j RaptorRes. 34(4):334-338 ¸ 2000 The Raptor ResearchFoundation, Inc.

DIET OF THE BAP,N OWL (TYTOALBA TUIDARA) IN NORTHWESTERNARGENTINE PATAGONIA

MARTAS. PILt•DO AND ANA TREJO CentroRegional Universitario Bariloche, Unidad Postal Universida& 8400 San Carlosde Bariloche,Argentina

KEy WORDS: ; Tyro alba; diet;,Patagonia; Argen- where n•was the nnmber of individualsof the ith species tina. and w, was the mean weight. We also determined the mean length of consunred after .Jaksi• et al. (1977): MLR = • fx•/m, wheref was the frequencyof Given its wide distribution and sedentary habits, the the i speciesin the diet, x, wasmean body length, and m diet of the Barn Owl (Tytoalba) has been studiedin more the total number of identified rodents. Mean weight of detail and more extensivelythan that of any other bird and mean body length of rodents were taken of prey (Everett et al. 1992). Rodents and other small from the literature (Redford and Eisenberg 1992, Pear- mammals are the main prey in the diet of Barn Owl in son 1995). all of its range along with variable proportionsof birds, Food-nichebreadth (FNB) wasestimated using Levins' reptiles, amphibians,fish, and arthropods (Taylor 1994). (1968) index: FNB = 1/(• p•9),where p•was the propor- The Barn Owl (T. alba tuidara) is widespread in conti- tion of prey taxon i in the diet. A standardized niche breadth value (FNBst)was then calculated,which ranged nental Argentina and occasionallyon islands (Canevari from 0-1: FNBst = (FNB - 1)/(n - 1), where nwas the et al. 1991). Food habits of the Barn Owl have been thor- total number of prey categories(Colwell and Futuyma oughly studied in agrosystemsin Argentina (Bellocq 1971). Evenness(J') wascalculated by the Shannon-Wie- 1990, Bellocq and Kravetz 1994), but little is known about ner function as follows:J' = H'/H'max, where H' was its diet in southern Argentina. In Patagonia,most studies the Shannon-Wiener function and H'max was the maxi- have focusedon the arid easternsteppes (De Santisand mum value of H'; that is, the logarithm of the number Pagnoni 1989, De Santiset al. 1993, 1996, GarciaEspon- of speciesin the sample (Krebs 1989). da et al. 1998). Our aim was to provide information on RESULTS AND DISCUSSION the diet of the Barn Owl in a somewhat different area with more aresicvegetation features and a small A total of 425 prey items was identified from 229 pel- fauna mixing typical steppe specieswitla others more lets. The mean number of prey/pellet was 1.9 _+ 0.9 characteristicof hunrid forestsnearby (Monjeau 1989). (+SD, range = 1-4) and the mean number of rodents/

STUDY AREA AND METHODS pellet was 1.8 _+ 0.9 (range = 1-4). Barn Owls preyed mainly on rodents (95.1%). Hares and insectsmade up The studysite was located in the ReserveArea of Na- 0.5% and 4.4% of prey, respectively.The two European huel Huapi National Park, in northwesternArgentine Pa- hares (Lepuseuropaeus) fbund in the diet were newborns. tagonia (71ø07'25'qN,40ø47'14%) at 700 m elevation above sea level. The area is an ecotone between the arid Insectswere all in the family Scarabaeidae(Table 1). Patagoniansteppe to the east and the southern beech By percent frequency,the most consumedsigmodon- (Nothofagusspp.) foreststo the west. The site was domi- tine specieswere Abrothrixlongipilis, Loxodontomys nated by bunchgrasses(Stipa speciosa) and cushionbushes micropus,and Oligoryzomyslongicaudatus. In ternis of bio- ( Mulinum spinosum)with scatteredtrees (Austrocedruschi- mass,Loxodontomys micropus was the most important prey lensis,Maytenus boaria, and Populusnigra). At tinres,wil- in the diet, followed by Abrothrixlongipilisand Oligoryzomys lows (Salixfragilis) formed small gallery forests. longicaudatus(Fig. 1). Owl roostswere located by observingareas of white- The Barn Owl feeds almostexclusively on small mam- wash or recording placeswhere pellets were found. Pel- mals throughout its range, although the proportions of lets were collected every two weeksfrom June 1993-May other prey may vary slightly(Taylor 1994). Barn Owls in 1994 at two lmown roost sites.Pellets were grouped into calendarseasons, oven-dried in 70øCfor 72 hr, and pro- our study preyed almost exclusivelyon rodents with ju- cessedfollowing standard methods (Marti 1987). Most venile hares and insects rarely appearing in the diet, prey were identified to species.Mammalian prey were mostlyin spring.We did not find birds, reptiles, nor am- identified and quantified on the basisof skullsarid den- phibians to be important prey aswas the casein La Pam- tanes using reference collections and keys (Pearson pa, Argentina (Noriega et al. 1993). 1995). Insectswere quantified by counting head capsules Based on the literature, the most important prey of and mandibles. Barn Owls in Argentine Patagoniaare Eligmodontiamor- Biomassof each rodent speciesin the total biomassof gani and Reithrodonauritus (De Santisand Pagnoni 1989, the diet was calculated by multiplying mean body mass of individualsby the number of individualsin pelletsand De Santis et al. 1993, Tiranti 1996, Travaini et al. 1997). expressedas a percentage of total rodent biomasscon- In our studyarea, neither speciesrepresented >4% and sunred.We calculatedthe geometric mean of weight of 8% of total prey items, respectively.This was not surpris- prey (Marti 1987): GMW = antilog (Z n,log w,/E n•), ing because the habitat characteristicsof our study area

334 DECEMBER 2000 SHORT COMMUNICATIONS 335 336 SHORT COMMUNICATIONS VOL. 34, NO. 4

40•

35•,

30•

25

• Z0

15

10

Ct A1 Ax Em Ech It Lm O1 Px Ra

Species

•Frequency(%) •Biomass (%) I

F•gure 1. Frequencyand biomassof rodent prey speciesin the diet of the Barn Owl. Biomassis expressedas the percentageof biomassof each speciescalculated on total rodent biomass.Ct--Ctenomys haigi, A1--Abroth•ix longipilis, Ax-•A. xanthorhinus,Em--Elig'modontia morgani, Ech-- chinchilloides, It--Irenomys tarsalis, Lm--Loxodontomys mzcropus,Ol--Oligoryzomys longicaudatus, Px-- xanthopyga, Ra--Reithrodon au•itus.

were not optimal for these rodents, which prefer the variableproportions (Don•zar et al. 1997,Trejo and Gri- more xeric and open habitatsof the Patagoniansteppe gera 1998). Evenjuvenile hares may not be very suitable (Pearson 1995). prey for Barn Owls since they are much smaller than The most common speciesin the diet, both in fre- Great Horned Owls (Everett et al. 1992). According to quency and biomass,Abroth•ix longipilis, Oligoryzomys lon- Jaksi• (1986), this is a common situation in southern gzcaudatus,and Loxodontomysmicropus are good climbers South America where some predators hunt mainly the and prefer brushy places, although the latter is also more abundant native rodents, often ignoring abundant found in shallowwet grasslands(Pearson 1983). Taking introduced lagomorphs.Jaksi• (1986) attributed thisfact th•s into account,we inferred that the most frequently- to an "escapein size." Maximum weight ofjuvenile hares usedhabitats in the Barn Owls' hunting range were those is about 300 g (Bonino and Montenegro 1997), which wxthgood vegetationcover and ample water. puts them beyond the size of prey more frequently con- Hares were only occasionallyeaten by Barn Owls de- sumed by Barn Owls. Rabbits, although smaller than spxte their relative abundance (approximately 4-18 hares,were probablynot in our studyarea. hares/ha; Novaro et al. 1992), their crepuscularor noc- Mean weightsand sizesof rodentswere approximately turnal habits, and their open nests (Bonino and Monte- the same during the four seasons,suggesting that in our negro 1997), all traitswhich might make them vulnerable study area the Barn Owl preyed more upon medium- to an aerial nocturnal predator like the Barn Owl. There sized (A. longipilis,L. micropus,and O. longicaudatus)than xsonly one citation for the Argentine Patagoniarecord- on the smaller-sized(A. xanthorhinusand E. morgani)ro- xng by Barn Owls on rabbits (Oryct01aguscunic- dents in the area. Mean weight of prey of the Barn Owl ulus,0.1% of total prey,Travaini et al. 1997). In central in Chilean Patagoniais smaller (29.9 g), due to a greater Chile, the proportion of rabbitsin the diet of Barn Owls consumptionof smaller species(Iriarte et al. 1990). xsalso very low (0.03% of total prey, Herrera and Jaksi• Food-niche breadth is intermediate, as has been shown 1980). In Chilean Patagonia,Iriarte et al. (1990) did not in Chilean Patagonia (Iriarte et al. 1990). This indicated record predation on hares by Barn Owls although they that the Barn Owls in our study behaved essentiallyas were eaten by Great Horned Owls (Bubovirginianus) in specialized rodent predators. Diets of sympatric Great DECEMBER 2000 SHORT COMMUNICATIONS 337

Horned Owls have been studied in two sites in north- EVERETT, M., I. PRESTT AND R. WAGSTAFFE. 1992. Barn and western Patagonia (Donazar et al. 1997, Trejo and Gri- Bay Owls 7}to, Pholidus.Pages 36-50 in J.A. Burton gera 1998) and, in both cases,a lower food niche breadth [ED.], Owls of the world. Eurobook, Italy. (0.20) was found to be due to lower speciesevenness in GARCiAESPONDA, C.M., L.J.M. DE SANTIS,J.I. NORIEGA, the diet. G.O. PAGNONI,G.J. MoREIRA,AND N.M. BERTELLOTTI 1998. The diet of Tytoalba (Strigiformes:Tytonidae) RESOMEN.--Enel presente trabajo se estudi6 la diem de in the lower Chubut valley (Argentina). Neotropica44' Tyroalba tuidara en el noroestede la Patagoniaargentina. 57-63. Los roedores sigmodontinosfueron el componente prin- HERRERA,C.M. ANDEM. JAtcSI&1980. Feedingecology of cipal de la dieta, tanto en nfimero como en biomasa.Las the Barn Owl in central Chile and southernSpain. A liebres y los insectosfueron poco consumidos.Las espe- comparativestudy. Auk 97:760-767. cies de roedores mas consumidasfueron Abrothrixlongi- IRIARTE,J.A., W.L. FRANKLIN,AND W.E. JOHNSON.1990. pills, Loxodontomysmicropus y Oligoryzomyslongicaudatus. De los datos de la diem y teniendo en cuenta los habitats de Diets of sympatricraptors in southern Chile. J. Raptor Res. 24:41-46. las presasse infiere que la actividad de caza de T. albaen el area de estudio se desarrol16preferentemente en am- JAIqSIC,F.M. 1986. Predation upon small mammals in bienteshamedos o m6sicoscon buena coberturavegetal. shrublandsand grasslandsof southern South Amen- [Traducci6n de los autores] ca: ecologicalcorrelates and presumableconsequenc- es. Rev. Chil. Hist. Nat. 59:209-221. LITERATURE CITED , R. PERSICO,AND J. TORRES.1977. Sobre la parti- BELLOCQ,M.I. 1990. Composicitn y variacitn temporal citn de recursospot las Strigiformesde Chile central. de la dieta de Tytoalba en ecosistemasagrarios pam- An. Mus. Hist. Nat. Valparaiso10:185-194. peanos, Argentina. Vida Silv. Neotrop.2:32-35. KREBs,CJ. 1989. Ecological methodology. Harper and -- ANDEO. KRAVETZ.1994. Feedingstrategy and pre- Row, New York, NY U.S.A. dation of the Barn Owl (Tyt0alba) and the Burrowing LEVINS,R. 1968. Evolution in changing environments: Owl (Speotytocunicularia) on rodent species,sex, and some theoretical explorations. Princeton Univ. Press, size, in agrosystemsof central Argentina. Ecol.Aust. 4: Princeton, NJ U.S.A. 29-34. MARTI,C.D. 1987. Raptor food habit studies.Pages 67- BONINO,N. ANDA. MONTENEGRO.1997. Reproductionof 80 in B.A. Giron Pendleton, B.A. Millsap, K.W. Cline, the European hare in Patagonia,Argentina. Acta Ther- and D.M. Bird lEDs.], Raptor management tech- iol. 42:47-54. niques manual. Sci. Tech. Set. 10. Natl. Wildl. Fed., CANEVARI,M.P., P. CANEVARI,G.R. CARRIZO,G. HARms,J. Washington, DC U.S.A. RODRiGUEZMATA AND R.J. STRANECK.1991. Nueva MONJEAU,J.A. 1989. Ecologiay descripcitngeografica de guia de las avesargentinas. Fundacitn Acindar, Buen- los pequefiosmamiferos del Parque Nacional Nahuel os Aires, Argentina. Huapi y areas adyacentes. Ph.D. dissertation, Univ. COLWELL,R.K. ANDD.J. FUTUYMA.1971. On the measure- Nac. de La Plata, La Plata, Argentina. ments of niche breadth and overlap. Ecology52:567- NORIEGA,J.I., R.M. ARAMBURt),E.R. JUSTO,AND LJ.M. DE 576. SANTIS.1993. Birdspresent in pelletsof Tytoalba (Stn- DE SANTIS,LJ.M. ANDG.O. PAGNONI.1989. Alimentacitn girofmcs,Tytonidae) from Casade Piedra, Argennna. de Tytoalba (Aves:Tytonidae) en localidadescosteras J. RaptorRes. 27:37-38. de la Provinciadel Chubut (Reptblica Argentina). NovARo, A., A. CAPURRO,A. TRAVAINI,M. FUNES,AND J. Neotropica35:43-49. RABINOVICH.1992. Pellet-count sampling based on , I.M. PElqA COZZARIN, AND M.F. GROSSMAN. 1993. spatialdistribution: a casestudy of the European hare Vertebrados depredados pot Tyto alba (Aves, Tytoni- in Patagonia. Ecol.Aust. 2:11-18. dae) en las proximidades del rio Corintos (Provincia PEARSON,O.P. 1983. Characteristics of a mammalian fau- del Chubut, Argentina). Neotropica39:53-54. na from forestsin Patagonia, southern Argentina. J , C.M. GARCIAESPONDA, AND GJ. MOREIRA.1996. Mammal. 64:476-492. Vertebrados depredados por Tyto alba (Aves:Tytoni- --. 1995. Annotated keysfor identifying small mam- dae) en el sudoestede la provincia de Chubut (Ar- mals living or near Nahuel Huapi National Park or gentina). Neotropica42:123. Lanin National Park, Southern Argentina. Mastozool. DON/i,ZAR, J.A., A. TRAVAINI,O. CEBALLOS,n. DELIBES, Neotrop.2:99-148. AND F. HIRALDO. 1997. Food habits of the Great REDFOP,D, K.H. ANDJ.F.EISENBERG. 1992. Mammals of the Horned Owl in northwestern Argentine Patagonia: Neotropics,the southern cone. Vol. 2. Univ. Chicago the role of introduced lagomorphs.J. RaptorRes. 31: Press,Chicago, IL U.S.A. 364-369. TAYLOR,I. 1994. Barn Owls. Predator-preyrelationships 338 SHORTCOMMUNICATIONS VOL. 34, NO. 4

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