfuBukauz(Aegoliusfunereus) im at elevationsover 1000 masl, where it is allopatric Nationalpark "SSchsischeSchweiz". Mitt. Ve,:Sdichs. with Ural Owls, is needed. Ornithol. 8:465-474. The altitudesoccupied by the Tawny Owl are un- BENUSSI,E. AND E GENERO.1995. L'Alloccodegli Urali suitable for Boreal Owls. At high altitudes, harsh (Strixuralensis macroura) nel Trnovskigozd (Slovenia) conditions prevent the Tawny Owl from competing censimento in un'area campione. Suppl.Ric. Biol. Sel- with the Ural Owl; an advantage for the Boreal vagginaXXH:563-568. BEZZEL,E. 1982. V6gel in der Kulturlandschaft.Verlag Owl, which was capable of survivingharsh condi- tions within Ural Owl territories. Boreal Owls are Eugen Ulmer, Stuttgart, . CekRBONE,C. ANDJ.L. G•TrLEM•. 2002. A common rule able to breed in Ural Owl territories, although for the scalingof carnivoredensity. Science 295:2273- their densityand breeding activitycan be reduced 2276. (Hakkarainen and Korpim/iki 1996). On the con- CZUCHNOWSm,R. 1997. Diet of the Ural Owl (Strix ura- trary, as shownwith altitudinal segregationin this lensis)in the Niepolomicka Forest, S-E . Buteo and some other studies from Central and Southern 9:69-75. Europe (Pedrini 1982, Glutz von Blotzheim and Dm•ER, L.V. AND D.M. THOME. 1999. Population density Bauer 1994, Locker and Flfigge 1998, Augst 2000), of northern Spotted Owls in managedyoung-growth Tawny Owls seem to exclude Boreal Owls from forestsin coastalnorthern California.J. RaptorRes. 33' their territories. Further studies are needed to ex- 275-286. DvoP,A•, M., A. RqNNER, AND H.-M. BERG. 1993. Atlas der amine the competitive and predation interactions Brutv6gelOsterreichs. Umweltbundesamt, Wien, Aus- among these two owls. tria. Few studies have dealt with the altitudinal distri- FORSMAN,E.D. 1983. Methods and materialsfor locating bution of owls.The present studyshowed that owl and studying Spotted Owls. USDA For. Serv. Gen densityor abundancecan vary substantiallyaccord- Tech. Rep. PNW-162, Portland, OR U.S.A. ing to altitude in somespecies (Fig. 1). Owl survey FUP,•AN,I. 1988. Prime•jalne raziskavezoocenoz karabi- studiesin areaswith elevationsranging more than dov (Carabidae, Coleoptera) v razli•nih variantah ras- 500 m should record altitude. For accurate esti- tlinske zdruibe Abieti-Fageturndinaricum (Comparative mations of owl densities or abundance, all altitudes study of carabid zoocenosis(Carabidae, Coleoptera) with suitablehabitat should be surveyed.Normally, in different variants of associationAbieti-Fagetum dz- such surveyscover very large areas, which are im- naricum).M.S. thesis,Univ. Ljubljana, Ljubljana, Slo- venia. possibleto searchintensively. A samplingapproach GAI.EOTTI,P. 1994. Patternsof territory sizeand defence stratified by altitude should be used. Although the level in rural and urban tawny owl (Strix aluco)pop- accuracyof density estimatesbased on broadcast ulations.J. Zool.Lond. 234:641-658. sampling permit comparisonswith other studies GILL, LB. 1995. Ornithology. W.H. Freeman & Go., New (Table 4), the reliability of this method should be York, NY U.S.A. tested more thoroughly to determine the relation- GLUTZ VON BLOTZHEIM, U.N. AND K.M. BAUER. 1994 ship of resulting estimatesto absolutepopulation Handbuch der V6gel Mitteleuropas,Bd. 9. Aula-Ver- size. lag GmbH, Wiesbaden,. MARCH 9003 URAL, TAWNY, AND BOPreALOWL DENSITY 61

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