IN many birds, females copulate with males other than their social mate, resulting in extra-pair fertilizations (EPFs)(1-7). It is still unknown, however, why females seek EPFs(7,8). In one study, males that accounted for most EPFs had higher survival(6), but neither the characteristics revealing male quality nor the benefits accruing to females selecting attractive males were identified. Great reed warblers, Acrocephalus arundinaceus, are socially polygynous, and females base their mate choice on territory quality(9) and song-repertoire size(10), both of which predict harem size and reproductive success(11,12). By DNA fingerprinting(13), we demonstrate that female great reed warblers obtain EPFs from neighbouring males with larger song repertoires than their social mate. In addition, the relative post-fledging survival of offspring was positively correlated with their genetical fathers’ song repertoire size. These data support the hypothesis that females, by engaging in extra-pair fertilizations, seek genetic benefits for their offspring(7,8).