Unusual Concentration of Black-chested Buzzard-Eagles in Central Argentina

Abstract

In birds, gregariousness has been traditionally explained as a way to reduce individualpredation pressure or to enhance foraging, although bird aggregations can also be theresult of reproductive or social behavior. Despite being generally solitary, some raptorspecies congregate in groups for feeding, roosting or migrating. The Black-chestedBuzzard-eagle (Geranoaetus melanoleucus) can aggregate in moderate numbers inroosts or predictive foods sources. Here we describe an unusual aggregation of 113-126 eagles for at least a month in a 30 km road section in western La Pampa, CentralArgentina. The unusual aggregation seems to be related to a population explosion ofCommon Yellow-toothed Cavies (Galea musteloides) after an unusually rainy yearlinked to "El Niño" phenomenon. These and other records of Black-chested BuzzardEagle aggregations suggest the existence of some migratory/dispersive movements inthis species. Further studies are needed to assess the factors determining theseunusual eagle concentrations in this species.