BREEDING BIOLOGY OF A GREY EAGLE-BUZZARD POPULATION IN PATAGONIA

摘要

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/19 km~2 (N = 44), with higher densities occuring in the plains (1 pair/13.2 km~2, N = 17) than in the mountains (1 pair/22.7 km~2, 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) (58.2%), 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.