Older male reed buntings are more successful at gaining extrapair fertilizations

The importance of extrapair paternity (EPP) as an aspect of mixed reproductive strategies is currently the focus of many studies. Since females have at least some control over the occurrence of inseminations, they are expected to engage in extrapair copulations only if they benefit, for instance through gaining high-quality or compatible genes for their offspring, or insurance against infertility. To distinguish between these benefits, we studied characteristics related to intermale variation in fertilization success as well as differences in fitness between half-siblings in the reed bunting, Emberiza schoeniclus, a socially monogamous passerine with high levels of EPP (50% of young). We found that older males were more successful at siring offspring in other broods and a nonsignificant tendency for them to be less cuckolded in their own broods. The success of older males does not support the fertility insurance hypothesis: a male's sperm storage capacity did not predict his fertilization success, the occurrence of infertile eggs was unrelated to the occurrence of EPP in a nest and older males had more infertile eggs in nests with no EPP. Extrapair young had longer tarsi at fledging than their maternal half-siblings, but we found no other differences between maternal or paternal half-siblings in several presumed quality traits. Owing to the absence of long-term fitness data, we are unable to determine whether the difference in tarsus length is related to a difference in fitness benefits. At this time, we cannot exclude the possibility that females do gain genetic benefits through EPP.