J. RaptorRes.34(2):108-119 ¸ 2000 The Raptor ResearchFoundation, Inc. A REVIEW OF THE TROPHIC ECOLOGY BARN OWL IN ARGENTINA OF THE M. Is•a•Ei• BEI•I•OC(2 Departamento de CienciasBiol6gicas, FCEN-Universidad de BuenosAires, Ciudad Universitalia,Pab. 2, BuenosAires1428, Argentina ABSTRACT.--Ireviewed the literature on the trophic ecology of the Barn Owl (Tyt0 alba) in 40 areasof Argentina representingsevenvegetationtypesin the species'range. Mammals (rodents,marsupialsand bats) representedan averageof 90.2 -+ 13.6% (_+SD) of the total prey found in pellets, and most mammalianpreywere sigmodontinerodents(77.0 -+ 19.6%). Birdswere the mostcommonsecondary prey, and insects,reptiles and amphibianswere negligible in the diet. The number of mammalian genera found in pellets (dietary richness)was similar among vegetation typesbut higher in subtropicalthan in temperate regions, showing a negative correlation with latitude. Mean weight of dominant prey speciesrangedfrom 12.6-326.0g. Averagefood niche breadth (FNB) wassimilarin subtropical(4.07) and temperate (4.03) regions. Standardized FNB was 0.33 -+ 0.16 and it was similar among vegetation typesbut lowerin subtropicalthan in temperateregions.The diet of Barn Owlsmayreflectsomehuman alterations to the habitat. KEYWO}•DS: Barn Owl;Tyro alba; diet;,trophicecologs; Argentina. Un resumen de la ecologiatrofica de Tyroalbaen Argentina RESUMEN.--Serevis6 la literatura acerca de la ecologia tr6fica de Tyroalba (lechuza del campanario) proveniente de 40 localidadesde Argentina, que representan siete tipos de vegetaci6n en el •trea de distribuci6n de la especie.Los mamiferos (roedotes,marsupiales,murcirlagos) representaronun promedio de 90.2 -+ 13.6% (_+DE) del total de presaspresentesen regurgitadosy la mayoria de estos mamiferoseran roedoressigmodontinos(77.0 _+ 19.6%). Las avesfueron la presasecundariam•ts comfin y los insectos,reptiles y antibiostuvieron una representaci6ninsignificanteen la dieta. E1 nilmero de grinerosde mamlferos representadosen los regurgitados(riqueza) fue similarentre los distintos tipos de vegetaci6n, pero mayor en regiones subtropicalesqueen templadas,presentandouna correlaci6n negativacon la latitud. E1peso medio de la especiepresa dominante en la dieta vari6 entre 12.6-326.0 g. La amplitud de nicho tr6fico (ANT) promedio rue similar entre regionessubtropicales (4.07) y templadas(4.03). La ANT estandarizada totalfue 0.33 _+0.16.La ANT estandarizada fue similar entre los tipos de vegetaci6n,pero menor en regionessubtropicalesque templadas.La dieta de Tyto alba puede reflejar algunasalteracionesantr6picasdel h•tbitat. [Traducci6n del autor] Food is a major dimension of ecologicalniches (Schoener1974). Competitionfor food resources, resulting in specializationand food partitioning, has been suggestedas a primary mechanismallow- predator models in studiesof community ecology (e.g., Jaksi• 1985, Marti et al. 1993), and the Barn Owl (Tyro alba) has been frequently included in major trophic studiesof raptor assemblages (Marti ing coexistence of speciesin assemblagesof am- et al. 1993 and references therein). phibians(Toft 1985), reptiles(Pianka1973),birds (Lack 1946) and mammals (Dickman 1988). De- The Barn Owl is a common and widely distributed nocturnal predator that feeds primarily on scriptionsof what animals eat are essentialto identify energy paths through trophic websand to understand how species divide food resources. Furthermore, knowledge of trophic ecologyis fundamental to understand feeding strategies(selective and opportunistic behaviors) and niche dynamics. Raptors have frequently been used as small mammals. In Argentina, it is one of the most common and best-studied raptors. It occurs throughout the country and is particularly common in agrosystems,grasslandsand open areas. Studies of the breeding biology have been conducted in temperate grasslandsin the eastcentral part of the country (Fraga 1984, Nores and Gu- 108 JUNE2000 B• OWL DIET IN ARGENTINA ti6rrez 1986, Bellocq and Kravetz 1993). Two additional studieshave focusedon feeding strategies of breeding and nonbreedingBarn Owls,and prey selectionon rodent species(Bellocq and Kravetz 1994, Bellocq 1998). Food habits of the Barn Owl have been studied in numerous areas throughout its geographic range (Clark et al. 1978). Over 40 studieshavedescribedthe food habitsof Barn Owlsin Argentina and several additional studies have been conduct- ed in Chile (e.g.,Jaksi• and Yafiez1979, Iriarte et al. 1990). Here, I presenta synthesisof the trophic ecologyof the Barn Owl in Argentina based on a compilationand analysisof publishedinformation in 40 areasof the country. MATERIALS AND METHODS 109 cus, Schizachyrium consanguineum, Setaria mendocina).De- forestationand ranching has partially modified the natural vegetation. Warm semidesertsinclude the driest landsof Argentina where annual precipitation ranges from 80-200 mm Typically, they are dominated by thorn-scrub communities with shrubssuch as Larrea divaricata,L. cuneifolia,L nitida, Montteaaphyllaand Bougainvillae spinosa. Bromehads are also common. The main human activity of the region is ranching. In cold semideserts,mean annual temperaturerangesfrom 5.0-13.4øCand annual precipitation ranges from 155-500 mm. It is a steppe where common speciessuch as Malinum spinosum, Brachyclados caespitosus, Junellia tridensand Nassauviaglomerulosaare adapted to a dry and windy climate. Further details on thesevegetationtypescan be found in Ragonese(1968) and Cabrera (1971). Data Analysis.Pellet analysiswasused by all authors to study diets of Barn Owls. The objectives of the studies, however, were varied (Table 1). While some studies fo- cusedon feeding habits of the owls, others emphasized ied in over 40 areasrepresentingsevenmajor vegetation small mammal distributions and descriptions of new types: subtropical rainforest, humid forest, the delta of small mammal races (e.g., Contreras and Rossi 1981). I the Paran/t River, grasslands,savannas,and warm and used the number of mammalian genera found in pellets to providean estimateof dietaryrichnessfor eachlocality cold semideserts(Table 1, Fig. 1). Subtropicalrainforestsare characterizedby warm and and I estimatedthe frequencyand recorded the average weight (taken from Redford and Eisenberg,1992) of the humid weather, with mean annual temperatures of 2021øCand annual precipitationrangingfrom 1500-2000 dominant prey species. For each localityand prey type, data were summarized mm. The vegetationhasseveralstrataand a high richness as the percent frequency of the total number of prey of speciesand life forms (e.g., epiphytes).The delta of the Paran• River has elementsof the subtropicalrainfo- found in pellets. Because most publications provided rest. However, I considered it as a separate region be- more detailson mammalianprey than on anyother prey, I was also able to obtain frequencyestimatesof rodent cause it has distinct environmental characteristics and numerous endemisms. The vegetation physiognomyis families in the diets. Although there were relatively redominated by pastures,willows and marshesin the lower cent changesin small mammal taxonomy,I maintained zones,and by open shrublandsand humid forests(where the old taxonomyasit appearedin literature. As a result, I refer to Cricetidae (insteadof Sigmodontinae) and Munative speciesare common) in the upper zones. Subtropicalhumid forestswere originallyxerophyllous ridae (instead of Murinae). Marsupials,bats, birds, repand dominated by Schinopsis balansaeand Aspidosperma tiles, amphibiansand arthropodswere considereddifferquebracho in the upper canopy.There are typicallyone or ent categories. Finally, I estimated the average two subcanopytree stratafollowed by shrubs (e.g., Pro- percentagefrequency for each prey category by vegetasopisruscifolia,Acaciapraecox,Castelacoccinea) and herbs tion type. Food-niche breadth (FNB) was estimated for each lo(e.g. Bromeliaserra,Leptochloa virgata, Gomphrena pulchella). The landscapehas been widelymodified by logging cality following Marti's (1988) criteria. Thus, mammals (rodents,marsupialsand bats) were categorizedby genus and by ranching. Temperate grasslandshave a moderate climate where whereasbirds, reptiles, amphibians and insects(includannual precipitationvariesfrom 600-1100 min. The most ing other invertebrates)were catagorizedby class.Levins' common native grassesare Stipa,Piptochaetium, Aristida, index was used to estimate food-niche breadth as FNB = Melica,Briza,Bromus, Eragrostis and Poa.These highlypro- 1/Ep?, where p, is the proportionof item i in the diet ductive grasslandshave gradually been converted to ag- Hurlbert's standardization was employed to allow meanriculture over the last two centuries. Currently, the nat- ingful comparisons,and it was calculated asFNBs = (Bob• ural plant community is composedof both native and -- Bmin)/(Bmax - Bmin), where Bobs = FNB (as calculated introducedspecies(e.g., Lolium,Briza,and Bromus).Pri- above), Brain = 1 (minimum possibleniche breadth) and mary usesof the land are for cereal crops and livestock Bm•x = N (maximum possible niche breadth or total number of prey categories). breeding. ANOVA or Kruskal-Wallis (H, when the data set d•d Savannashave a moderate to dry climate with xerophyllousvegetation,where Prosopis caldeniais the most not reach homogeneityof varianceseven after transforcommon tree species.Other tree speciesinclude P. flex- mation) tests were used to test for differences in FNBs uosa, Geoffroea decorticans, Jordina rhombifolia,Schinusfas- and in the number of mammalian genera found in diet ciculatusand Ximeniaamericana.Savannasare open wood- among vegetationtypes.Correlation and regressionanallands with some shrubs (e.g., Condalia microphylla, yseswere used to testfor associations and explain spatial Atamisquea emarginata, Ephed,zz triandra,Maytenus spinosus) variationsof food-niche parameters.All means are preand diverse grasses(e.g., Trichloriscrinita,Elionurusmuti- sented +__1SD. Habitat Descriptions. The diet of Barn Owls was stud- 110 B•nnoc(2 Vote. 34, No. 2 Ju•E 2000 B.• OWLDIET IN ARGENTINA 111 112 BELLOCQ VOL. 34, NO. 2 usually abundant in the diet. Mammals represented an averageof 90.2 + 13.6% (range = 36.3100%) of the total prey found in pelletsand most mammalian prey were cricetine (now sigmodontihe) rodents (77.0 +_19.6%, range = 27.5-100%). Barn Owls took a wide variety of mammalian species depending on the distribution and, presumably, the local availabilityof prey. The differential predation on rodent speciesin agrosystems reported by Bellocq and Kravetz (1994) may be attributed to habitat 17-18 13 14-15 24 .16 '• 19 27-28 26 21-23 32-34. selection. There is evidence that Barn Owls feed selectivelyon large rodents when prey abundanceis high during the breeding season,but not consistentlyduring the nonbreeding season when rodent abundanceis low (Bellocq 1998). Small marsupialswere taken by Barn Owlswhen available.They represented23.8%of the totalprey in Bonpland but, in Diego Gaynor,they were not found in pellets or in smallmammal trapping conducted during severalyearsusing severaldifferent trapping methods.Marmosapusillawasconsistently found in pellets from the Prosopis savanna.Marsupials such as Marmosaagilisare abundant in the humid forest and Massoia (1983) suggestedthat they may be a preferred prey of Barn Owls in the delta of the Paranti River. Although marsupials such as Lutreolina crassicaudata occurred in almost all study areas, they were only occasionallyfound in pellets,presumablybecauseof its large sizeand aggressivebehavior. Other marsupialsrepresented in diets included Thylamys agilis,T. elegans and Monodelphishenseli. Birdswere the most common secondaryprey but the percent of birds found in pelletsvaried greatly among localities. The averagepercent of birds in the diet was7.2 - 12.8% (range = 0-63.7%). The following orders were identified in decreasingorFigure 1. Geographicallocation of siteswhere the diet of Barn Owls was studied in Argentina. Numbers correspond to locationsgiven in Table 1. RESULTS AND DISCUSSION Overall Diet. The Barn Owl is a specialiston small mammals (Dean 1975, Morton and Martin 1979,Jaksi•et al. 1982,Lenton 1984,Marti 1988). Mammals were also the most common prey found in pelletsin all areasof Argentinawith the exception of San Miguel and Castelarwhere passerine birds were more abundant (Table 2). In Castelar, C6rdoba, San Miguel and Ensenadita (urban or suburban areas), birds, Muridae and bats were un- der: Passerifirmes, Columbiformes, Tinamiformes, Charadriiformes, Anseriformes and Gruiformes. Bats were occasionallyfound and represented <56.6% of the diet. Batsmostcommonlytaken included speciesof Lasiurus,Molossus, Eumops,Myot•s and Sturnira. Reptiles such as snakesand iguanaswere only found in pellets from Diego Gaynor,Laguna Bianca, Punta Este and Luan Cura Hu6. Amphibians were found in pellets from S.M. Tucumtn (Leptodactyluschaquensis), Campo Ram6n (Batrachia), Desaguadero(Batrachia), San Miguel (Leptodactylidae, Bufonidae,Ceratophrynidae),Alta Italia, Villa Regina (Batrachia), Trevelin (Pleurodema bufonina) and Junin de los Andes. Ju•E 2000 B,a•,• OWL DIET IN ARGENTINA 113 Arthropods were likely taken opportunistically and their role in fulfilling energetic requirements appeared to be negligible in Argentina. Insects were reported in the diet in five localities.In Lobos, 15% of the pellets collectedyear round contained remains of insectsand the percent of pellets showing insects was higher in spring-summer (50.3%) than in fall-winter (1.8%) (Faverin 1989). g) and Scapteromys tumidus(146 g) which were the primary prey found in two subtropical localities (Desaguaderoand the Ibicuy Islands). ComparisonamongRegions.The mean percent frequencyof the dominant prey speciesin the diet was similar between subtropical (39.5 _+15.0) and temperate (34.7 -+ 10.2) localities(F = 0.941, P > 0.05). Although the primary prey speciesvaried within most Although33.5% of the food itemsfound in pellets among localities,there wasconsistency in Diego Gaynor were arthropods,their contribu- regions (Table 2). tion in terms of biomass(<1%) was negligible in The number of mammalian genera found in comparisonto that of vertebrates(Bellocq 1990). diet was similar among vegetation types (H = Massoia and Vetrano (1988a) identified spiders 1.110, P > 0.05), but higher in subtropical(13.6 +and insects (Carabidae, Scarabaeidae, Coccinelli- 4.3) than in temperate (8.8 _+ 2.5) regions (F-dae, Rutelidae, Gryllidae, Acridiidae, Mantidae, 19.008, P < 0.001) (Table 3). In contrast, more Mantispidae, Cicadidae, Blattidae) in pellets from classesand rodent families were represented in diVilla Regina which represented9.5% of the total em in temperate (3-4 classesand 4-6 rodent families) than in subtropical (2-3 classesand 3 rodent prey. Food-niche Parameters. Dietary richness was families) regions.This may be explainedby the dinegativelycorrelated with latitude. The number of versityof primary prey in the wild. Speciesdiversity mammalian genera found in pelletsvaried from 4- of Cricetine rodents is higher in subtropicalthan 20 (Table 2) and was correlated with latitude (r = in temperate regions (Redford and Eisenberg -0.456). I excluded from the analysisone outlier 1992), and they representedover 60% of the diet correspondingto Junin de los Andes. The coeffi- of Barn Owls in natural regions of Argentina (Tacient of determination showed that latitude exble 3). Barn Owls seem to prey on a wide variety plained 18% of the variation in the number of of Cricetine specieswhen availablebefore explormammaliangenera found in pellets (F = 7.862, P ing other prey taxa. < 0.01). Given that speciesrichnessis highly and FNB wassimilar in subtropical(4.07 _ 1.22) and negativelycorrelated with latitude (Rohde 1992), temperate (4.03 _+1.51) regionsof Argentina (F= the spatialvariation in dietary richnessmay be par- 0.005, P > 0.05). Marti et al. (1993) obtained simtially explained by differential availabilityof prey at ilar values for temperate and tropical regions of the regional scale. This expected general pattern South America (Table 4). The averageFNBs was identified in southern regions of the Neotropics 0.33 (Table3). It washigherthananyvalueestiwas not found among diets of raptor assemblages mated in the southern portion of the Barn Owl's in northern latitudes (Marti et al. 1993). range in the Neotropics (Table 4) but similar to I also expected to find an associationbetween the average value obtained from Marti's (1988) FNB and latitude, where declining FNB should be compilation of nine areas most of them from associatedwith increasinglatitude. However, there southern latitudes (FNBs = 0.27, F = 0.27, P > was no significant correlation between FNB and 0.6). My FNBs value was also similar to the mean latitude. Average FNB was 4.05 _+ 1.40 ranging value estimated from 19 areas around the world from 1.37 in Alta Italia to 7.49 in Saladillo and (FNBs = 0.26, F -- 0.96, P > 0.3) (Marti 1988). FNBs was 0.33 - 0.16 ranging from 0.05 in Alta In Argentina, FNBs was similar among natural Italia to 0.69 in the Toay and Loventu• Districts regions (H = 3.552) (Table 3). However, the av(Table 2). The dominant prey in the diet repre- erage FNBswaslower in subtropical(0.25 - 0.04) sented>30% of the total prey in 70% of the lo- than in temperate regions (0.35 _ 0.19, F = 4.423, calities and >40% in 47% of the localities. P < 0.05). Marti et al. (1993) estimated a higher Mean weightof the dominantpreyspeciesvaried diet diversityin tropical than in temperate regions from 12.6-326.0 g (Table 2). The overall geometric of South America. Management Implications. Diets of Barn Owls mean of prey weight of Barn Owls in temperate Neotropical regions was estimated to be 45.1 g may reflect opportunistic feeding becauseof the (Marti et al. 1993). It was unusual for Barn Owls effects of human impactson their habitat. For into take prey as heavyas Holochilusbrasilensis (326 stance, the diet of Barn Owls changed after dam 114 B•I•I•OC:(2 VoI•. 34, No. 2 JUNE2000 BARON OWLDIETIN ARGENTINA 115 116 BELLOCQ VOL. 34, No. 2 +1 +1 +1 +1 +1 • • +1 +1 +1 +1 • +1 • +1 +1 +1 +1 +1 • +1 +1 • • +1 +1 +1 +1 +1 • +1 • JUNE2000 BARNOWLDIETIN ARGENTINA Table 4. Food-niche breadth (FNB, based on proportion of species) and standardizedfood-niche breadth (FNBs) of the Barn Owl in regions of Europe, North Ainerica and South America. Source: Marti et al. (1993). FNB FNBs Mediterranean 3.84 0.20 West 4.43 0.06 East 2.23 0.03 West 7.88 0.03 Midcentral 9.61 0.29 Eastcentral 1.96 -- Southeastern 3.04 -- North South I thank 0.18 0.38 Insular 3.20 0.19 on the Colorado for comments on earlier versions of Aires. --AND CITED F.O. KRAVETZ. 1990. Practical and theoretical implicationsof perch use for avian predators on rodent populationsß Ecosur16:61-67. -- 4.28 4.61 Marti BELLOCQ,M.I. 1990. Composicitn y variacitn temporal de la dieta de Tyroalbaen ecosistemasagrariospampeanos,Argentina. Vida Silvestre Neotropical 2:32-35 ß1998. Prey selectionby breeding and non-breeding Barn Owls in Argentina. Auk 115:224-229. Ainerica Temperate Tropical C.D. this manuscript. Comments by F.M..Jaksi• and two anonymousreviewersalso helped to improve it. This work was supported by the Gonsejo Nacional de Investigaciones Gientfcas y Ttcnicas of Argentina and the University of LITERATURE Ainerica construction ACKNOWLEDGMENTS Buenos Europe 117 AND --'. 1993. Productividad de la lechuza del campanario (Tyt0 alba) en nidos artificiales en agros- istemaspampeanos.Hornero13:277-282. --AND --'. 1994. Feeding strategyand predation of the Barn owl (Tyt0 alba) and the Burrowing Owl (Speotyto cunicularia)on rodent species,sex, and size, in agrosystems of central Argentina. EcologiaAustral4' River in Casa de Pie- 29-34. A.L. 1971. Fitogeogratiade la Reptblica Argendra. Montalvo et al. (1985) reported diet compo- CABRERA, tina. Bol. Soc.Arg. Bot. 19:1-42. sition basedon pelletscollectedin October 1979 CLARK, J.R., D.G. SMITHANDL.H. KELSO.1978. Working and November 1983, prior to and after dam conbibliography of owls of the worldßNatl. Wildl. Fed struction (alsoin Noriega et al. 1993). Prior to dam Sci. Tech., Washington, DC U.S.A. construction, the diet consistedof 97.3% mammals CONTRERAS, J.R. ANDM.I. ROSSL1981. Una nueva subesand 2.7% birds.After dam construction,it changed pecie del rattn colilargo para la provincia de Mento 60.2% mammalsand 39.8% birds. This change doza: Oligoryzomys fiavescensoccidentalis(Mammaha, in diet may have been due to a change in the relative abundance of vertebrate populations or a higher vulnerabilityof birds due to increasingedge Rodentia, Cricetidae). l•istoria Natural (Argentina) 1 157-160. DF•N, W.RJ. 1975. Tytoalbaprey in southwestAfrica and the northern cape. Zool.Aft:.10:217-219. DE SANTIS,L.J.M. ANDg.o. PAGNONI.1989. Alimentaci6n Barn Owls are predatorsof small mammal spede Tyroalba (Aves:Tytonidae) en localidadescosteras cies considered harmful for agriculture and agrode la provincia del Chubut (Repfiblica Argentina). forestry. Calomys lauchaand Holochilusbrasilensis are Neotropica 35:43-49. known to damage crops and young tree planta- --, G.I. MONTALVOANDE.R. JUSTO.1983. Mamfferos tions, respectively.Furthermore, C. laucha,Oligoryintegrantes de la dieta de Tyro alba (Aves: Strigifbrzomys fiavescens and Ahodonazaraecan carry serious mes, Tytonidae) en la provincia de La Pampa, Argenhuman diseasessuch as hantaviruspulmonary syntina. Iaristoria Natural (Argentina) 3:187-188. effect. drome (Levis et al. 1995) and hemorrhagic fever (Kravetz et al. 1986). Increasing availability of roosting sitesfor raptorial birds in cropfieldsresulted in decreased short-term abundance of C. laucha (Bellocq and Kravetz 1990). 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