Feeding Habits of Barn Owls Along a Vegetative Gradient in Northern Patagonia

J. Raptor Res. 41(4):277–287 E 2007 The Raptor Research Foundation, Inc.

FEEDING HABITS OF BARN OWLS ALONG A VEGETATIVE GRADIENT IN NORTHERN PATAGONIA

ANA TREJO1 Centro Regional Bariloche, Universidad Nacional del Comahue, 8400 Bariloche, Argentina

SERGIO LAMBERTUCCI Laboratorio Ecotono, Centro Regional Bariloche, Universidad Nacional del Comahue, 8400 Bariloche, Argentina

ABSTRACT.—Barn Owls (Tyto alba) have been considered a useful tool for estimating extinct and extant distributions of small mammals by the analysis of their diets. To test Barn Owls’ sensitivity to environmental changes, we analyzed the trophic ecology of these owls in northern Argentine Patagonia, a region characterized by a marked west-east vegetative gradient. We based our study on new and published information on diets in 15 localities along this gradient, from the Andes to the Atlantic Ocean. We analyzed number of mammalian prey items, food niche breadth, and mean weight of prey. We used Barn Owls’ food habits to detect changes in the local composition of prey species, by means of correspondence and cluster analysis. Our results confirmed Barn Owls as small-mammal specialists (up to 99% of their total prey). The number of mammalian prey species and the mean weight of prey decreased from west to east, and food niche breadth was not correlated with longitude. Statistical analyses yielded an ordination of localities that corresponded to changes in vegetation and in small-mammal assemblages. Our results in northern Pata- gonia showed that prey selection along a vegetative gradient was associated with the rodent assemblages in each vegetation type. This suggests that the use of Barn Owl pellets is appropriate for study of the distribution of small mammals.

KEY WORDS: Barn Owls; Tyto alba; diet; Patagonia; vegetative gradient.

HA´ BITOS ALIMENTARIOS DE TYTO ALBA A LO LARGO DE UN GRADIENTE DE VEGETACIO´ NENEL NORTE DE LA PATAGONIA

RESUMEN.—Las lechuzas Tyto alba han sido consideradas una herramienta eficaz para estimar las distribu- ciones histo´ricas y actuales de pequenõs mamı´feros mediante el ana´lisis de sus dietas. Para probar la sensibilidad de las lechuzas a los cambios ambientales, analizamos la ecologıá tro´fica de estas aves en el norte de la Patagonia argentina, una regioń caracterizada por un marcado gradiente de vegetacioń en sentido oeste a este. Basamos nuestro estudio en informacioń nueva e informacioń publicada sobre dietas en 15 localidades a lo largo de este gradiente, desde los Andes hasta el Oceáno Atlańtico. Analizamos el nu´mero de presas mamı´feras, la amplitud de nicho tro´fico y el peso medio de las presas. El uso de los ha´bitos alimentarios de las lechuzas para detectar cambios en la composicioń local de las presas fue explorado por medio de ana´lisis de correspondencia y de conglomerados. Nuestros resultados confirmaron que T. alba se especializa en pequenõs mamı´feros (hasta un 99% de las presas totales). El nu´mero de presas mamı´feras y el peso medio de las presas decrecieron hacia el este del gradiente, y la amplitud de nicho tro´ficonosecorrelaciono´ con la longitud. Los ana´lisis estadı´sticos determinaron una ordenacioń de localidades que siguieron los cambios en las unidades de vegetacioń y en los ensambles de pequenõs mamı´feros. Nuestros resultados del norte de la Patagonia mostraron que la seleccioń de presas a lo largo de un gradiente de vegetacioń responde a los ensambles de roedores en cada unidad de vegetacioń. Esto favorece el uso de las egagro´pilas de T. alba para estudiar la distribucioń de pequenõs mamı´feros. [Traduccioń de los autores editada]

Owl pellets have long been used by mammalogists small-mammal communities ( Jaksic´ et al. 1981, Mas- and paleontologists to describe extant and extinct soia et al. 1987, Pearson and Pearson 1993, Bonvi- cino and Bezerra 2003, Pardin˜as et al. 2003). How- 1 Email addresses: [email protected], ana.r.trejo@ ever, to obtain acceptable data, the selected raptor gmail.com must be a widespread small-mammal specialist, but

277 278 TREJO AND LAMBERTUCCI VOL. 41, NO.4 have feeding habits flexible enough to vary accord- gives way to scattered grass-shrub steppes to the east (Maz- ing to changes in faunal composition and popula- zarino et al. 1998). Climate is cold-temperate with mean annual temperature ranging from 6uC in the Andean area tion fluctuations. The Barn Owl (Tyto alba) seems to to 14uC near the Atlantic Ocean at Valde´s Peninsula (Par- be a suitable tool for these kinds of studies, because uelo et al. 1998). it is a predator of small- and medium-sized small The gradient encompasses three major phytogeographic mammals (for Argentina, see review in Bellocq units (Table 1): subantarctic forest, Patagonian grass steppe, and typical austral monte, and two ecotones: for- 2000). In addition, this owl is known to shift its est-steppe ecotone, and Patagonian steppe-monte ecotone. primary prey when disturbances, such as fire, cause The following descriptions and data are based on Leoń et changes in the small-mammal fauna (e.g., Sahores al. (1998) and Paruelo et al. (1998). and Trejo 2004), which indicates a certain degree of Subantarctic forest biome (F) extends in Argentina from 35–54uS in a narrow stretch (maximum width ca. 75 km, opportunism in this species. However, there may be Dimitri 1982) along the eastern slopes of the Andes. The some biases in the use of Barn Owls’ diets to esti- area consists of lakes, glacial valleys, and mountain slopes mate the composition of small-mammal assem- dominated by deciduous and/or evergreen southern blages that should be considered, such as a tendency beech (Nothofagus spp.). Total annual rainfall ranges from 1000 mm to .3000 mm to the west. to underestimate the numbers of diurnal and/or The Patagonian grass steppe (S) extends to the east of larger rodents, and overestimate the numbers of 70uW between 38u and 46uS. Vegetative communities are species inhabiting open landscapes (Pardinãs et al. dominated by Stipa spp. grasses, with scarce cushion 2003, Torre et al. 2004). bushes (Adesmia campestris, Berberis heterophylla, Senecio fila- One way to test Barn Owls’ sensitivity to environ- ginoides, Mulinum spinosum). Vegetation cover is ca. 50%, and mean annual rainfall approximately 200 mm. mental change is to study their feeding habits along Typical austral monte (M) is an open shrubland with habitat gradients (e.g., Travaini et al. 1997, Leveau shrubs (Larrea spp.) 1–2 m tall, and scarce herbaceous et al. 2006). Vegetative changes usually are associat- cover (10–20%). The area includes eastern Rıó Negro ed with changes in the small-mammal assemblages and Chubut provinces. Mean annual rainfall is ca. 200 mm. (Pardinãs et al. 2003). Barn Owls are common in Patagonian steppe-monte ecotone (S-M E) is found in northern Argentine Patagonia (Neuqueń, Rıó Ne- the Valde´s Peninsula, northeastern Chubut province. Veg- gro and northern Chubut provinces), an area char- etation is dominated by shrubs 0.5–1.5 m tall (Chuquiraga acterized by a marked vegetative gradient (correlat- spp., Condalia microphylla) and grasses (Stipa spp.). Total vegetation cover is ca. 40%, a result of mean annual rain- ed with rainfall and temperature gradients), from fall .200 mm and the oceanic influence. moist Nothofagus forests in the Andean western re- Forest-steppe ecotone (F-S E), found in the transition gion to the arid grassland-shrublands in the east region between subantarctic forest and Patagonian grass (Leoń et al. 1998). Along this gradient, characteris- steppe has mean annual rainfall from 500–800 mm and vegetation cover ca. 60%. Vegetation consists of a mosaic tic small-mammal faunas are found in relation to of grasslands (Festuca pallescens, Stipa spp.), dispersed low vegetative types (Pearson and Pearson 1982, Pardi- bushes (Discaria articulata, Berberis buxifolia, Adesmia boro- nãs et al. 2003). Here we present a synthesis of the noides, Mulinum spinosum), and scattered patches of trees trophic ecology of the Barn Owl in northern Argen- (Austrocedrus chilensis, Lomatia hirsuta, Maytenus boaria, Schi- tine Patagonia, based on a compilation and analysis nus patagonicus). Diet Analyses. The diet of Barn Owls was studied in 15 of unpublished and published data along the west- localities along a west-east environmental gradient on east gradient. The objective of our study was to test a transect of approximately 700 km from the Andean whether habitat variations are reflected in prey se- mountains in the west to the Atlantic coastline in the east lection by Barn Owls. (Fig. 1). Mean annual temperature of the localities studied (west to east) varied from 6–13uC, and mean annual rainfall from 1100 mm to 170 mm. We studied variations in METHODS Barn Owls’ feeding habits by analyzing both published and Study Area. Argentine Patagonia extends in southern our own field data. We collected 111 pellets at the bottom South America from 39uSto55uS, and includes the land of nests and/or roosts of three pairs of owls at Sierra Paile- between the Andes Mountains and the Atlantic Ocean. mań during the 2005–2006 austral summer (Table 1), and Our study was centered in an area of northern Patagonia 118 pellets from one pair at Punta Delgada during the between 40–43uS (Fig. 1), characterized by both decreas- 2003–2004 austral summer (Table 1). Pellets were dried, ing precipitation and increasing temperature along a west- and then processed following standard methodology to-east gradient. From the Andes Mountains and eastward, (Marti 1987). Bone remains and arthropod exoskeletons total annual precipitation decreases exponentially (Par- were identified by comparison with reference collections uelo et al. 1998), from .3000 mm to 100 mm annual rain- deposited in the Centro Regional Bariloche (Universidad fall, which determines a marked vegetation gradient. Cold- Nacional del Comahue). temperate forests grow on the western mountain slopes, We also reanalyzed Barn Owl pellet data from Trejo et and a steady transition from open forests to grasslands al. (2005), discriminating the diet for each locality sepa- DECEMBER 2007 DIET OF BARN OWLS IN PATAGONIA 279

Figure 1. Study area in northern Patagonia, Argentina, showing the 15 locations surveyed along the west-east transect. Black circle 5 subantarctic forest, white circle 5 forest-steppe ecotone, black rectangle 5 Patagonian grass steppe, triangle 5 typical austral monte, white rectangle 5 Patagonian steppe-monte ecotone.

rately. We selected published data with a minimum of 50 weights were taken from the literature (e.g., from Redford identified prey items, from localities along the gradient. and Eisenberg 1992), and from our own records. For each locality, the percent frequency of each prey spe- Pearson correlation analyses (Zar 1996) were performed cies was calculated for the total number of prey found in to test for associations between food-niche parameters and all the pellets. localities along the longitudinal gradient. We also used Statistical Analyses. For each locality we estimated three correspondence analysis (CA; Greenacre 1993, Lebart et trophic characteristics: (1) number of mammalian prey al. 1995, Cuadras 1996) to analyze changes in the compo- species (NMPS); (2) food-niche breadth (FNB, Levins sition of the Barn Owl diet along the longitudinal gradi- 1968); and (3) geometric mean of prey weight (GMPW). ent. Data were organized in a matrix of species of prey To estimate FNB, we identified mammalian prey (ro- (columns) versus localities (rows). The values included dents, marsupials, bats, and lagomorphs) to species level. in each cell of this matrix were the frequency of appear- Birds, reptiles, amphibians, and invertebrates (insects and ance (%) of each prey category over the total number of other invertebrates) were identified to class level. Levins’ pellets for each locality. We recorded the mean annual index was used to estimate food-niche breadth as FNB 5 precipitation, mean temperature, and longitude for each 2 1/Spi , where pi is the proportion of item i in the diet. To locality. Precipitation and temperature were taken from allow meaningful comparisons, we used the FNB st 5 WorldClim database (www.worldclim.org), a digital global (FNB-FNBmin)/(FNBmax-FNBmin), where FNB min 5 1 climate database that provides information at low spatial (minimum possible niche breadth) and FNBmax 5 N resolution (ca. 1 km2; Hijmans et al. 2005), and we com- (maximum possible niche breadth or total number of prey plemented this information with data from published rec- categories). ords (APN 1996). The geometric mean of prey weight in the diets was Climatic variables and longitude were considered as sup- calculated following Marti (1987). We also determined plementary (illustrative) in the correspondence analysis the mean prey weight of the primary prey species. Prey and do not contribute to the total dispersion of the data. 280 TREJO AND LAMBERTUCCI VOL. 41, NO.4

Supplementary variables do not participate in the confor- mation of the axes, but they have a measurable position relative to the principal axes (Lebart et al. 1995). We performed cluster analysis to corroborate the groups obtained with the correspondence analysis. This analysis groups similar entities together into hierarchic classes (Gauch 1999). OURCE ed pooled in Trejo S The Ward’s method was used to partition the data in the correspondence analysis; this method uses an analysis of variance approach to evaluate the distances between clusters, attempting to minimize the sum of squares of any two clusters that can be formed at each step (for details see Ward 1963). We used software SPAD 4.5 (2000) for multi-

Patagonian steppe-monte ecotone. variate analyses. 5 RESULTS Mammals represented an average of 95.7 6 5.4% (range 5 77.9–99.7%) of the total prey found in

ATE pellets; most mammalian prey were sigmodontine D rodents (88.4 6 8.3% of total mammal prey, range 5 75.2–100%). Small marsupials were eaten where available and, very occasionally, bats (Table 2). Oth-

OLLECTION er prey were birds and arthropods. Birds were eaten C typical austral monte, S-M E by some individuals, and, therefore, relatively high 5 rates of bird consumption were observed only in some localities (Table 2). Number of mammalian prey species varied from 6–16 (Table 2), and was significantly correlated with longitude (r 5 0.592, P 5 0.026). The value corre- sponding to Junı´n de los Andes (N 5 16) was con-

ABITAT sidered an outlier (probably because that study does s were studied. Data from localities marked with an asterisk, were present H not represent a single locality, but a pool of more

Patagonian grass steppe, M than 30; see Travaini et al. 1997). Hence, it was not W S Spring to Summer, 1991, 1992, and 1994–95 Travaini et al. (1997) 5 9 included in the present analysis. Mean FNB was 30 W F All seasons, 2001–2002 Trejo and Ojeda (2004) W F-S E Spring to Autumn, 2001–2002, and 2002–2003 This study * W F-S E Winter 1993 to Autumn 1994 Pillado and Trejo (2000) W F-S E Spring to Autumn, 2001–2002, and 2002–2003 This study * W F-S E Spring to Autumn, 2001–2002, and 2002–2003 This study * W F-S E Spring to Autumn, 2001–2002, and 2002–2003 This study* W F-S E Spring to Autumn, 2001–2002, and 2002–2003 This study * W F-S E Spring to Autumn, 2001–2002, and 2002–2003 This study * W S Summer 2000 and Summer 2001 Andrade et al. (2002) W S Unidentified season, 2001–2002 Teta and Andrade (2002) W M Summer 2005 and Summer 2006 This study W M Winter 1986 to Summer 1987 De Santis and Pagnoni (1989) W M Spring and Summer, 1987 De Santis and Pagnoni (1989) W S-M E Summer 2003 and Summer 2004 This study u 9 9 9 9 9 9 9 9 9 9 9 9 9 9 0.384 (SD 5 0.127), and was not significantly corre- 16 08 07 06 06 59 48 48 22 13 46 08 56 38 –71 u u u u u u u u u u u u u u

9 lated with longitude (P . 0.05). ONGITUDE 30 L u Mean weight of primary prey species varied from 17 g (Eligmodontia morgani) to 164 g (Ctenomys sp.). The overall geometric mean of prey weight was 40.7 S70 9 6 2.5 g (range: 24–60 g), and geometric mean of 20 S71 S71 S71 S71 S71 S70 S70 S70 S69 S68 S65 S65 S64 S63 u 9 9 9 9 9 9 9 9 9 9 9 9 9 9

forest-steppe ecotone, S prey weight was significantly correlated with longi- 15 45 47 49 18 03 54 58 44 13 11 49 49 48 –40 u u u u u u u u u u u u u u 5 9 tude (r 5 0.631, P 5 0.012). ATITUDE 30 L u The first two axes generated by the correspondence analysis accounted for 52.0% of the variance in the diet (Fig. 2). Representation of localities and prey categories on the plane defined by the first two

ń 41 axes clearly segregated the western part of the gradient (which tended toward the negative zone of ń 41 ó 41 axis 1 and the positive zone of axis 2) from the subantarctic forest, F-S E OCALITIES ń de Creide 40 L west-central area (the center area of axis 1 and neg- ń de los Andes 39 5 ative zone of axis 2), and from the eastern area (the positive zone of axes 1 and 2). Those areas are con- sistent with areas of subantarctic forest and western 1 Challhuaco 41 2 Valle Encantado 40 3 Rinco 4 La Lipela 41 5 El Desafı 6 San Ramo 7 Pipilcura 40 8 Cooperativa Escuela 40 9 Junı 10 Estancia Calcatreo 41 11 Sierra Talagapa 42 12 Sierra Pailema 13 Laguna Blanca 42 14 Punta Este 42 15forest-steppe Punta Delgada 42 ecotone (cluster 1), eastern forest- UMBER

Table 1.et Localities al. in (2005). northern F Argentine Patagonia where Barn Owl pellet N steppe ecotone and Patagonian steppe (cluster 2), D ECEMBER 07D 2007 Table 2. Diet composition in 15 localities situated on a west-east transect in northern Patagonia. Frequencies of prey are expressed as percentages of the total number of prey. Coordinates and habitat of localities (1–15) are detailed in Table 1.

PREY ITEM 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Mammals Lestodelphys halli 2.9 1.0 Thylamys sp. 1.0 6.3 14.7 19.5 Dromiciops gliroides 1.0 Histiotus montanus 0.1

Tadarida brasiliensis 0.9 OF IET Abrothrix longipilis 20.4 19.2 13.9 30.8 14.2 13.7 10.9 2.6 Abrothrix olivaceus 0.4 1.7 4.6 2.8 10.9 5.9 10.8 29.1 17.2 14.6 B

Akodon spp. 21.1 ARN Akodon iniscatus 0.5 4.4 15.3

Akodon molinae 6.3 2.1 1.8 O Chelemys macronyx 15.2 2.4 1.5 11.7 1.2 5.0 1.7 1.6 2.2 IN WLS Geoxus valdivianus 6.2 1.7 0.8 0.7 0.6 Notiomys edwardsii 0.9 0.5 P

Calomys musculinus 2.2 4.5 1.7 ATAGONIA Eligmodontia sp. 2.1 7.9 6.2 3.6 14.8 8.3 0.7 14.3 16.3 23.8 17.4 36.9 37.9 50.9 61.0 Euneomys sp. 1.0 0.8 9.4 2.9 6.9 1.1 9.8 33.8 Graomys griseoflavus 18.0 15.3 16.8 5.9 Irenomys tarsalis 6.5 1.5 1.6 0.8 0.2 Loxodontomys micropus 29.1 23.7 4.6 24.8 18.0 8.7 19.4 10.9 9.8 Phyllotis xanthopygus 0.7 3.8 12.3 0.4 6.3 1.4 4.6 8.6 3.6 3.9 Reithrodon auritus 2.1 0.7 4.6 8.0 18.0 28.5 22.3 10.3 22.7 21.6 11.8 5.4 5.3 11.9 Oligoryzomys longicaudatus 14.5 25.4 7.7 16.5 0.8 14.2 0.7 7.4 Galea musteloides 0.6 4.5 Microcavia australis 0.1 1.3 0.8 0.5 0.4 0.9 Ctenomys sp. 6.9 5.8 3.4 7.8 5.9 19.4 8.0 5.8 17.6 10.9 11.7 1.6 0.4 Lepus europeaus 0.5 2.4 0.1 Oryctolagus cuniculus 0.1 281 8 T 282

Table 2. Continued.

PREY ITEM 1 2 3 4 5 6 7 8 9 101112131415 Birds Unidentified birds 0.6 0.8 1.1 0.5 1.8 3.4 Columbidae 0.7 Mimidae 11.1 0.9 Emberizidae 3.9 0.4 7.9 2.7 Icteridae 1.1 AND REJO Tyrannidae 0.3 0.4 1.1 Furnariidae 0.4 0.8 0.5

Reptiles1 0.1 0.9 0.5 2.2 L AMBERTUCCI Amphibians2 0.2 Arthropods 0.7 1.5 4.9 3.9 2.0 2.2 2.9 2.7 Total prey (N) 289 291 65 387 128 253 139 175 2299 1249 1021 111 190 226 118 NMPS3 12 11 9 11 8 12 11 9 16 10 12 9 8 7 6 FNB4 0.265 0.394 0.313 0.372 0.737 0.481 0.438 0.550 0.356 0.426 0.319 0.367 0.359 0.255 0.235 GMPW5 52 42 48 39 44 47 60 33 44 47 51 35 29 24 24

1 All reptiles were lizards (Lacertilia). 2 All amphibians were toads (Anura). 3 Number of mammalian prey species. 4 Food-niche breadth. 5 Geometric mean of prey weight. V OL 1 N 41, . O .4 DECEMBER 2007 DIET OF BARN OWLS IN PATAGONIA 283

Figure 2. First factorial plane of the correspondence analysis showing the variation in the composition of the diet of the Barn Owl in northern Patagonia, Argentina. Arrows in the plane indicate the direction of the increase of the supplementary variables (longitude, mean precipitation, and mean temperature). We include only the species name of the prey that contribute to the constitution of the axis. Filled circles represent localities and filled triangles species. and typical austral monte and Patgonian steppe- nomic floristic units (cluster 1–2, and cluster 3), monte ecotone (cluster 3; Table 1). Axis 1 was indicating that Barn Owls have a dissimilar diet strongly negatively correlated with longitude (r 5 composition in those areas. Inside the first group 20.94) and precipitation (r 520.82), and positive- (cluster 1–2), the first bifurcation separated subant- ly correlated with temperature (r 5 0.85). arctic forest and the western localities in the forest- Prey selection differed among the western (clus- steppe ecotone (cluster 1: Challhuaco, Rincoń de ter 1), central (cluster 2) and eastern (cluster 3) Creide, La Lipela and Valle Encantado) from more areas of Patagonia (Table 3). Species that were eastern forest-steppe ecotone and Patagonian clearly consumed in the western area were Abrothrix steppe localities (cluster 2). The next bifurcation longipilis, Oligoryzomys longicaudatus and Loxodontomys inside cluster 2 separated only Junıń de los Andes micropus. In the central area, Abrothrix olivaceus, Rei- from the rest of the localities. After those two throdon auritus, Euneomys sp. and Ctenomys sp. were groups, the final separation was between the eastern the primary species consumed. Finally, in the east- localities in the forest-steppe ecotone (El Desafıó, ern area, nearly half the diet consisted of Eligmodon- Cooperativa Escuela, Pipilcura, and San Ramoń) tia sp.. Graomys griseoflavus, Thylamys sp., Akodon in- and those situated in Patagonian steppe (Sierra Ta- iscatus, and Passeriformes were also consumed in lagapa and Calcatreo). On the other side of the relatively high numbers in the east (Table 3). dendrogram (inside cluster 3), the first bifurcation The cluster analysis used with five factors yielded was between typical austral monte and Patagonian three clusters or groups, which agreed with the ar- steppe-monte ecotone (Punta Delgada, inside Pen- rangement obtained from the correspondence anal- insula Valdez Natural Reserve). The last bifurcation ysis. However, the dendrogram obtained (Fig. 3) was inside typical austral monte, separating Paile- showed six groups that had significant differences mań from two localities ca. 200 km away, in the (P , 0.05). The first bifurcation in the dendrogram southern Valde´s Peninsula (Punta Este and Laguna separated two groups of places in different physiog- Blanca; Fig. 3). 284 TREJO AND LAMBERTUCCI VOL. 41, NO.4

Table 3. Composition of the three first clusters deter- DISCUSSION mined by correspondence analysis, according to the prey The large proportion of small mammals in the species taken by Barn Owls. Barn Owls’ diet in Patagonia supported the current classification of this owl as a small-mammal special- TEST CLUSTER GENERAL ist (Bellocq 2000). Excepting rare cases, this owl VALUE P (%)a (%)b SPECIES does not replace its main prey (small- and medi- Cluster 1 um-sized rodents) with other type of prey. Among 3.33 0.000 16 6 Oligoryzomys these exceptions, ornithophagy is known in some longicaudatus sites (probably due to local circumstances) from 3.09 0.002 20 8 Abrothrix longipilis published (Noriega et al. 1993) and unpublished 2.73 0.006 27 13 Loxodontomys micropus sources (P. Wallace pers. comm.). Similarly, other 2.36 0.018 2 1 Irenomys tarsalis strigiforms, such as Magellanic Horned Owl (Bubo 2 2.16 0.030 6 20 Eligmodontia sp. magellanicus), may consume higher proportions of Cluster 2 alternative prey such as lagomorphs and/or other 2.68 0.008 14 7 Abrothrix olivaceus 2.44 0.014 12 7 Ctenomys sp. vertebrates when faced with local variations in rela- 2.23 0.026 11 4 Euneomys sp. tive abundance of available prey (Dona´zar et al. 2.21 0.028 18 12 Reithrodon auritus 1997, A. Trejo unpubl. data). Cluster 3 The number of mammalian prey species in the 3.48 0.000 14 4 Graomys griseoflavus diet decreased with decreasing rainfall and increas- 3.36 0.000 47 20 Eligmodontia sp. ing temperature. This may reflect variations in 2.89 0.004 1 0 Lacertilia small-mammal diversity along the west-east gradient. 2.81 0.004 10 3 Thylamys sp. The diversity of small mammal (,200 g) species in 2.62 0.008 2 1 Akodon molinae 2.18 0.028 5 1 Akodon iniscatus the subantarctic forests and in the forest-steppe eco- 2.13 0.034 8 3 Passeriformes tone is greater than the diversity of small mammal species in the forests of most other regions of the a Percentage of each prey item in a cluster. world (Pearson and Pearson 1982). Pardin˜as et al. b Percentage of each prey item in all samples.

Figure 3. Cluster analysis dendrogram of the 15 studied sites in northern Patagonia, based on the similarity in diet composition. The values of Ward’s index of similarity are indicated at each bifurcation. F 5 subantarctic forest, F-S E 5 forest-steppe ecotone, S 5 Patagonian grass steppe, M 5 typical austral monte, S-M E 5 Patagonian steppe-monte ecotone. DECEMBER 2007 DIET OF BARN OWLS IN PATAGONIA 285

(2003) also found decreasing rodent diversity to- such as Punta Delgada vs. Laguna Blanca and ward the east, in the extra-Andean Patagonia. Punta Este. Interestingly, these relatively close re- The geometric mean weight of prey was estimated gions have different vegetation types, steppe-monte to be 45.1 g (Marti et al. 1993) for Barn Owls in ecotone and typical austral monte, respectively. Al- temperate neotropical regions, which was similar though Pailemań is far from these localities, it has to our results. However, considering the variation a similar vegetation and is positioned closer to La- of mean prey weight along the gradient, it seemed guna Blanca and Punta Este in the dendrogram that Barn Owls did not select a particular size class than to Punta Delgada. of rodent, but that the energetic needs were appar- Our results in northern Patagonia agreed with ently met by taking the small-mammal species that those of Leveau et al. (2006) for central Argentina, was the most abundant or the easiest to catch in in that prey selection along a vegetative gradient each locality. The decrease of geometric mean corresponded to the rodent assemblages in each weight of prey in the eastern regions of the gradient vegetative region, suggesting that Barn Owl pellets (monte and Patagonian steppe-monte ecotone) was may be appropriately used to study the distribution due to the greater consumption of small (,20 g) of small mammals. As a caveat, we note that our Eligmodontia sp. This species, although present in all study was limited by three important considerations. the localities studied, is typical of open habitats with First, we pooled data from 1989–2005, so we were high proportion of bare soil (Pearson et al. 1987). unable to analyze any possible temporal variations. Typical austral monte and Patagonian steppe-monte Second, we analyzed very small samples in some ecotone are the areas with the least vegetative cover localities. The influence of site-specific conditions along the gradient (10% and 40%, respectively) and on the diet of the resident owls could be significant, it is reasonable to assume that this species is partic- and the animals studied might not have been rep- ularly abundant there. Consumption of a small spe- resentative of the locality. Third, there were many cies, not very profitable in energetic terms, is possi- more samples for western localities in Patagonia ble only when the species is very abundant or easy than for eastern ones, which may have biased our to catch ( Jaksic´ and Marti 1984). In Mediterranean analyses. There is a need for additional studies on environments, Barn Owls were more specialized and the trophic ecology of owls (and other birds of prey) preyed on larger rodents in habitats with high ro- in the Patagonian steppe and monte regions. dent diversity (Varuzza et al. 2001). ACKNOWLEDGMENTS The diet groups resulting from the multivariate analysis corresponded to the different phytogeo- Lorenzo Sympson collected the Punta Delgada pellets. Ulyses Pardinãs and Pablo Teta shared reference speci- graphic regions generated by the rainfall and mens, bibliography, and useful information. Marcelo temperature gradients in northern Patagonia. Kun, Susana Seijas, and Rubeń Ba´rquez kindly identified According to Pardinãs et al. (2003), these regions insects, birds, and a bat, respectively. We are grateful to determine different rodent assemblages that are Nora Baccala´, Hernań Pastore, and Adriana Ruggiero for their valuable help. 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