Microsatellite Markers for Paternity Testing in the Willow Warbler Phylloscopus Trochilus: High Frequency of Extra-Pair Young in an Island Population

Four microsatellite markers from the willow warbler Phylloscopus trochilus: were isolated and characterised. The combined exclusion probability of the three most polymorphic markers (Phtr1, Phtr2 and Phtr3) was 0.96. These markers were used to test for the occurrence of extra‐pair young (EPY) in twelve willow warbler families sampled from the island of Gotland in 1992 and 1993. The mean frequency of extra‐pair young was found to be 28% and the mean frequency of families with extra‐pair young 58%. These results contrast with an earlier study in a mainland Swedish population, where no EPY were found. The island population in this study has a higher breeding density than the mainland population, rendering support for density dependent factors affecting the frequency of EPY. A cross‐species study showed that the willow warbler microsatellite markers can also be used in other European passerines as well as distantly related species, such as the Australian magpie Gymnorhina tibicen.