Inferring the ecology of willow warblers during their winter moult by sequential stable isotope analyses of remiges

We present a comparison of feather stable isotope ( δ 13 C, δ 15 N) patterns representing the habitat and diet conditions for two subspecies of willow warblers Phylloscopus trochilus that breed in parapatry, but winter in diff erent regions of subSaharan Africa. Previous analyses have shown that on average winter moulted innermost primaries (P1) show subspecifi c diff erences in δ 15 N values, although individuals show substantial variation for both δ 13 C and δ 15 N within the subspecies. We examined whether corresponding variation in the timing of the winter moult, as refl ected by consistent intra-wing correlations for individual ’ s δ 13 C and δ 15 N values, could explain some of the previously observed isotopic variation. Further, diff erential subspecifi c adaptations to winter precipitation patterns across Africa might result in a variable degree of site fi delity or itinerancy during moult. We found no consistent trend in isotopic values from innermost to outermost primaries, thus inter-individual variation in the timing of moult does not explain the subspecifi c isotopic variation for P1. Patterns in wing feather δ 13 C and δ 15 N values indicated that 41% of the individuals from both subspecies shifted their diet or habitats during winter moult. Importantly, despite well-documented itinerancy in willow warblers during the winter, 59% of the individuals had feather isotope values consistent with stable use of habitats or diets during winter moult. Repeatability analyses suggest that individuals of both subspecies initiate moult in similar habitats from year-to-year while feeding on isotopically similar diets.