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

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Abstract
The C and N isotopes of feathers from two subspecies of willow warblers (Phylloscopus trochilus trochilus and Phylloscopus trochilus acredula) are isotopically distinct. Our analysis of 138 adult males from sites distributed across Sweden shows that the mean δ15N and δ13C values of subspecies acredula (from latitudes above 63° N) were significantly higher than the mean δ15N and δ13C values of subspecies trochilus (from latitudes below 61° N). The analysed willow warbler feathers had been moulted in the winter quarters and the observed isotopic signatures should thus reflect the isotopic pattern of food assimilated in Africa. The isotopic data observed in Sweden match the cline in morphology, both showing abrupt changes around 62° N.This result agrees with data from ringing recoveries indicating that the two subspecies occupy geographically and isotopically distinct wintering grounds in Africa. Our isotopic data suggest that analysis of stable isotopes of C and N is a promising method to track wintering quarters of European birds that migrate to Africa.