Non-invasive DNA monitoring unveils the reproductive strategy of an endangered and elusive top predator, the Chaco Eagle

Population genetics is an invaluable tool for studying and conserving scarcely researched and elusive threatened species. The Chaco eagle ( Buteogallus coronatus ) is one of the rarest and most severely threatened raptors in the Neotropical region, and due to its elusiveness, many aspects of the species' life history, with important conservation implications, remain mostly unknown. Here, relying primarily on non-invasive DNA sampling at a large spatio-temporal scale, we investigate the reproductive strategies of the Chaco eagle. Over twelve years (2002–2014), we collected 87 moulted feathers from territorial individuals and blood samples from 32 nestlings at 30 nesting sites distributed across a large region (approximately 30,000 km 2 ) of semiarid habitats in Argentina. A primary aim was to elucidate whether this species breeds on a two-year cycle basis, a crucial but still unclear aspect of its life history since, if confirmed, this delay would impose a significant limitation on the lifetime productivity of Chaco eagles across its entire range. We also investigated the occurrence of pair substitutions and territory changes between reproductive events, factors likely intertwined with adult mortality rates and anthropogenic pressures. We found conclusive evidence that eagles may breed annually. In addition, our data suggest that the replacement of a pair member and/or the breeding dispersal to another territory (up to 30 km away) is not uncommon in the species, presumably due to human-related mortality. These spatial patterns of breeding territory selection and breeding dispersal have never been described for large eagles in the Neotropics and highlight the potential ecological consequences of human pressure on this keystone species. Overall, these findings bear direct implications for the demographic dynamics, management, and conservation strategies of the Chaco eagle.