Extrapair paternity, song, and genetic quality in song sparrows

Most songbirds are socially monogamous, yet molecular studies have found that in most species, some offspring in the nest are sired by males other than the social mate of the female. The functional significance of extrapair paternity (EPP) in social monogamy is poorly understood, despite numerous theoretical and empirical studies in the last decades. We have examined EPP in the song sparrow (Melospiza melodia) using microsatellites and tested whether females choose as extrapair mates males that 1) had larger song repertoires, 2) shared more songs with their neighbors, 3) were more heterozygous, or 4) were less related to the females than the social mate of these. We found that 24% of offspring were sired by extrapair males and that the extrapair sires were invariably neighbors. However, neither song repertoire size nor song sharing with neighbors predicted a male's EPP success. Furthermore, neither heterozygosity of a male nor his relatedness to the female predicted EPP success. At the same time, males that did not lose paternity in their own nest or gain paternity in other nests tended to be younger. These results indicate that females are not using song repertoire size or song sharing as a basis for extrapair mate choice and are not likely to accrue significant genetic benefits from EPP. Instead, the occurrence and level of EPP in this population might be primarily governed by behavioral trade-offs between mate guarding and pursuing extrapair copulations. We suggest that detailed behavioral studies are needed to understand extrapair mating in this species. Copyright 2011, Oxford University Press.