Data from: Across a migratory divide: divergent migration directions and non-breeding grounds of Eurasian reed warblers revealed by geolocators and stable isotopes

Migratory divides represent narrow zones of overlap between parapatric populations with distinct migration directions and, consequently, expected divergent non-breeding distributions. The composition of the mixed population at a migratory divide and the corresponding non-breeding ranges remain, however, unknown for many Palaearctic-African migrants. Here, we used light-level geolocation to track migration direction and non-breeding grounds of Eurasian reed warblers (Acrocephalus scirpaceus) from three breeding populations across the species’ migratory divide. Moreover, by using feathers grown at non-breeding grounds, we quantified stable isotope composition for individuals with known southwestern (SW) and southeastern (SE) migration directions. On a larger sample per population, we then assessed the proportions of SW- and SE-migrating phenotypes in each of the three populations. All tracked reed warblers from Germany and two thirds of the birds tagged from the Czech population headed initially SW. Nevertheless, about one third of the birds from the Czech site migrated towards SE. No tracking data have been obtained for the Bulgarian population. The initial migration direction determined by geolocators was a strong predictor of the non-breeding region, with SW migrants staying in West Africa and SE migrants in Central Africa. Feather δ34S and δ15N values confirmed the predominance of SW migrants in the German population, the co-occurrence of SW and SE migrants in the Czech population, and indicated a high (72%) proportion of SE migrants in the Bulgarian population. Thus, the combined approach of geolocator tracking and stable isotopic assignments provided clear evidence for the existence of a migratory divide in the southeast of Central Europe and predicted non-breeding range in Central and Central-Eastern Africa for the eastern population