Stable isotopes examined across a migratory divide in Scandinavian willow warblers (Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula) reflect their African winter quarters

The C and N isotopes of feathers from two subspecies of willow warblers (Ph ylloscopus trochilus trochilus and Phylloscopus trochilus acredula) are iso topically distinct. Our analysis of 138 adult males from 14 sites distribut ed across Sweden shows that the mean delta(15)N and delta(13)C values of su bspecies acredula (from latitudes above 63 degrees N) were significantly hi gher than the mean delta(15)N and delta(13)C values of subspecies trochilus (from latitudes below 61 degrees N). The analysed willow warbler feathers had been moulted in the winter quarters and the observed isotopic signature s should thus reflect the isotopic pattern of food assimilated in Africa. T he isotopic data observed in Sweden match the dine in morphology, both show ing abrupt changes around 62 degrees N. This result agrees with data from r inging recoveries indicating that the two subspecies occupy geographically and isotopically distinct wintering grounds in Africa. Our isotopic data su ggest that analysis of stable isotopes of C and N is a promising method to track wintering quarters of European birds that migrate to Africa.