Origins of Nearctic migratory landbird vagrants recorded in Europe revealed by feather isotopic analysis

Understanding the occurrence of vagrant individuals away from their usual distribution range can shed light on animal navigation systems. In autumn, migratory Nearctic landbirds often occur as far as Western Europe, and while the link between these occurrences and wind conditions over the Atlantic Ocean is largely established, the drivers and natal origins of these transatlantic vagrants remain elusive. We conducted feather hydrogen isotopic (δ²H_f) analyses from 72 Nearctic individuals representing 26 landbird species captured in the Azores (Portugal), Iceland and France over the last century to infer their likely origins. While potential origins of several individuals came from northeastern Nearctic populations (e.g., Blackpoll Warbler, Northern Parula), we also found that some individuals likely originated from northwestern (e.g., American Redstart, Gray-cheeked Thrush) or southern (Yellow-billed Cuckoo) populations. Our results confirm that northeastern populations migrating along the coast or undertaking over-water flights are more likely to be displaced. However, they also show that even northwestern populations migrating southeast towards the eastern coast of North America have the potential to be displaced across the Atlantic Ocean. Further research is needed to fully understand the drivers of these transatlantic vagrancy events and to determine whether wind is the only factor that can push migratory individuals off their intended course. Overall, this study sheds light on the broad geographic origins of transoceanic vagrants and the potential implications for the colonization of new regions.