Stopover-site feather isotopes uncover African non-breeding grounds of migratory passerines

The survival of migratory passerines depends considerably on conditions experienced on their non-breeding grounds. However, these critical non-breeding sites are generally poorly known, especially for species and populations using the eastern Afro-Palearctic flyway. To fill this gap, we measured hydrogen stable isotopes in winter-grown feathers (δ ²H_f) of five long-distance migratory passerines (Eurasian Blackcap, Eastern Olivaceous Warbler, Eurasian Reed Warbler, Olive-tree Warbler, and Barred Warbler) collected during spring migration at a stopover site in Israel, a major migratory bottleneck in the Afro-Palearctic Flyway. We determined non-breeding origins of these species, for the entire migrating population and for early- and late-arriving individuals separately. We used a probabilistic model based on feather isotopes and isotopic distribution of precipitation δ ²H (δ ²H_p) in Africa, as well as range maps derived from species distribution models and expert opinion. While our results suggested that Reed Warbler and Olive-tree Warbler occupy most of their known range during the non-breeding season, Blackcaps migrating through Jerusalem, Israel, likely spent the non-breeding season specifically in Ethiopia and near areas, and Eastern Olivaceous Warbler concentrated in two regions in eastern tropical and central Africa. Barred Warblers’ non-breeding grounds were estimated in Kenya, but the species distribution model approach suggested additional regions. Our results further suggested that early- and late-arriving Reed Warblers spent the non-breeding season in separate areas, whereas early- and late-arriving Blackcaps migrated to the same area. Given the rapid decline in many migratory species, our results are important for a more accurate evaluation of the conditions experienced during the non-breeding season and our study is a template for refining migratory connectivity estimates for species using this important flyway.