MICROSATELLITE MARKERS FOR PATERNITY TESTING IN THE WILLOW WARBLER PHYLLOSCOPUS-TROCHILUS - HIGH-FREQUENCY OF EXTRA-PAIR YOUNG IN AN ISLANDPOPULATION

Four microsatellite markers from the willow warbler Phylloscopus troch ilus: were isolated and characterised. The combined exclusion probabil ity of the three most polymorphic markers (Phtr1, Phtr2 and Phtr3) was 0.96. These markers were used to test for the occurrence of extra-pai r young (EPY) in twelve willow warbler families sampled from the islan d 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-pai r young 58 %. These results contrast with an earlier study in a mainla nd Swedish population, where no EPY were found. The island population in this study has a higher breeding density than the mainland populati on, rendering support For density dependent factors affecting the freq uency of EPY. A cross-species study showed that the willow warbler mic rosatellite markers can also be used in other European passerines as w ell as distantly related species, such as the Australian magpie Gymnor hina tibicen.