Breeding Biology of a Grey Eagle-buzzard Population in Patagonia

-we studied the breeding performance of a Grey Eagle-Buzzard (Geranoaetus melanoleucus) population in a region of northern Argentinian Patagonia where two different habitats occur; the mountains near the Andean cordillera and the plains where shrub-steppes predominate. Mean eagle density was 1 pair/l9 km2 (N = 44), with higher densities occuring in the plains (1 pair03.2 km2, N = 17) than in the mountains (1 pairI22.7 km2, N = 27). We located four communal roosts, with a maximum number of 14 immature birds roosting together. Of 1254 prey items identified from pellets, most were European hares (Lepus europaeus) (5X.2%), rodents (19.1%), and birds (16.5%). Diet varied between pairs and between those breeding in the mountains and those in the plains. The number of nests per pair was significantly higher in the plains (5.1 nests/pair, N = 9 pairs) than in the mountains (3.2 nests/pair, N = 14 pairs). Breeding success was similar in both breeding seasons. About 80% of the pairs laid eggs and over 60% successfully raised at least one young. The number of fledgling young per pair varied between one and three. The mean number of fledglings per successful pair was 1.8. The values of breeding density and success are the highest published for this species (and among the highest for other large eagles), probably because the species exploits the abundant European hare, a mammal introduced to Patagonia at the beginning of the century. Received 12 Sept. 1994, accepted 10 May 1995. The Grey Eagle-Buzzard (Geranoaetus melanoleucus) is the largest Buteoninae, weighing 1670-3170 g (Jimenez and Jaksic 1990). Its range extends from Venezuela to Tierra de1 Fuego. It prefers open country with steep landscapes. It is seen more rarely in the plains (Brown and Amadon 1968, Jimenez and Jaksic 1990). In Patagonia the Grey Eagle-Buzzard is the largest raptor species, except for Cathartidae, and the birds are relatively undisturbed by man (Donazar et al. 1993). The most significant recent change in the ecology of this region has been the introduction of two lagomorph species: the European hare (Lepus europaeus)-introduced in 1888 in central Argentina (Godoy 1963) and around the beginning of this century in our study area (authors unpubl.)-and the European rabbit (Oryctolagus cunicuZus) that recently (1945-l 950) expanded its range across the Argentinian border from Chile (Howard and Amaya 1975). There are few studies on the Grey Eagle-Buzzard, and none have provided quantitative data (see review in JimCnez and Jaksic 1990). Food I Estaci6n Biol6gica de Doiiana, CSIC, Avda. Maria Luisa s/n, 41013 &villa, Spain. 2 Grupo de Estudios Biol6gicos Ugarra, Carlos III 19, 31002 Pamplona, Spain, 3 Nationalparkverwaltung Berchtesgaden. Doktorberg 6, 83471 Berchtesgaden, Germany. 4 Centro de Ecologia Aplicada de1 NeuquBn, Casilla de Correos 92, 8371 Junh de 10s Andes, NeuquBn, Argentina. ’ Present address: Estaci6n Biol6gica de DoAana, CSIC, Avda. Maria Luisa s/n, 41013 &villa, Spain.