Patterns of stable isotope signatures in willow warbler Phylloscopus trochilus feathers collected in Africa

We conducted stable isotope analyses of nitrogen and carbon on feathers obtained from willow warblers in Africa to find an explanation for a previously observed pattern of different delta N-15 and delta C-13 values across a migratory divide in central Scandinavia. A new data set confirms that north Scandinavian birds of the subspecies P. t. acredula have higher delta N-15 values than south Scandinavian birds of the subspecies P. t. trochilus. In Africa, we found significant differences for both delta N-15 and delta C-13 values among feathers collected from major geographical regions as well as between countries within regions. Isotope signatures of delta N-15 and delta C-13 in Scandinavian acredula matched well with those of willow warblers sampled in southern parts of Africa, but differed from samples obtained in East and West Africa. Isotope signatures in Scandinavian trochilus did not agree with the pattern in any of the three African regions (West, East or South). However, a more detailed analysis of the isotopic data in feathers from countries within West Africa, which is the wintering region of Swedish trochilus based on ringing recoveries, revealed a correspondence with samples from Liberia, the Ivory Coast and Nigeria.