Using juvenile movements as a proxy for adult habitat and space use in long-lived territorial species: a case study on the golden eagle

Effective conservation management of wildlife species depends on understanding their space and habitat use. Telemetry has become the primary source of data for information on how species use space and habitats. However, animals can be difficult to capture, leading to limited sample sizes and thus low quality inferences. As some individuals may be easier to capture than others, it may be tempting to use them to make inferences about the studied population as a whole. Juvenile birds, in contrast to adults, are easy to capture while they are still in the nest. However, there are few studies on when and how they might serve to obtain a representative characterization of the habitat or space use of adults. This study investigated this by using GPS-tracking data of 35 adult/juvenile dyads of golden eagles Aquila chrysaetos, with the juvenile and adult in a dyad sharing the same home-range. We assessed juvenile-to-adult home-range overlap and also compared their relative use of habitats within that space. We also analysed how these metrics evolved throughout the post-fledging dependence period (PFDP). During this period, juvenile-to-adult similarity was more than 80% for the entire home-range, whereas it was lower for the core area (approximately 60%). Habitat-use similarity was high, at approximately 90% for both the home-range and core area, both in land-cover and topography. The similarity increased following the improvement of juvenile flight skills over a period of two months, to the extent that two months after fledging and until the end of the PFDP, habitat and space use of juveniles can be used to infer the home-range and habitat requirements of adults. It would be valuable to study this 'adult-by-juvenile replacement' approach in other species to determine whether it could be generalized, notably for species with a shorter dependence period or more complex social interactions.