Owls Along an Elevational Gradient in Andean Argentine Patagonia

J RaptorRes. 31 (!) :59-64 ¸ 1997 The Raptor ResearchFoundation, Inc.

FOOD HABITS OF COMMON BARN-OWLS ALONG AN ELEVATIONAL GRADIENT IN ANDEAN ARGENTINE PATAGONIA

ALEJANDROTRAVAINI, Jos• A. DONAZAR EstacitnBioltgica de Do•ana, ConsejoSuperior de InvestigacionesCientfficas, Apartado1056, 41080 Sevilla,Spain

OLGA CEBALLOS Grupode Estudios Bioltgicos Ugarra, CarlosIII 19, 31002 Pamplona,Spain

ALEJANDRORODRIGUEZ, FERNANDO HIRALDO, MIGUEL DELIBES EstacitnBioldgica de Dohana, ConsejoSuperior de InvestigacionesCientificas, Apartado1056, 41080 Sevilla,Spain

ABSTRACT.--Weevaluated the diet of Common Barn-owls (Tyt0 alba) along an elevational gradient in Argentine Patagonia.Small mammals(mainly rodents) were the main prey accountingfor 93.2% of total prey items. Consumptionof rodentsappeared to be dependent on their availability.Sizes of mammalian prey were variablebut most ranged from 10-100 g in body mass.We concludedthat the diet of these barn owlscould be used as an index of cricetid rodent populationsalong the gradient. KEyWORDS: CommonBarn-owl; Tyto alba; prey;Patagonia; selectivity; gradient.

Diem de la lechuza (Tyt0 alba) a lo largo de un gradiente altitudinal en la Patagonia Andina Argentina. REStJMEN.--Seestudi6 la dieta de la lechuza (Tyto alba) y se la contrast6 con la composici0n especifica de la comunidad de micromamiferos a lo largo de un gradiente altitudinal en la PatagoniaArgentina. La principal presala constituyeronpequefios mamlferos (fundamentalmenteroedores) alcanzandoun 93.2% del total de presasconsumidas. La masa corporal media de los mamlferospresa se concentr6 fuertemente en un rango comprendido entre 10 y 100 g. La diem de la lechuza result6 ser un bufin indicador de la composici0nde Cricetidosa lo largo del gradiente estudiado.E1 consumode cada uno de los roedorespresa dependi6 de su disponibilidaden el terreno. [Traducci6n Autores]

Few of the many studies of Common Barn-owl forest, while another seven rodent species (Cteno- (Tyro alba) food habits have examined dietary re- mys haigi, Akodon xanthorrinus,Reithrodon auritus, sponsesto elevational distribution of prey species Eligmodontiatypus, Phyllotis darwini, Euneomyssp, (Herrera 1974, Brunet-Lecomte and Delibes and a marsupial, Lestodelphishally) were in the dry 1984). In northwestern Patagonia, a steep eleva- shrubsteppe.They also analyzed the diets of Com- tional gradient (600-3000 m) occursover just a few mon Barn-owls at three different localities at the km. The abrupt change in elevation and the asso- forest-steppetransition and found that the propor- ciated change in precipitation (300-3600 mm) tions of speciescaptured by traps and by owlswere causesa distinct shift in the vegetation from shrub- different. steppe to montaneous forest habitat within a few Besidesproviding new information on the tro- km. Pearson and Pearson (1982) qualitativelyde- phic niche of Common Barn-owls in Argentina, scribed the small mammal species composition here we test the feasibilityof using barn owl food along this gradient. They found six speciesof ro- habits to describe changes in composition and dents (Aconnaemysfuscus, Dromiciops australis, No- abundance of small mammals along an elevational- tzomysmacronyx, Notiomys valdivianus, Irenomys tar- precipitation gradient. We considered the data salis and Akodon olivaceous)occurred in the humid provided by Pearsonand Pearson (1982) as the ac-

59 60 TRAVAINIET AL. VOL. 31, NO. 1

tual representation of the small mammal commu- mass of all small mammals (MWSM) in the diet; H'NGG, nity along this gradient and the barn owl prey as trophic diversity in relation to the number of individuals contributed by each higher taxonomic unit (mammals, its descriptor.Additionally, Pearson (1986) provid- birds, amphibians,invertebrates); H'NM, trophic diver- ed data on relative abundanceof cricetid species sity in relation to the small mammal component of the in eight different habitatsranging from steppeto diet (rodents, lagomorphs,marsupials); and H'NR, tro- forest over an 8-yr-period. phic diversity in relation to the number of individuals We made two comparisonsbetween estimatesof contributedby each rodent species.The latter three pa- rameters were computed by means of Shannon's infor- rodent availabilityand their occurrencein the diet mation function (Ludwig and Reynolds 1988). Corre- of common barn owls:(1) on a broad scale,along sponding valuesof evenness(J = H'/H'max) were also the elevational-precipitationgradient, using data calculated. from Pearson and Pearson (1982), and (2) on a The use of the barn owl food habits to detect and describe changes in the composition and abundance of fine scale,along a segmentof the complete gra- smallmammals along a gradientwas explored using cor- dient, using abundance estimatesfor rodent spe- respondence analysis(SAS 1987). This is a multivariate ciesgiven by Pearson(1986). We predictedthat the ordination method (Digby and Kempton 1987, Pielou barn owl diet would reflect gradientchanges in ro- 1984), applied on a data matrix that included frequen- dent community composition (Herrera 1974, Pear- ciesof appearance(%) of severalprey categories for each barn owl locality.This typeof analysispermits one to plot son and Pearson 1982, Taylor 1994), but it would points for both rows and columns (here localities and be lessaccurate in reflectingavailability of prey at small mammal prey categories)on the sameplane. Cor- a fine scale 0aksic and Yafiez 1979). respondenceanalysis is especiallyappropriate for matri- ceswith numerical frequencies(Cuadras 1980) and does STUDY AREA AND METHODS not normally require previous transformation of data (Digby and Kempton 1987). We used this analysisto ver- Located in northwesternPatagonia (70ø30'-71ø30'W; ify if the pattern in rodent speciesdistribution derived 39ø30'-40ø20'S),the studyarea constitutesa portion of from barn owl dietswas similar to that describedby Pear- the Precordilleragradient and partiallyoverlaps with the son and Pearson (1982) along both the elevationaland area studied by Pearson and Pearson (1982) and Pearson precipitation gradients. For this analysis,we restricted (1986). The greatestdistance between our site and that of Pearson was under 150 kin. The climate of the area is ourselvesto those localities (N = 23) with >30 identified prey items. dry and cold with frost throughoutmost of the year and Selection among potential prey specieswas studied frequent snowfall in winter. Topographically,the area consistsof plains from 800-900 m above sea level that only in intermediateand highland Piedmont.Barn owl selectivitywas evaluatedby comparingthe species'rank are dissectedby steeprugged valleysand large rivers.In for each cricetid rodent in the barn owl diet with the general, the study area consistsof lowland which coin- species'abundance in the field as estimatedby Pearson c•des with river valleysand highland Piedmont with an (1986). We used a Spearman Rank Correlation Coeffi- intermediate area between them. Pearson (1986, 1987, 1988) has described five different habitats in intermedi- cient (Siegel and Castellan1988) for this analysis.It was calculated from the sum of the ranks obtained for each ate and highland Piedmont: steppe or scatteredbushes, speciesin each of the five habitatsconsidered. Compar- usually< 1 m tall, usuallymixed with bunch grass,much ison was restricted to cricetids because these were the of the ground is devoidof vegetation;bunchgrass or hab- rodent speciesfor which Pearson (1983) estimated rela- itatswith relativelypure standsof one or more speciesof tive abundance. bunchgrass;weeds or areaswith denseweeds and grasses, usually growing in moist places;rocks or cliffs with tum- RESULTS bled rocks large enough to provide refuge for rodents; and bare ground or large areasnot vegetatedwith a sub- A total of 2447 prey itemswere identified from strate of fine screeor rock and very few rocky shelters. barn owl pellets. Small mammals were the main Barn owl pellets were collected from September-Feb- ruary 1991, 1992 and 1994-95. Pellets were collected at prey in northwestern Patagonia, accounting for 32 •solatedlocalities more than 2 km apart. On thisbasis, 93.2% of the total. Birds,amphibians and inverte- we assumedthat barn owls at each localitywere different bratesmade up the remaining6.8% (Table 1). Cal- individuals.Only fresh pelletswere included in our sam- culated from a random sampleof 71 pellets, the ples in order to restrict our study to the spring season and avoid seasonalvariation. All pellets were dissected mean number of prey/pellet was 1.85 (SD -- 1.09, using standard techniques (Yalden 1977). Small mammal range 1-6). remains were identified using taxonomic keys (Pearson Among mammals,rodents most frequently oc- 1986), reference specimenscollected in the study area curred in the diet, representingthe 98.9% of the and museum collections. Small mammal biomass was de- total (Table 1). The MWSM was54.2 g (SD = 36.7, rived from Pearson (1983, 1984) and Redford and Eisen- berg (1992). range 17.5 g for Calomysmusculinus, 286.1 g for FollowingHerrera andJaksic (1980), we characterized Microcavia australis) but most small mammals were barn owl food habitsby the followingparameters: mean between 10-100 g (Fig. 1). MARCH 1997 BARN OWL DIET IN PATAGONIA 61

Table 1. Composition of the Common Barn-owl in the lowland, midland, and highland piedmont, northwestern Argentine Patagonia.

TOTAL LOWLAND MIDLAND HIGHLAND PREYTYPE PIEDMONT(%) PIEDMONT(%) PIEDMONT(%) N %

MAMMALS

Rodents Hystricognath Ctenomyshaigi 13.1 3.1 3.0 134 5.5 Galea musteloides 1.7 0.3 0.0 13 0.5 Microcavia australis 0.3 0.0 0.0 2 0.1 Cricefids Akodonspp 9.6 21.4 25.0 484 19.8 Auliscomys micropus 0.2 6.3 18.7 226 9.2 Chelemysmacronix 0.0 1.0 3.1 37 1.5 Eligmodontiatypus 31.6 13.1 6.7 374 15.3 Euneomyssp 0.0 0.1 2.8 25 1.0 Geoxus valdivianus 0.2 0.3 1.0 13 0.5 Irenomystarsalis 0.0 0.1 0.5 5 0.2 Oryzomyslongicaudatus 6.6 8.2 6.0 171 7.0 Phyllotisdarwini 4.0 12.8 5.6 198 8.1 Reithrodon au•itus 23.0 16.9 24.4 522 21.3 Calomysmusculinus 5.4 1.9 0.0 51 2.1 Marsupials Marmosapusilla 1.8 1.1 0.0 22 1.0 Lagomorphs Oryctolaguscuniculus 0.0 0.2 0.1 3 0.1 Birds 1.3 0.8 0.3 19 0.8 AMPHIBIANS 0.0 0.5 0.0 5 0.2 INSECTS 0.3 11.9 2.8 143 5.8

TOTAL PREY (595) (980) (872) (2447)

H'NGG O.28 JNGG 0.20 H'NM 0.06 JNM 0.05 H'NR 2.06 JNR O.78

These barn owls had a relatively narrow diet as ysisaccounted for 49.5% of the variancein the diet shownby the H'NGG value (Table 1). The low even- (Fig. 2). Representationof localities and prey cat- ness index was largely due to their concentration egorieson the plane defined by the two axesclear- on mammalprey (Table 1). The diversityand even- ly segregatedthe dry, lowland Piedmont localities ness of the small mammal component (H'NM) (which tended to the positive zone of Axis I and showedthat the diet wasbased upon a smallnum- negativezone of Axis II) from the rest of localities ber of small mammal species,most of which were (which occupied all the space defined by Axis I rodents.Among rodents,H'NR (2.06) and evenness and the positivezone of Axis II). Prey in the low- (0.78) reached the highest values, denoting both land Piedmont localities included Ctenomyshaig't, the high number of rodent prey speciesconsumed Microcavia australis, Galea musteloides,Eligmodontia (N = 14, 82% of total presentin the area) and their typusand Calomysmusculinus. Prey in most of the overalleven representationin the diet. other localities, included in mid- and high Pied- Two axesgenerated by the correspondenceanal- montswere Akodonsp, Auliscomysmicropus, Chelemys 62 TRAVAINIET •L. VOI•. 31, NO. 1

25-

•15

•)

o ; I I 10 1 O0 1.000 Body mass (g)

Figure 1. Relativefrequencies (%) of smallmammal prey in the diet of CommonBarn-owls in northwesternAr- gentinePatagonia, ordered along a logarithmicaxis of bodyweights.

macronix and Geoxusvaldivianus. In a few localities, (Table 2) indicated that barn owlsfed on prey ac- Phyllotisdarwini and Euneomyssp. alsooccurred. cording to their availability in intermediate and The similarity between the expected and ob- highland Piedmont areas (r• = 0.91, P< 0.01). The served rank order in cricetid speciesin the diet most important differencewas the greater impor- tance of Reithrodon auritus and the overall absence of Akodonsp. in the diet. (+) II DISCUSSION EUN Barn owlsin our studypreyed almost exclusively on rodents, as found in most other studies (Smith and Cole 1989, Bellocq 1990, Iriarte et al. 1990, De Santis et al. 1994, Taylor 1994). This suggeststhat these barns owls do not behaveas opportunistic, nonselective predators as suggestedby Mikkola (1983). Taking this into account, •H EO care should be taken when interpreting the unusually high predationon birdsreported by Noriega et al. (1993) in the Patagonian zoogeographicdomain (Ringuelet • _. c^LBB•, (*) 1961). Their data may reflect individual differences of individual barn owls. • MIC Our mean body massestimate of small mammals in QAL the diet (56.2 g) wasintermediate between that reported in Spain (21.2 g) and Chile (70.7 g) (Herrera andJaksic (-) 1980). Becausethe largestprey taken in the three areas was the European rabbit, (Oryctolagntscuniculus), the F•gure2. Variationin the taxonomiccomposition of the smallerMWSM in Argentina when comparedwith Chile d•et of the CommonBarn-owl in northwesternArgentine was a consequenceof both the high concentrationof Patagonia.Plot of the CorrespondenceAnalysis, areas of smallerprey than in Chile (30-500), and the greatercon- each delimited by minimum poligon. See Table 2 for sumption of smaller rodents than in Chile. In Argentine Patagonia,only one of the three availablerodent species most speciesabbreviations. MAR, Marmosapusilla; CAL, weighingmore than 150 g (Ctenomyshaigi) was readily Calomysmusculinur,, GAL, Galeamusteloides;, CTE, Ctenomysconsumed by owls,while in Chile three of four suchspe- hatgi.Circles correspond to highlandPiedmont, quadrats cies were consumed in amounts similar to that of our C. to lowland Piedmont and trianglesto intermediatePied- haigi (Herrera and Jaksic 1980). mont. H'NGG diversityand evennessindexes were similar to MARCH 1997 BAm'q OWL DIET IN PATAGONIA 63

Table 2. Expected composition of Common Barn-owl diet expressedas a percentage for five habitats, based on Pearson (1983, 1986).

BUNCH SPECIES1 STEPPE GRASS WEEDS ROCKS BARE OWL DIET OWL DIET FIELD RANK

AKO 42.0 76.0 49.0 41.0 36.0 27.0 AKO AKO AUL 2.0 12.0 16.0 2.0 23.0 12.0 REI ELI CHE 0.0 0.0 1.0 0.0 10.0 1.9 ELI AUL ELI 43.0 12.0 2.0 3.0 3.0 13.0 AUL ORY EUN 0.0 0.0 0.0 1.0 15.0 2.8 PHY REI GEO 0.0 0.0 2.0 0.0 2.0 1.0 ORY PHY IRE 0.0 0.0 0.0 1.0 0.0 0.3 EUN EUN ORY 4.0 0.0 19.0 2.0 8.0 10.0 CHE CHE PHY 4.0 0.0 0.0 48.0 1.0 11.0 GEO GEO REI 5.0 0.0 11.0 2.0 2.0 21.0 IRE IRE • AKO, Akodonsp.; AUL, Auliscomysmicropug CHE, Chelemysmacronix;, ELI, Eligmodontiatypus;, EUN, Euneomyssp.; GEO, Geoxusvaldz- vzanur,IRE, Irenomystarsalis; ORY, Oryzomyslongicaudatus; PHY, Phyllotisdarwini; REI, Reithrodonauritus. those found by Herrera and Jaksic (1980) for Chilean vin 1984). There, barn owlsfrequently select voles in the barn owls.The inclusion of amphibiansand the absence presenceof other prey, probablybecause the former are of reptilesby the Argentine owlswere the main reasons heavier and easier to catch (Taylor 1994). Three factors for these differences. provide a plausibleexplanation. First, R. auritus(80.4 g) Among mammals, the preponderance of rodents is 3 times heavier than Akodonsp. (28.0 g), even a bigger among the diets of Argentine owlsmade the H'NM di- difference to that found between voles and mice (Marti versityand evennessindices very low compared to those 1974, Colvin 1984). Second,Akodon sp. showsgreater ac- obtained for Chilean owls (Herrera and Jaksic 1980), tivityduring the daytime (Rau et al. 1981). The opposite which were more equally represented.In Chile, Spain is true for R. aurituswhich is a typical nocturnal species and Argentina, the low consumption of the European (Pearson 1988), thus overlappingits activityperiod with rabbit is likely related to its large body size. that of the barn owl. Finally, R. auritusfeeds in open The correspondenceanalysis results concurred with re- grassyhabitat where it may be more exposedto owl pre- sultsobtained by Pearsonand Pearson (1982) and our dation, while Akodonsp. prefers dense bushes in the prediction with respect to the gradient with which cri- steppe (Pearsonand Pearson1982), perhapsgaining pro- cetid rodents are associated.Both speciesand localities tection againstowl predation. were segregatedby an aridity gradient. Irenomystarsalis occurred in the barn owl diet only near ACKNOWLEDGMENTS the Nothophagusforest. Auliscomysmicropus was eaten only We are very grateful to Martin Funes, Obdulio Mort- in forest or dense cover habitatsbut Oryzomyslongicau- salvo and Miguel Angel Pineda for help with the field datus was eaten in all the habitats considered for this work. Our thanks to Gerardo Alefia, Gabriel and Miguel analysis.Eligmodontia typus occurred in the diet of owls Anz, Paul Cordero, Victor Solerio, the administrators of only in open habitatswith bare soil and scattereddesert the Estancias Cerro los Pinos, Serranias de Lolfn, Collon- shrubs, Ctenornyshaigi occurred in the diet in open areas Cura and Chimehuin, who permitted us to work on their with sandy soils, and Calomysmusculinus and Reithrodon land. Logistic support was provided by the Centro de auritus occurred in the diet of birds associated with the Ecologia Aplicada del Neuqufn (Argentina); we thank arid portion of our gradient. Alejandro del Valle and Antonio Guifiazfi for their con- Observed consumption of Reithrodonauritus was, both stant kind assistance. We also want to thank Marta Pian- in percent frequency and rank order, higher than ex- tanida from the Museo de Ciencias Naturales Bernardino pected. A similar situation was observedby Jaksic and Rivadaviafor providing uswith a completesmall mammal Rau (1986) for the Great Horned Owl (Bubovirginianus) reference collection. D.W. Holt, C. Marti and two anon- in a Chilean Patagonian steppe with about the same ymous refereesimproved the manuscript.Financial sup- mammalian composition that we report. As in Europe port wasprovided by Instituto de Cooperacitn Iberoam- and North America, comparison of the abundance of ericariaand the Ministerio de AsuntosExteriores (Spain) smallmammal speciesis often susceptibleto problemsof throughout the Programade Cooperacitn Cientifica con differentialtrapability. On the other hand, Akodonsp. was Iberoamfrica. consumedless than expected by chance in all five habitats.Perhaps their more diurnal activityperiod and small LITERATURE CITED body weight contributed to these results (Taylor 1994). Basedon our knowledge, the ecologyof Reithrodonauritus BELLOCQ•M.S. 1990. Composiciony variacion temporal and Akodonsp. resembles that of voles and mice de- de la dieta de Tyroalba en ecosistemasagrarios Pam- scribedin North America and Europe (Marti 1974, Col- peanos,Argentina. Vida SilvestreNeotropical 2:32-35. 64 TRAVAINI ET AL. VOL. 31, NO. 1

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