We analysed stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios in Marsh Warbler Acrocephalus palustris feathers sampled in Europe and Africa to assess non-breeding habitat selection and location of wintering grounds of different breeding populations. Feather δ¹³C values showed that Marsh Warblers occupy a biome dominated by C₄ vegetation during the stopover in northeastern Africa, whereas C₃ habitats are used during the complete moult in southern Africa. East European Marsh Warblers differed in their stable isotope profiles from other European regions, suggesting a certain level of population segregation in southern Africa. A dual-isotope assignment approach confirmed this difference and helped us restrict the autumn staging areas to lower elevations of the Ethiopian Highlands west of the Rift valley. Available ring recoveries, however, suggested high levels of population mixing both on migration through East Africa and in the final wintering grounds.