Feeding Habits of Barn Owls Along a Vegetative Gradient in Northern Patagonia
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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 charac- terized 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 distri- bution 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 pequen˜os mamı´feros mediante el ana´lisis de sus dietas. Para probar la sensibilidad de las lechuzas a los cambios ambientales, analizamos la ecologı´a tro´fica de estas aves en el norte de la Patagonia argentina, una regio´n caracterizada por un marcado gradiente de vegetacio´n en sentido oeste a este. Basamos nuestro estudio en informacio´n nueva e informacio´n publicada sobre dietas en 15 localidades a lo largo de este gradiente, desde los Andes hasta el Oce´ano Atla´ntico. 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 composicio´n 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 pequen˜os 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 ordenacio´n de localidades que siguieron los cambios en las unidades de vegetacio´n y en los ensambles de pequen˜os mamı´feros. Nuestros resultados del norte de la Patagonia mostraron que la seleccio´n de presas a lo largo de un gradiente de vegetacio´n responde a los ensambles de roedores en cada unidad de vegetacio´n. Esto favorece el uso de las egagro´pilas de T. alba para estudiar la distribucio´n de pequen˜os mamı´feros. [Traduccio´n 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 Leo´n 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 (Pardin˜as 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 Rı´o Negro ed with changes in the small-mammal assemblages and Chubut provinces. Mean annual rainfall is ca. 200 mm. (Pardin˜as et al. 2003). Barn Owls are common in Patagonian steppe-monte ecotone (S-M E) is found in northern Argentine Patagonia (Neuque´n, Rı´o 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 (Leo´n 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- n˜as 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 rain- fall 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. ma´n 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.