EXTRA-PAIR FERTILIZATIONS IN THE SEDGE WARBLER

Parentage of 201 young (from 44 broods) in a population of Sedge Warbl ers Acrocephalus schoenobaenus in South Central Sweden, 1990-1992, was determined by using multilocus DNA fingerprinting. The frequency of e xtra-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 ex tra-pair young, nine contained only one extra-pair sire and the remain ing brood two extra-pair sires. The frequency of EPF varied among year s (1.8-11.8%). The seasonal timing of broods with and without extra-pa ir young did not differ, and the occurrence of extra-pair fertilizatio ns (EPF) was not related to the length of the pair male's mate-guardin g period. EPF were not related to breeding synchrony (estimated as the mean number of fertile females per day during a female's fertile peri od). Pairs whose nests contained extra-pair young had more territories within 100 m of their nest than pairs without extra-pair young. Femal es that engaged in EPF had fewer attractive (i.e. singing) males to ch oose 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 th at fertilized extra-pair young tended to arrive earlier and to have hi gher pairing success than both males that were cuckolded and other mal es. 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 bene fits to their offspring, but we cannot exclude the alternative explana tion that attractive males are more efficient in forcing females to ac cept EPF.