Publisher: Federation of European Neuroscience Societies and Blackwell Publishing Ltd
Parentage of 201 young (from 44 broods) in a population of Sedge Warblers Acrocephalus schoenobaenus in South Central Sweden, 1990-1992, was determined by using multilocus DNA fingerprinting. The frequency of extra-pair young (EPY) was 7.5% and they occurred in 23% of the broods. For 11 out of 15 extra-pair young we could identify the true father; in all of the cases he was a close neighbour. Among ten broods with extra-pair young, nine contained only one extra-pair sire and the remaining brood two extra-pair sires. The frequency of EPF varied among years (1.8-11.8%). The seasonal timing of broods with and without extra-pair young did not differ, and the occurrence of extra-pair fertilizations (EPF) was not related to the length of the pair male's mate-guarding period. EPF were not related to breeding synchrony (estimated as the mean number of fertile females per day during a female's fertile period). Pairs whose nests contained extra-pair young had more territories within 100 m of their nest than pairs without extra-pair young. Females that engaged in EPF had fewer attractive (i.e. singing) males to choose among the day before and at the day they formed their pair bend. Moreover, in all but one case the extra-pair male was not singing (i.e. not available as pair mate) the day the EPF-female settled. Males that fertilized extra-pair young tended to arrive earlier and to have higher pairing success than both males that were cuckolded and other males. Hence, female Sedge Warblers engaged in extra-pair fertilizations with attractive male neighbours. These results are consistent with the hypotheses that females participating in EPF are seeking genetic benefits to their offspring, but we cannot exclude the alternative explanation that attractive males are more efficient in forcing females to accept EPF.