Unusual aggregations of immature Chaco eagles (Buteogallus coronatus) in northern Argentina

Abstract

Birds can spatially aggregate in order to exploit food resources or to avoid predators, thus increasing their chances of survival. Raptors are usually solitary and territorial, but some species may aggregate for colonial breeding, roosting, or to join large flocks during migration. The Chaco eagle (Buteogallus coronatus) is one of the rarest and most threatened raptors in the Neotropical region, being enlisted as “Endangered” and with an estimated population of less than 1000 individuals. It presents low-density populations and it inhabits fragmented environments. Furthermore, after the only fledgling achieves independence, it may traverse hundreds of kilometers during natal dispersal, being very difficult to track in the field. In fact, there are not many records of immature Chaco eagles at online databases of bird observations (e.g., eBird), and only two of them show more than one individual at the same time. Here we present four records where more than one Chaco eagle immature was seen at the same place and time, in Santa Fe province (northern Argentina). In all cases, individuals were not interacting negatively (i.e., agonistic behavior) with each other, which suggests that immatures of this species may tolerate close presence of conspecifics during aggregations, although the ultimate causes of this behavior need to be further studied. Moreover, the fact that this region constitutes one of the most important areas for the conservation of the Chaco eagle calls for urgent research to determine the social and environmental factors contributing to individual aggregations such as the one described here.