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Home , Abrothrix longipilis, Chelemys macronyx, Geoxus valdivianus, Irenomys, Lagomorpha, Leporidae

j. RaptorRes. 32(4):306-311 ¸ 1998 The Raptor ResearchFoundation, Inc.

FOOD HABITS OF THE GREAT HORNED (BUBO VIRGINIANUS) IN A PATAGONIAN STEPPE IN ARGENTINA

ANA TREJOAND DORA GRIGERA CentroRegional Universitario Bariloche, Universidad Nacional del Comahue,Unidad Postal Universida& 8400 Bariloche,Argentina

ABSTRACT.--Westudied seasonal variation in the diet of the (Bubo virginianus) through pellet analysis.Pellets were collectedevery month during 1995-96 from a steppearea in north- westPatagonia, Argentina. We identified1216 prey itemsin 522 pellets. accounted for 98.5% of the diet while the remainder consistedof a variety of birds and insects.Rodents most frequently found in pelletswere Eligmodontiamorgani, longipilis, A. xanthorhinus,Oligoryzomys longicaudatus, Reithrodonauritus, and Ctenomyshaigi. In terms of biomass,the most important specieswere R. auritus, A. longipilis,C. haigi,and E. morgani.Food-niche breadth was greatest in winter.Within the studyarea, the Great Horned Owl should be considered to be a specialistall year round. KEYWORDS: GreatHorned Owl; Bubo virginianus;diet;, rodents; Patagonia.

Hfibitos alimentariosdel Bubovirginianus en un firea estepariadel noroestede la PatagoniaArgentina RESUMEN.--Seestudi6 estacionalmentela dieta de Bubovirginianus mediante el anfilisisde egagr6pilas recolectadasmensualmente durante los aftos 1995 y 1996, en un '•treaesteparia del noroestede la PatagoniaArgentina. Fueron analizadas522 egagr6pilasque contenian1216 presas.E1 98.5% de las presaseran roedores,mientras qe el 1.5% restanteeran principalmenteaves e insectos.Entre los roe- dores consumidosse encontraronen mayor nfimero ejemplaresde Eligmodontiamorgani, Abrothrix lon- gipilis,A. xanthorhinus,Oligoryzomys longicaudatus, Reithrodon auritus y Ctenomyshaigi. En t6rminosde biomasa, las mayorescontribuciones corresponden a R. auritus,A. longipilis,C. haigiy E. morgani.La amplitud tr6fica alcanzael valor m•tximo en el invierno. En el firea estudiadaB. virginianuspuede considerarseun especialistaen roedoresdurante todo el afio. [Traducci6n de Autores]

The Great Horned Owl (Bubovirginianus) is dis- changes in diet composition and food-niche tributed widely throughout the Americas and it breadth over two years (1995-96). lives in a variety of different habitats (Burton STUDY AREA AND METHODS 1992). Its food habits have been studied at many different sites in North America. In South America, Our studywas conducted in northwestPatagonia, east severalquantitative studieshave been carried out of the city of Bariloche,Argentina (41ø08'-41ø08'45"S, 71ø12'-71ø13'20%V).The study site was located in a in Chile (Jaksicet al. 1978,Ygtfiez et al. 1978,Jaksic steppe area of the transition zone between the subant- and Yfifiez 1980, Jaksicand Marti 1984,Jaksic et al. arctic and the Patagonian steppe. The area is 1986, Iriarte et al. 1990) and Argentina (DonSzar dominated by bunchgrassessuch as Stipa speciosaand et al. 1997). Marti et al. (1983) reviewed studiesof Acaenasplendens and scatteredbushes ( Seneciofilaginoides, the owl's diet in North and South America. Most Baccharislinearis, Colletia hytstrix and the exotic species Rosarubiginosa). A road lined by exotic conifers (Pinus of these studiesreported Great Horned main- spp. and Cupressusspp.) ran through the area. These ly preying on rodents and lagomorphs,although trees provide roostsfor the Great Horned Owl. there were regional, seasonal,yearly and long-term The small community in the area has been differences in diet. studied by Guthmann (1996) and Guthmann et al. (1997). Accordingto them, the fauna consistsof repre- Our studyanalyzed the food habitsof the Great sentativesof forestand steppespecies dominated by Elig- Horned Owl in a steppe area in northwest Pata- modontiamorgani, Reithrodon auritus and Abrothrixxanthor- gonia, Argentina, and described the seasonal hinus,which are typical of semiarid steppe. A. longipilis

306 DECEMBEI• 1998 GREAT HORNED OWL DmT IN ARGENT•N^ 307 inhabits areas of dense to bushy steppe, and Oli- 1). The remaining 1.5% consistedof birds, insects, goryzomysIongicaudatus is abundant in brush areasand the edgesof forests(Pearson 1995). Smaller numbers of Lox- one , and one Iagomorph (a young Lepus odontomysmicropus inhabit humid or mesic brushy habi- about 0-6 months old accordingto cranial sutures tam, and Ctenomyshaigi inhabits open areas with sandy describedby Gonzfilez [1993]). We found one in- soils (Pearson 1995). There were so far no records of dividual each of the following birds in the diet: other nocturnal raptor specieswithin the study site, al- Tachycinetaleucopyga, Troglodytes aedon, Sicaris tuteota, though Barn Owls (Tyt0 alba) were probablyin the area. Owl roostswere located by observingareas of white- Zonotrichiacapensis, Anthus sp., and one unidenti- wash or recording placeswhere pellets were found. Pel- fied Furnariidae. Insects that could be identified lets were collectedmonthly from February 1995-Novem- were Coleopterans (one of them Scarabaeidae) ber 1996 at six known roost sites. Pellets were air dried and Lepidopterans. and their length and width was measuredwith an elec- Great Horned Owls preyed mainly on Eligmodon- tronic caliper to the nearest 0.01 mm. The pellets were dissectedusing standard techniques (Yalden 1990). Var- tia morganiover both years of the study,followed iations in measurements were related to the number of by Abrothrixlongipilis, A. xanthorhinus,Oligoryzomys prey contained in the pelletsby means of a one-wayAN- long•caudatus,Reithrodon auritus, Ctenomys haigi, and OVA. Prey biomasswas calculated only for rodents.Mass Loxodontomysmicropus. The number of rodents con- estimates for each prey taxon were either determined from individualscaptured in the studyarea or taken from sumedvaried seasonallyand waslower during win- literature. ter. There were significantdifferences between the Prey were identified to the finest possibletaxonomic number of prey of different specieseaten in 1995 level. Mammalian prey were identified and quantified on and 1996 (X2 = 14, df = 6, P • 0.05). The greatest the basisof skullsand dentary pairs using reference col- difference between the two years was the lower lectionsand keys(Pearson 1995). Insectswere quantified by counting head capsulesand mandibles.For other prey than expected consumption of L. micropusand O. items, reference collectionswere used and they were longicaudatusin 1996. There were no significant quantified by assumingminimum number of individuals differencesin the number of different speciescon- (e.g., feathers or scalesof a given specieswere deemed sumed between winters (X2 -- 10.01, df = 6, P < to represent only one individual). Diet compositionwas 0.05), but consumptionof prey did differ signifi- comparedbetween seasons and yearswith chi-squareand G tests. canfly between summers(X 2 = 35.93, df = 6, P < The contribution of each rodent speciesto the biomass 0.05), autumns (G = 29.64, df = 6, P < 0.05) and of the owls'diet wascalculated by multiplyingmean body springs(X • = 33.74, df = 6, P < 0.05). massof individualsby number of individualsin the pel- The mean weight of rodent prey ranged from lets. Values were expressedas a percentage of total ro- dent biomass consumed. 15.3 g for A. xanthorhinusto 146.2 g for C. haigi Food-nichebreadth (FNB) wasestimated using Levins' (Table 2). R. auritus, C. haigi and E. morganicon- (1968) index:FNB = 1/(E pi2), wherepi is the propor- tributed most to the prey biomass and all three tion of prey taxon I in the diet. A standardized-niche were consumed in a greater proportion in 1996 breadth value (FNBst) wascalculated, which ranged from than in 1995. In the pellets collected during 1995, 0 to 1: FNBst -- (FNB - 1)/(n - 1), where n is the total number of prey categories(Colwell and Futuyma1971). the proportion of R. auritusin the diet fell consid- Evennessof prey numberswas measured using the Shan- erably in spring, while that of A. longipilisand C. non-Wiener function J' (Krebs 1989): J' = H'/log n, haigi rose. where H' is the Shannon-Wiener formula and n is the Food-niche breadth was greatest in winter and total number of prey categories. smallestin spring during both 1995 and 1996. Al-

RESULTS though the number of prey typeswas highest in spring, evennesswas lower than in winter (Table A total of 1216 prey items was identified from 1). The standardized-niche breadth calculated for 522 pellets. The mean number of prey/pellet was the two years (FNB,t = 0.202) was slightly lower 2.3 (SD = 1.1; range = 1-7). Pelletmeasurements than for breeding seasons(spring and summer) ranged from 2.3-8.8 cm long (i = 4.5; SD = 1.1; for both years (FNBst= 0.218, N = 4). N = 516) and from 1.4-4.3 cm wide (• = 2.7; SD = 0.4; N = 516). Significantdifferences (P < 0.05) DISCUSSION were found for both length (F = 17.365, df = Because of their small size, Great Horned Owls 4,507) and width (F = 20.365,df = 4,506) and they that occur in southern South America have been appeared to be related to the number of prey in placed in their own subspecies(B. v. magellanicus, each pellet. Traylor 1958) and it has even been suggestedthat Rodentsaccounted for 98.5% of the prey (Table they in fact belong to their own species(Bubo ma- 308 TREJOAND GRIGERA VOL. 32, NO. 4

Table 1. Seasonaldiet of Great Horned Owls in northwesternPatagonia, Argentina. N = number of prey in each taxon; % calculatedover the total number of prey for each.

1995

SUMMER AUTUMN WINTER SPRING

PREY TYPE N % N % N % N %

MAMMALS Rodents

Muridae Abrothrixlongipilis 7 5.8 10 11.1 40 14.5 97 35.9 Abrothrix xanthorhinus 7 5.8 I 1.1 32 11.6 12 4.4 Eligodontiamorgani 39 32.5 19 21.1 70 25.5 58 21.5 Loxodontomysmicropus 2 1.7 10 11.1 12 4.4 10 3.7 Chelemysmacronyx ...... I 0.4 Reithrodon auritus 25 20.8 24 26.7 60 21.8 9 3.3 Oligoryzomys 27 22.5 18 20.0 25 9.1 43 15.9 valdivianus ...... Irenomystarsalis ...... Unidentified 7 5.8 5 5.6 30 10.9 21 7.8

Ctenomyidae Ctenomyshaigi 3 2.5 4 4.4 6 2.2 15 5.6 Lagomorphs Lepuseuropaeus ...... 0.0 BIRDS Passeriformes I 0.8 .... 2 0.8

REPTILES Liolaemussp. -- ...... INSECTS 2 1.7 .... 2 0.7

SPIDERS ...... Total prey 120 90 275 270 Total pellets 42 37 130 114 FNBst 0.446 0.667 0.672 0.308 J' 0.773 0.880 0.892 0.700 FNBst = food-niche breadth measuredwith standardizedLevins' index (see text for explanations). preysnumber evennessby Shannon-Wienerfunction. gellanicus,K6nig et al. 1996). Owing to their small gape of the owlslimited the sizeof the pelletsthey size, the averagelength and width of their pellets regurgitated. The low correlation between biomass are among the smallestreported for Great Horned and size of pellets could be a consequenceof es- Owls. Yfifiez et al. (1978) studied Great Horned timating biomassas average prey weight, without Owl pellets from two regions in Chile, and found consideringthat the predator might selectthe size that those that contained remains of rodents were of its prey. significantlywider than those containing arthro- Studiesin Chile (Yfifiezet al. 1978, Jaksicet al. pods. We could not verify this relationshipin our 1986, Iriarte et al. 1990) have found that, in some study becausethe pellets contained almost exclu- seasons, Great Horned Owls eat birds, insects, sively rodents. Nevertheless, we did find that, in arachnids,and lagomorphs(up to 17% in Torres terms of the number of prey contained in pellets, del Paine National Park [Iriarte et al. 1990]). In there wasless variation in pellet width than length. our study,these prey made up a negligible part of This could have been related to the fact that the the diet. Near Junin de los Andes,Argentina, Don- DECE•4BER 1998 GP,r•T HORNEt) OWL DIET IN ARGENTINA 309

Table 1. Extended.

1996

SUMMER AUTUMN WINTER SPRING TOTAL

N % N % N % N % N %

49 25.0 12 14.5 11 15.7 31 27.4 257 49.2 25 12.8 3 3.6 11 15.7 7 6.2 98 18.8 59 30.1 31 37.2 16 22.9 23 20.4 315 60.3 3 1.5 .... 4 3.5 41 7.9 ...... 1 0.2 19 9.7 6 7.2 10 14.3 21 18.6 174 33.3 17 8.7 9 10.8 7 10.0 7 6.2 153 29.3 -- -- I 1.2 .... 1 0.2 ...... I 0.9 I 0.2 12 6.1 16 19.3 8 11.4 10 8.8 109 20.9

6 3.1 5 6.0 5 7.1 3 2.7 47 9.0

I 1.4 -- -- I 0.2

2 1 -- -- 1 1.4 -- -- 6 1.2

...... 1 0.9 1 0.2

2 1.0 .... 4 3.6 10 2.0

...... 1 0.9 1 0.2

196 83 70 113 1216 73 33 39 54 522 0.394 0.429 0.672 0.376 0.202 0.746 0.794 0.885 0.764 0.597

ftzar et al. (1997) found the diet of Great Horned mammal in shrublands and grasslands Owlsconsisted of 11.9%Lepus europaeus and 27.3% of southern South America, with predators hunt- arthropodsbut, in terms of biomass,the two main ing mainly the most abundant native rodents, prey items (55.2% of total prey) werejuveniles of sometimes"ignoring" dense populations of intro- introduced lagomorphs (L. europaeusand Oryctola- duced lagomorphs. gus cuniculus). The proportion of lagomorphswe found in the Lagomorphsare consideredto be the best prey diet did not support the generalizationby Donftzar for horned owlsbecause their large body massbest et al. (1997) that lagomorphsrepresent 15% by suitsthe daily energy requirementsof owls (Don- number of the diet of Great Horned Owls in Ar- ftzaret al. 1989). In our study,the number of lago- gentine Patagonia. However, our resultsreinforce morphs in the diet wasremarkably low despitethe their explanation for the low frequency of lago- apparentabundance of L. europaeusin the area (7- morphs in the diet of Patagonian Great Horned 12 individuals/ha, Novaro et al. 1992). According Owls as compared to horned owls at similar lati- to Jaksic (1986), this situation is common for small tudes in the northern hemisphere, where they 310 TREJOAND GmGEP,A VOL. 32, NO. 4

Table 2. Biomassof rodents in Great Horned Owl diets in Argentina expressedas a percent of the total biomassof rodents consumedin each season.Mean prey weightswere obtained froln Pearson (1983) for C. macronyx,G. valdi- v•anusand I. tarsalis;from Pearson (pers. comm.) for C. haigi, and from Trejo (unpubl. data) for the remaining species.

PREY ABL ABX ELI LOX CHE REI OLI GEO IRE CTE

1995 Sintuner 5.2 2.8 17.1 3.0 -- 42.2 18.1 -- 11.6 Autumn 7.5 0.4 8.4 15.0 -- 40.9 12.2 -- 15.6 Winter 12.8 5.6 13.2 7.7 -- 43.5 7.2 -- 10.0 Spring 32.6 2.2 11.5 6.7 0.8 6.9 13.1 -- 26.2 1996

Summer 25.4 7.0 18.0 3.1 -- 22.3 8.0 -- -- 16.2 Autumn 14.9 2.0 22.7 -- -- 16.9 10.1 1.2 -- 32.2 Winter 13.5 7.3 11.6 -- -- 27.9 7.8 -- -- 31.9 Spring 24.3 3.0 10.7 6.3 -- 37.4 5.0 -- 1.2 12.2 TOTAL % 18.9 3.9 13.7 6.0 0.2 29.0 10.2 0.1 0.1 18.0 Prey mean 28.1 15.3 16.6 56.2 66.8 63.8 25.4 27.8 41.3 146.2 ABL, Abrothrixlongipilis; ABX, Abrothrixxanthorhinus; ELI, Eligmodontiamorgani; LOX, Loxodontomysmicropus; CHE, Chelemysmacronya;, REI, Reithrodonauritus; OLI, Oligoryzomyslongicaudatug GEO, Geoxusvaldivianug IRE, Irenomystarsalis; CTE, Ctenomyshaigi. weigh on average30-40% more. Donfizar et al. salis, macronyx and Geoxusvaldivianus were (1997) suggestedthat the large size of adult lago- found in the pellets, their occurrence was note- morphs could constrain Patagonian horned owls worthy becausenone of them were captured dur- from preying on them, while large rodents and ing the three years over which Guthmann (1996) young lagomorphsmay be more easilyhandled. systematicallytrapped rodents at the same site E. morgani,the mouseconsumed most frequently where we collected pellets. All three speciesare numerically and whosebiomass had least seasonal typical of the forest and environmentswith high variation, would not seem to be a profitable prey plant coverage (Pearson 1983, 1995). Their occur- due to its low weight (20 g). The energetic costof rence in the diet indicated that they may havebeen capturing and handling thesemice may exceedthe present in the area in low numbers,or perhapsthe actual gains (Jaksicand Marti 1984). Nevertheless, fossorial habits of G. valdivianusand C. macronyx E. morganiwas abundant in the area (Guthmann made them difficult to capture. 1996) and vulnerable, the two conditionsthatJak- There was seasonal variation in food-niche sic and Marti (1984) considerappropriate for such breadth. Both the overall food-niche and breeding small prey to be included in the diet of Buboowls. season (spring and summer) food-niche breadths It is easyto catch becauseit inhabits siteswith little were similar to those calculatedby Don•zar et al. plant cover or bare ground and it runs in the open (1997) in their studyin Junin de losAndes. They for prolonged periods (Trejo pers. obs.). The prey estimated a standardized food-niche breadth of that supplied the greatestbiomass in our studywas 0.20 for the breeding season.Jaksic et al. (1986) Reithrodonspp. According to Pearson (1988), its found that the horned owl diet breadth in Chile ,long hours of feeding, open habitat, declined from north to south based on standard- and unwary behavior seem to expose it to preda- ized food-niche breadth measurements obtained tion by owlsand other nocturnal predators.Cteno- for three Chilean locations at different latitudes: myshaigi, the largestrodent in the area, was eaten La Dehesa, 33ø21'S (FNBst = 0.66), Puerto Ib•fiez, in low numberslikely due to its fossorialhabits. O. 46ø18'S (FNBst= 0.62) and Torres del Paine, 51øS longicaudatusand L. micropus,which inhabit areas (FNB•t = 0.24). However, the FNB•t = 0.20 ob- coveredby bushyvegetation, are scansorial(Pear- tained both for Junin de los Andes (39ø30'- son 1983, 1995), which could facilitate their detec- 40ø20'S, 70ø30'-71ø30'W, Donazar et al. 1997) and tion and capture by owls. for Bariloche(41ø08'S, present study) does not fit Although only one specimeneach of Irenomystar- within the latitudinal trends proposed for Chile. DECEMBER 1998 GRF•kT HORNED OWL DIET IN ARGENTINA 311

The FNBst = 0.60 obtained by Iriarte et al. (1990) J•a•si½,F. 1986. Predation upon small in shrub- for Torres del Paine is also at odds with the pro- lands and grasslandsof southern South America: eco- posed latitudinal trend. Consideringthe seasonal logical correlates and presumable consequences.Rev. Chil. Hist. Nat. 59:209-221. fluctuations in the composition of the diet, com- -- ANDJ. Y•d•EZ.1980. Differential utilization of prey parisonsbetween different locationsshould prob- resourcesby Great Horned Owls and Barn OMs •n ably be done using data from the same time of Central Chile. Auk 97:895-896. year. --AND G.D. MARTI. 1984. Comparative food habits In our study,the five speciesmost consumedby of BubooMs in Mediterranean-typeecosystems. Condor horned owls had minimum population levels in 86:288-296. winter, while in other seasonsthey reached peak , J.R. 1L•u ANDJ. Yk•EZ. 1978. Oferta de presasy numbers (Guthmann et al. 1997). This would ex- predaci6n por Bubo virginianus (Strigidae) en el plain the increase of the food-niche breadth in Parque Nacional "Torres del Paine." Ans.Inst. Pat 9: 199-202. winter caused by the greater evenness,since the , J.L. YA•rz ANDJ.R. l•u. 1986. Prey and troph•c availabilityof all prey species. ecology of Great Horned OMs in western South ACKNOWLEDGMENTS America: an indication of latitudinal trends. Raptor Res. 20:113-116. We are grateful to J. Noriega for the identificationof K(•NIG, G., P. HEIDRICH AND M. WINK. 1996. Zur Taxon- birds in the pellets and to L. De Santis,M. Kun, E Cres- omie der Uhus (Strigidae: Bubo spp.) im sfidlichen po, H. Planas,N. Ibarg/iengoytia,D. Afitn Sufirez,and J. Sfidamerika.Stuttgarter Beitr. Naturk. Ser. A 540. Puntieri for their help at different stagesof this study.We would also like to thank the refereeswho revised the orig- KREBS,GJ. 1989. Ecological methodology. Harper and inal manuscriptfor their valuablecontributions and sug- Row, New York, NY U.S.A. gestions. LEVINS, R. 1968. Evolution in changing environments. some theoretical explorations. Princeton Univ. Press, LITERATURE CITED Princeton, NJ U.S.A. MARTI,G.D., E. KORPI 'MAinAND F.M. JAKSIC.1983. Trophic BURTON,J.A. [ED.]. 1992. Owls of the world. Peter Lowe, structure of raptor communities: a three-continent Eurobook, Italy. comparisonand synthesis.Pages 47-137 in D.M. Pow- COLWELL,R.K. ^NODJ. FUTUYMA.1971. On the measure- er [ED.], Current Ornithology. Vol. 1. Plenum Press, ment of niche breadth and overlap.Ecology 52:567- New York, NY U.S.A. 576. Nov•3to, A., A. CAPURRO,A. TRAVAINI,M. FINESAND j DON•ZAR,J.A., F. HIRALDO,M. DELIBESAND R.R. ESTREL- R2mINOVICH.1992. Pellet-count sampling based on I•. 1989. Comparativefood habits of the Owl spatialdistribution: a casestudy of the European Bubobubo and the Great Horned Owl Bubovirginianus in Patagonia.Ecol. Aust. 2:11-18. in six Palearctic and Nearctic biomes. Ornis Scand. 20: PEARSON,O.P. 1983. Characteristics of a mammalian fau- 298-306. na from forests in Patagonia, Southern Argentina. J. , A. TRAVAINI, O. CEBALLOS,M. DELIBESAND E HIR- Mammal. 64:476-492. ^LDO. 1997. Food habits of the Great Horned Owl in 1988. Biology and feeding dynamics of a South northwestern Argentine Patagonia:the role of intro- American herbivorous rodent, Reithrodon. Stud. Neo- duced lagomorphs.J. RaptorRes. 31:364-369. trop.Fauna Environ.23:25-39. GONZAI•EZ,S. 1993. Estudio craneomttrico de la liebre --. 1995. Annotated keysfor identifying small mam- (Lepussp.) introducidaen el Uruguay (: mals living in or near Nahuel Huapi National Park or ). Bol. Soc.Zool. Uruguay 8:304-312. Lanin National Park, Southern Argentina. Mastozool- GUTHMANN, N. 1996. Estudio de un ensamble de roe- ogiaNeotrop. 2:99-148. dores en la estepa ecotonal patagtnica. TesisDoctor- TRAYLOR, M.A. 1958. Variation in South American Great al, Universidad Nacional del Comahue, Centro Re- Horned Owls. Auk 75:143-148. gional Universitario Bariloche, S.C. de Bariloche, Y)mD•N, D.W. 1990. The analysisof owl pellets. Occas Argentina. Publ. Mammal. Soc. No. 13, London, U.K. --, M. LOZAD^,J.MONJE^U ^ND K. HEIN•M•N. 1997. YJdqEZ,J., J.R. P•u ANDF. Jamsic.1978. Estudiocompar- Population dynamicsof five sigmodontinerodents of ativo de la alimentacitn de Bubo virginianus(Stng•- northwesternPatagonia. Acta Theriologica42:143-152. dae) en dos regionesde Chile. An. Mus. Hist. Nat. 11' I•l•OtT•,J.A., W.L. FW•4I•IN •D W.E.JOHNSON. 1990. Di- 97-104. ets of sympatricraptors in Southern Chile. J. Raptor Res. 24:41-46. Received21 November 1997; accepted27 July 1998