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Diet of the Burrowing Owl (Athene cunicularia) and its seasonal variationin Patagonian steppes: implications for biodiversity assessments in theSomuncurá Plateau Protected Area, ArgentinaAnalia Andradea; Marcela J. Nabtea; Marcelo E. Kunb

a Centro Nacional Patagónico (CENPAT-CONICET), Puerto Madryn, Chubut, Argentina b CentroRegional Universitario Bariloche (CRUB), Universidad Nacional del Comahue, Bariloche, Río Negro,Argentina

Online publication date: 30 July 2010

To cite this Article Andrade, Analia , Nabte, Marcela J. and Kun, Marcelo E.(2010) 'Diet of the Burrowing Owl (Athenecunicularia) and its seasonal variation in Patagonian steppes: implications for biodiversity assessments in the SomuncuráPlateau Protected Area, Argentina', Studies on Neotropical Fauna and Environment, 45: 2, 101 — 110To link to this Article: DOI: 10.1080/01650521.2010.502010URL: http://dx.doi.org/10.1080/01650521.2010.502010

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Studies on Neotropical Fauna and EnvironmentVol. 45, No. 2, August 2010, 101–110

ISSN 0165-0521 print/ISSN 1744-5140 online© 2010 Taylor & FrancisDOI: 10.1080/01650521.2010.502010http://www.informaworld.com

NNFEORIGINAL ARTICLE

Diet of the Burrowing Owl (Athene cunicularia) and its seasonal variation in Patagonian steppes: implications for biodiversity assessments in the Somuncurá Plateau Protected Area, ArgentinaStudies on Neotropical Fauna and EnvironmentAnalia Andradea*, Marcela J. Nabtea & Marcelo E. Kunb

aCentro Nacional Patagónico (CENPAT-CONICET), Puerto Madryn, Chubut, Argentina; bCentro Regional UniversitarioBariloche (CRUB), Universidad Nacional del Comahue, Bariloche, Río Negro, Argentina

(Received 25 July 2009; accepted 15 June 2010)

A wide literature exists on the food habits of the Burrowing Owl (Athene cunicularia) in South America. However,this information is scarce for the arid lands of Argentina. Owl’s diet composition in the Somuncurá Plateau andits seasonal variations were estimated. The Somuncurá landscape is a volcanic plateau conformed by superim-posed layers of basalt flows. The climate is arid and cold and the vegetation is characterized by a grassy steppe ofthe Patagonian Phytogeographical Province. Pellets were collected during late spring and late summer aroundtwo lagoons from upper elevations (41°25′35″S, 66°57′27″W, 1412 m a.s.l.). These were measured, dissected andprey were identified and quantified. Sample richness, biomass and food-niche breadth were calculated as dietaryparameters. Small mammals availability was estimated by trapping. The Burrowing Owl showed a generalist diet,including a wide variety of prey from invertebrates to vertebrates. Coleopterans were dominant in the spring diet,while rodents and arachnids became important in the late summer diet. The high species richness detected in thisstudy through the analysis of raptor pellets is valuable for biodiversity assessments of unexplored areas such asSomuncurá.

Keywords: Argentina; arid environments; biodiversity; food habits; Strigiformes

Introduction

The Burrowing Owl, Athene cunicularia, is found inNorth as well as in South America, from Canada tothe southern end of Argentina and Chile (Burn 1999).In Argentina it inhabits almost all grassy plains, andit is the most abundant owl in agroecosystems of theArgentinean Pampa (Bellocq 1987).

Available literature exists on the food habits ofthis owl in South America, mainly in Brazil (e.g.,Silva Porto & Cerqueira 1990; Soares et al. 1992) andChile (e.g., Yañez & Jaksic 1979; Schlatter et al. 1980,1982; Jaksic & Marti 1981; Torres-Contreras et al.1994; Zunino & Jofré 1999), and some references arefound in Paraguay (Andrade et al. 2004a). Moreover,the diet of A. cunicularia has been thoroughly studiedin agroecosystems in Argentina (Bellocq & Kravetz1983; Bellocq 1987, 1988). For natural environments,such as the extensive Patagonia, limited informationis available. Furthermore, works were conducted onlyin coastal areas (Massoia et al. 1988; De Santis et al.1997; Nabte et al. 2008). These studies concluded thatA. cunicularia is a generalist predator that consumes awide range of prey, from invertebrates to vertebratetaxa. In addition, some of them showed an opportunisticstrategy as was evidenced by a seasonal variation inits diet according to the abundance of prey in the field

(Schlatter et al. 1982; Bellocq & Kravetz 1983; Bellocq1988; Silva Porto & Cerqueira 1990; Torres-Contreras etal. 1994). In particular, Torres-Contreras et al. (1994)observed that during fall and winter the BurrowingOwl consumed mainly insects, while mammals andarachnids were better represented in the diet duringspring and summer. An inverse relationship wasobserved in agroecosystems of Argentinean Pampa(Bellocq & Kravetz 1983; Bellocq 1988), with themajor rodent consumption during winter, whileinsects were homogeneously consumed throughoutthe year. The authors attributed this result to a reductionin the abundance of rodents towards the spring due toagricultural practices, therefore decreasing their con-sumption by the owl.

Few studies were done on food habits of the Bur-rowing Owl in the vast central desert areas of Patagonia.All of them were made in warm areas, such as theMonte Desert (Massoia et al. 1988; De Santis et al.1997; Nabte et al. 2008). A preliminary work on thePatagonian steppes was conducted about the predationof this owl on vertebrate prey in the SomuncuráPlateau (Andrade et al. 2004b), but so far no evalua-tion of its diet or of its seasonal variation has beenmade. Cold desert environments, such as the extran-dean Patagonian steppes, are characterized by low

*Corresponding author. Email: andrade@cenpat.edu.ar

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102 A. Andrade et al.

precipitation and wide annual and day thermal varia-tions. This fact could alter the prey composition avail-able to this owl during all seasons. An opportunisticand generalist predator such as Burrowing Owlscould reflect those faunal replacements through theseasonal changes in its diet.

The arid land of Patagonia is, by far, the lessprotected biome in Argentina. Somuncurá Plateauwas declared a protected area by the Government ofRío Negro Province in 1986. However, there are nobiodiversity assessments and a management plan hasnot yet been implemented. The Plateau harbors anumber of endemic species including fishes, amphibi-ans, reptiles, and mammals (Cei 1969a, 1969b; Crespo1963; Donoso-Barros & Cei 1971; Cei & Castro1973). In addition, cattle are not under any type ofcontrol and overgrazing could lead to desertificationand poaching of wildlife species.

The objectives of this study were to estimate thediet composition and food-niche breadth of theBurrowing Owl in the Somuncurá Plateau as well asits seasonal variation. We believe that the study willcontribute to the understanding of the trophic ecol-ogy of this owl in the cold desert lands of Argentina.Finally, it also attempts to conduct a biodiversityassessment in an important biogeographic area suchas Somuncurá to serve as a baseline work in order toplan future conservation actions.

Materials and methods

Study area

The study area is located in the upper levels of theSomuncurá Plateau Protected Area, a 25,000 km2

volcanic plain located in the middle of the Río Negroand Chubut provinces. The effusive core is in thecenter of the plateau conforming a line of isolatedhills called “Alta Sierra del Somuncurá”, with CerroCorona being the highest elevation (1644 m a.s.l.)(Croce 1963). The climate is arid and cold and thevegetation is characterized by a grassy steppe of thePatagonian Phytogeographical Province (sensu Leónet al. 1998). Mean annual precipitation in the area isless than 150 mm (Paruelo et al. 1998). Highest andlowest temperatures for July (winter) are 17.2°C and−24.3°C, respectively. For January (summer) theseare 37.5°C and −2.5°C (Maquinchao weather station,data given by Departamento Provincial de Aguas(DPA), Río Negro).

Pellet collecting sites were surrounding two basins,Laguna Blanca and Laguna Carribal (41°25′35″S,66°57′27″W, 1412 m a.s.l.), rodent collecting sites werearound Laguna Blanca and Laguna Silvano (41°25′49″S,66°57′27″W, 1411 m a.s.l), all of these located in front

of Cerro Corona. Blanca, Silvano and Carriballagoons are separated from each other by only 100 m.

Collection of owl pellets

The diet of the Burrowing Owl was studied by analyzingpellets collected beneath roosts. The field study wasconducted during the months of March 2005,November 2005 and April 2006 at Laguna Blancaand in April 2006 at Laguna Carribal. The lagoons’sperimeter was traveled, while identifying and count-ing the burrows and registering vegetation structurearound them. Because of weather conditions, somepellets were disintegrated and prey remains accumu-lated around the burrows forming a bone assemblage.This also incorporated partially consumed prey thathad not passed through the digestive system. Theassemblage was collected during March 2005. It waspicked up thoroughly – with shovel and paintbrush –and sifted with a 2 mm mesh. This assemblage wasnot incorporated into the diet analysis, but wasincluded into the biodiversity assessment conductedon the Somuncurá Plateau, adding relevant informationabout species that were not recorded in fresh pellets.

Diet analyses

Length (cm) and width (cm) of pellets were measuredusing a caliper. Intact and fragmented pellets weredissected using a diluted solution of sodium hydroxideaccording to Reise (1973). This solution disintegratesthe hair and feathers while conserving the skeletalpieces (bones, teeth and chitinous insect parts).

Prey items were identified at the finest taxonomiclevel using reference collections and identificationkeys (Scholtz 1990; Borror et al. 1992; Morrone &Roig-Juñent 1995; Pearson 1995; Roig-Juñent 1995;Flores 1997; Morrone & Coscarón 1998; Marvaldi &Morrone 2000; Flores & Vidal 2001; Roig-Juñent2002) and with the aid of specialists. Lizards that livearound the lagoons were divided into three categoriesbased on size: large (Diplolaemus sp., Leiosauridae),medium-sized (Liolaemus petrophilus, L. rothie andPhymaturus sp., Liolaemidae), and small (L. bibroniand L. boulengeri, Liolaemidae). This classificationwas made because taxonomical differentiation inSomuncurá lizards is primarily based on genetic andexternal characters. Additionally, there is little varia-tion in osteological characters. Pellet prey remainswere assigned to one of the above three categories.

Vertebrates were identified and quantifiedthrough cranial and jaw remains, arthropods throughchitinous parts. The number of arthropod specimenswas estimated through heads, thorax, and elytra incoleopterans; chelicera fangs in spiders; telson and

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Studies on Neotropical Fauna and Environment 103

pedipalps in scorpions, and heads and mandibles inother arthropods.

Field abundance of rodent prey

During March 2005 (sampling one) and April 2006(sampling two), jointly with the collection of pellets,small mammals abundance in the field was evaluatedusing Sherman traps. We marked two trap-lines duringsampling one and three during sampling two, eachwith 50 traps per line. They were set along the detritusbank and the basaltic rockface around LagunaBlanca and Laguna Silvano near the owl’s burrows.Sherman traps were set for two consecutive nightsduring March 2005 and four nights during April2006. There was an effort of 200 trap-nights duringsampling one and 600 trap-nights during samplingtwo. Traps were baited with vanilla-flavored oats andwere checked every morning. Animals collected wereprocessed as specimens according to standard prepa-rations (Díaz et al. 1998). This included museumskins and skeletons or fluid-preserved carcasses.

Statistical and quantitative analyses

In order to quantify the diet, the minimum number ofindividual prey (MNI), relative frequency and thepercentage biomass of each taxon were calculated asfollows: Biomass % = (100 wi × Ni)/Σ(wi × Ni), wherewi is the average weight of prey i and Ni is the numberof prey i. Average weights were obtained from thebibliography (Redford & Eisenberg 1992) as well asfrom field measurements.

We estimated dietary parameters by calculatingsample richness (defined as the number of recordedfamilies) and food-niche breadth (FNB) measured bythe Levins index (Levins 1968; FNB = 1/Σpi2, where piis the proportion of each prey on the total sample) andthe standardized index (FNBs = B − Bmin/Bmax − Bmin,where Bmin = 1 and Bmax is the number of differentprey classes) for each pellet sample. To estimate FNBs,order taxonomical level was consigned.

The relationship between length and width of pel-lets as well as the number of prey included was evalu-ated through a Spearman Rank Correlation. Thebiomass of the prey taxa contained in the pellets wascalculated and the median biomass was estimatedonly for the March 2005 pellet sampling because of alarger sample size. The significance in the distributionof the biomass scores around this median biomasswas tested by a binomial test applied to a WilcoxonSigned Ranks Test. Only whole pellets were consid-ered in those analyses. Taxonomically ranked dietcomposition was compared between seasons using achi-square test.

All statistical analyses were performed usingStatistica 6.0 software (Statsoft Inc., 2001).

Results

A total of three burrows at Laguna Blanca and 10 atLaguna Carribal were counted corresponding to atleast three couples of Burrowing Owls. These werelocated in the detritus bank over the lagoon floodlevel, in elevated patches with creeping vegetation. Atotal of 235 pellets were collected. They included: 110in March 2005, 60 in November 2005, and 65 in April2006. Prey remains and partially consumed prey(amphibians and reptiles) were also gathered.

Diet composition

The number of prey items per pellet was significantlycorrelated with pellet length (rs = 0.23; n = 76; P = 0.03)but not with its width (rs = 0.03; n = 76; P = 0.78).Median biomass of the prey items identified in thepellet sample was 20 g. There were no significantdifferences between biomass scores around thismedian biomass (Wilcoxon Signed Ranks Test, n = 76;P = 0.885). We considered only vertebrate prey formedian biomass calculation because of their largerweight.

Amphibia, Aves, Chelicerata, Insecta, Mammalia,and Reptilia were identified among a total of 2762prey items (Table 1). Insects and rodents were themost frequent prey in the Burrowing Owl’s diet(between 28 and 69% for insects and between 14.5and 54% for rodents, per sample), followed by Cheli-cerata (between 8.7 and 30% per sample). However,in comparison with the other taxa, rodents comprisedthe bulk of the biomass in the diet (around 90%).

Considering only vertebrates in the analysis (fromthe four pellet samples), sigmodontine rodents werethe dominant prey based on frequency of occurrence(between 67.3 and 97%) and biomass (between 82.8and 98%). Eligmodontia sp., Euneomys chinchilloides,and Notiomys edwardsii were the dominant species(between 9.6 and 46.5%, between 18.9 and 29.6%and between 9.5 and 34.3%, respectively), followedby Abrothrix olivaceus (between 8.9 and 18.1%).Phyllotis xanthopygus, A. longipilis, Calomys musculinusand the caviomorph Ctenomys sp. were scarcely rep-resented in the pellets (Table 1). Reithrodon aurituswas observed in the diet through the bone assem-blage but it was absent from pellet samples. Euneo-mys chinchilloides was the prey item that mostlycontributed to the total biomass in the diet (between36.9 and 51.2%).

Birds, reptiles, and anurans were much less fre-quent in the Burrowing Owl’s pellets, but the three

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104 A. Andrade et al.T

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106 A. Andrade et al.

size categories of lizards were well represented. Iso-lated remains of only two passeriform birds, similarin size to a Mockingbird (Mimus sp., Mimidae) andto a Rufous-collared Sparrow (Zonotrichia capensis,Emberizidae), were found in the samples. Oneanuran species, Pleurodema bufoninum, typical ofthe basaltic lagoons from the upper levels of theSomuncurá Plateau, was recovered. We also foundanurans at the entrance of the burrows. These wereeviscerated, suggesting a partial consumption by theBurrowing Owls.

A high diversity of arthropods was recorded in theBurrowing Owl’s pellets. Considering only inverte-brates in the analysis, coleopterans were the domi-nant prey (frequency of occurrence: 48.7–86.7%), themost frequent families being Tenebrionidae, Curcu-lionidae, and Carabidae.

Food-niche breadth and seasonal diet variation

In general, standardized food-niche breadth (FNBs)was narrow (between 0.13 and 0.37 per sample),demonstrating that the diet of the Burrowing Owlwas composed of relatively few prey taxa and une-qually distributed (Table 1). However, there weredifferences between seasons and years. FNBs wasbroadest in the late summer pellet sample that wascollected at Laguna Blanca (April 2006, FNBs = 0.37)and narrowest in the spring pellet sample (November2005, FNBs = 0.13). If consecutive years are com-pared for the same lagoon and season (March 2005and April 2006 Laguna Blanca pellet samples), dif-ferences in FNBs are found (March 2005: FNBs = 0.21vs. April 2006: 0.37). FNBs for the Laguna Carribalsample (0.19) was narrower than those for LagunaBlanca.

Diet composition was different among seasons(April 2006 vs. November 2005, c2 = 129.9, df = 5, P< 0.00001 and March 2005 vs. November 2005, c2 =1063.9, df = 5, P < 0.00001). The spring sample exhib-ited the highest proportion of insects (69.5%), whilethe diet in late summer was primarily based onrodents, arachnids, and coleopterans (26.3, 18 and48.1% for March 2005 and 36.5, 30 and 29.2% forApril 2006, respectively). However, comparing onlyvertebrate prey, the diet during spring showed ahigher proportion of reptiles and anurans (9.6 and19.2%, respectively), while the diet in late summerhad a high quantity of rodents (89–97%). AlthoughFNBs from the Laguna Carribal summer sample wasnarrow, the high frequencies for both rodents (54%)and coleopterans (25%) were maintained. Diet com-position was also different between summers fromdifferent years (March 2005 vs. April 2006, c2 =389.4, df = 5, P < 0.00001).

Rodent prey availability in Somuncurá

In the field, 11 individuals were caught during March2005 and 33 individuals during April 2006, with atrap success of 5.5% in both samplings. Five smallmammal species were captured around LagunaBlanca and Laguna Silvano: A. olivaceus, A. longipilis,Eligmodontia sp., Eu. chinchilloides and P. xanthopygus.Phyllotis xanthopygus was captured only in the basal-tic rockface while Eu. chinchilloides and A. longipiliswere also caught in the rocky and bare habitat in thesurroundings of the lagoons. Abrothrix olivaceus andEligmodontia sp. were trapped in the rocky and barehabitat.

A differential predation apparently occurredamong those recorded species. Abrothrix longipilisand P. xanthopygus were underrepresented in theowl’s pellets (Figure 1). On the contrary, N. edwardsii,one of the most frequent prey taxon in the owl’s pellets,was not captured in the trappings. High proportions ofEligmodontia sp., A. olivaceus and Eu. chinchilloideswere found in both the pellet samples and in the traps.As rodent trapping data were limited, no statisticaltests were applied.

Discussion

Feeding habits

Burrowing Owls had a generalist diet in theSomuncurá Plateau. It included a wide variety ofprey, from invertebrates to vertebrates, in agreementwith other studies (e.g., Marti 1974; Schlatter et al.1980; Bellocq & Kravetz 1983; Bellocq 1988; SilvaPorto & Cerqueira 1990; Torres-Contreras et al. 1994;Zunino & Jofré 1999; York et al. 2002; Andrade et al.2004a; Nabte et al. 2008). However, the low values ofstandardized food-niche breadth (FNBs) obtainedfrom all samples demonstrate a selection for a fewprey taxa. Athene cunicularia is active during everyhour of the day (Marti 1974). Still, our data suggestthat this predator primarily feeds during twilight andnight, based on the known activity patterns of themain prey items, in agreement with other regionalstudies (e.g., Nabte et al. 2008). The most frequentlyconsumed rodents such as Eu. chinchilloides, Elig-modontia sp. and A. olivaceus are nocturnal. Further-more, the owls ate a large number of scorpions, anocturnal invertebrate.

Seasonal diet variation in the Somuncurá Plateau

Differences in FNBs between seasons showed that thediet of the Burrowing Owl was more uneven in springthan in late summer. Differences in FNBs betweenlagoons, for the same season, could be related to the

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Studies on Neotropical Fauna and Environment 107

Figure 1. Comparison between the proportion (%) of small mammals recorded in the pellets of Burrowing Owls and by trappingin the Somuncurá Plateau Protected Area for (a) March 2005 and (b) April 2006. Caviomorphs were not included in thisfigure. Echi, Euneomys chinchilloides; Aoli, Abrothrix olivaceus; Alon, Abrothrix longipilis; Nedw, Notiomys edwardsii; Elig,Eligmodontia sp.; Pxan, Phyllotis xanthopygus.

Pro

port

ion

(%)

Species

Echi Aoli Alon Nedw Elig Pxan

Pro

port

ion

(%)

Species

b)Diet Field

0

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Echi Aoli Alon Nedw Elig Pxan

a)Diet Field

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108 A. Andrade et al.

low environmental heterogeneity observed in LagunaCarribal compared with the one in Laguna Blanca(Andrade, personal observation).

The frequencies of insects, rodents, and arachnidsthat turned out to be the main prey in the BurrowingOwl’s diet, depended on both the season and the year.For instance, coleopterans were the dominant prey inlate spring while rodents and arachnids became moreimportant in late summer. Beetles are common inextrandean Patagonian steppes (Cheli et al. 2010) andthey were the insect prey most frequently consumedby this owl in Somuncurá as in other Patagonianareas (Nabte et al. 2008). They tend to be concen-trated in dark sites such as the Burrowing Owl’s bur-rows (Torres-Contreras et al. 1994). Moreover, it wasrecently demonstrated that Burrowing Owls use dungas a bait to attract their prey (Levey et al. 2004). Burrowareas were surrounded by dung of Upland Gooses aswell as by those of horses. This bait tool could be usedby Somuncurá owls and may explain the high fre-quency of beetles in their diet.

On the other hand, rodents were the most significantprey in terms of biomass. These differences accountedfor the variation in the food-niche values, showingthat Burrowing Owls in the Somuncurá Plateau as wellas in other regions and/or countries (Schlatter et al.1982; Bellocq & Kravetz 1983; Bellocq 1988; SilvaPorto & Cerqueira 1990; Torres-Contreras et al.1994) exhibit a seasonal variation in their diets, atleast for our study period. The highest abundance ofrodents in Somuncurá Plateau was observed in latesummer and early fall (March to April) in compari-son with late spring and early summer (November toDecember, personal observation). Scorpions gener-ally increase their activity during summer months(Warburg & Ben-Horin 1981). The high frequenciesof rodents and arachnids in pellets as well as the highabundance of rodents in the field during the hot sea-sons indicate that the owls showed an opportunisticstrategy, consuming prey in relation to their abun-dance in the field. We agree with Torres-Contreras etal. (1994) and other authors who called this feedingstrategy “seasonally opportunist”. Differences in theBurrowing Owl’s diet between Somuncurá Plateauand other areas may be correlated with the harshenvironmental conditions prevailing in the area.Frosts are common until November and even earlyDecember in the upper levels of Somuncurá, so thecold season extends in the study area up to late springand early summer.

The consumption of frogs, and secondarily lizardsand birds, was notably increased during spring. Bellocq& Kravetz (1983) pointed out that Burrowing Owls com-plete their energetic requirements with other vertebrateprey when rodent density in Pampa agroecosystems

decreases. This may well be the case in the SomuncuráPlateau. The decrease in the density of rodents in thefield during the cold months (winter and spring)could reduce prey availability and consequently pro-voke a shift towards this alternative prey. Partiallydigested frogs recovered from the surroundings of theburrows suggest that they could have been consumedin a higher frequency than that registered in the pelletsamples. This habit was observed in other studies(Schlatter et al. 1980; Torres-Contreras et al. 1994;Andrade et al. 2004a) suggesting a differential con-sumption of viscera and muscles.

Rodent prey selection

Together, only nine species of rodents were recordedin the samples. This represents a low species richnesscharacteristic of the cold desert at high elevations incomparison with the higher richness observed forintermediate and lower levels of the Somuncurá Plateau(Andrade 2007, 2009). However, Burrowing Owlsconsumed all small mammal species recorded at thosehigh elevations (Andrade 2007, 2009). With theexception of N. edwardsii, the most consumed species,Eligmodontia sp., A. olivaceus, and Eu. chinchilloideswere also the most frequent species in the field duringthe study period, as was evidenced by traps. Euneomyschinchilloides was the major contributor of biomass tothe diet due to its high frequency and its bigger size.The absence of N. edwardsii in the trap sampling canbe explained by its habits. It is a semi-fossorial species(Pearson 1984) and its subterranean activity couldreduce the likelihood of being captured by traditionalsampling methods. On the other hand, the high abun-dance of N. edwardsii in the Burrowing Owl pelletsmay be due to its insectivorous feeding behaviour,being attracted by insects grouped around the bur-rows and thus favoring its consumption by the birds.

Phyllotis xanthopygus and A. longipilis were rarein the owl’s diet, although they were found in high fre-quencies by trapping. Similar remarkable differenceswere also obtained in Aucó, Chile (Torres-Contreras etal. 1994). They observed a lower consumption by theBurrowing Owl of A. longipilis and P. darwini thanexpected by their abundances in the field. They attrib-uted these discrepancies to a body size that limited thecapacity of this owl to catch them. This may not be theexplanation for the under-representation of A. longipilisin the Somuncurá Plateau because it is smaller thanthe individuals from Aucó and similar in size to the veryfrequently consumed A. olivaceus. Also, this could notbe the reason for the low frequency of P. xanthopygus inthe owl’s diet given that its weight is even lower thanthat of Eu. chinchilloides, the most preyed-upon species.The low frequency of P. xanthopygus in comparison

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Studies on Neotropical Fauna and Environment 109

with Eu. chinchilloides might be related to their differenthabitats. The latter has a less restrictive habitat thatcould increase the probability of encounter with theowls. Similar results (Eu. chinchilloides dominating thediet with an absence of P. xanthopygus and A. longipilisfrom the sample) were also found in a previous workabout predation of A. cunicularia on vertebrates inthe Somuncurá Plateau (Andrade et al. 2004b).

Biodiversity assessments

This study allowed us to list 43 taxa for the SomuncuráPlateau Protected Area (unidentified not included).The biodiversity corresponds to a total of nine smallmammals, at least two birds, three categories of lizards,one toad, three arachnids, and 25 insects. Three sigmo-dontine rodents (R. auritus, C. musculinus and N.edwardsii) and one caviomorph (Ctenomys sp.) inregurgitated pellets were not captured with live traps.

The bone assemblage, being a year-round sample,constituted an essential source of information on rarespecies that could have not been detected by a singlepellet sample under the seasonal diet variations factor.This assemblage was also important due to the pres-ence of species that were partially consumed by theowl but did not pass through the digestive system.For instance, the higher frequency and diversity of liz-ards in this assemblage compared to the pellet samplesis possibly associated with this kind of accumulation.Conversely, arthropod diversity was lower in the boneassemblage in comparison with the pellet samples. Thissmall representation may be associated with a greaterfragility of arthropod remains and a poor preserva-tion under adverse environmental conditions.

As a starting point for a conservation-orientedmanagement of the Somuncurá Plateau ProtectedArea, it is necessary to conduct a biodiversity assess-ment and a spatial description of its components. Inthis sense, studies like the one described here, usingthe raptor pellets analysis as a complementary meth-odology to the traditional trap sampling, couldbecome an important source of information for deter-mining the composition of the biodiversity in anunexplored area such us Somuncurá.

Acknowledgements

Mauricio Failla, Director de Fauna del Ministerio deProducción, Río Negro, granted the capture permits.Florencia del Castillo, Agustín Cordero, and DanielUdrizar Sauthier are thanked for their help duringfield and laboratory work. We greatly thank theQuiñeluf family for their hospitality. Nicolas Frutos,Nestor Basso, Gustavo Flores, and Luis Acosta helped uswith the determination of reptiles, anurans, Coleoptera,

and Araneae. We also would like to thank the Depar-tamento de Zoología, Centro Regional UniversitarioBariloche (Universidad Nacional del Comahue) forallowing access to the arthropod collection of theInvertebrates B chair. Leandro Becker, Rita LopezLaphitz, Lucrecia Pizarro, and Gary Fernandezhelped with the English translation. We appreciatethe improvements in the early draft of this articlemade by the editor Anne Zillikens and the anony-mous reviewer. This work was supported by ConsejoNacional de Investigaciones Científicas y Técnicas(CONICET, Argentina), PIP 6475.

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