Breeding biology of Grey-Eagle Buzzard (Geranoaetus melanoleucus) in southern Patagonia, Argentina

Abstract

The breeding biology of Grey Eagle-Buzzard Geranoaetus melanoleucus was studied at Deseado department, Santa Cruz province, Argentina, between September 1987 and January 1988. The breeding period, as observed, extended at least from September to late January, when all birds reached an age of eight weeks. Six breeding pairs were found in 130 km2, with a density of one pair per 21.6 km2. All nests were placed in basaltic cliffs, between 2.8 and 20 m height (mean 8.99 m). Alternative nests were found for some pairs in their nesting range. Clutch size was 2.5 eggs for six pairs. Incubation period for three nesting pairs ranged between 37 and 42 days. Hatching occurred between November 7 and 22, with a hatching success of 100%. Five pairs out of six (83.3%) successfully raised at least one chick. Breeding success was 1.67 nestlings per pair. Four vocalizations were recorded for adults and two for nestlings. Mating and nestling development are described. Eagles appeared to be tolerant to some raptors in the area, including other eagles, but suffered the attacks of Falco sparverius and Circus cinereus. Attacks of eagles upon these raptors were also noted. European hare (Lepus europaeus) was the main prey brought to the nestlings.