High frequency of extra-pair paternity in a dense and synchronous population of Willow Warblers Phylloscopus trochilus

Genetic parentage was studied in a Norwegian, sub-alpine population of Willow Warblers Phylloscopus trochilus by means of multilocus DNA fingerprinting. We found that 33% (36/109) of the offspring were unrelated to the putative father, and that one additional offspring was unrelated to both putative parents. Altogether 50% (10/20) of the broods contained illegitimate young. The distribution of extra-pair paternity was bimodal, with several broods containing many extra-pair offspring. These results contrast markedly with a previous study of the same species, reporting no cases of mismatched parentage in a Swedish lowland population. Potentially, the high frequency of extra-pair paternity could be attributed to a higher breeding density and/or synchrony in our study population. Cuckolded males had a lower body mass, but no shorter tarsi or wings, than non-cuckolded males. This suggests that a male's loss of paternity is somehow related to his body condition. The data set includes five broods in which the pair male was permanently removed on the day the first egg was laid. There was no tendency that the manipulation increased the frequency of extra-pair paternity.