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Females of many socially monogamous species accept or even actively seek copulations outside the social pair bond. As females cannot increase the number of offspring with promiscuous behaviour, the question arises why they engage in extra-pair mating. We used microsatellite data to determine paternity, heterozygosity and genetic relatedness in the reed bunting (Emberiza schoeniclus), a species with high levels of extra-pair paternity (EPP). We found that extra-pair young (EPY) were more heterozygous than within-pair young ( WPY ). The high heterozygosity of the EPY resulted from a low genetic similarity between females and their extra-pair mates. EPY were heavier and larger when compared with their maternal half-siblings shortly before they left the nest. Recapture data indicated a higher fledgling survival of EPY compared with WPY. Our data suggest that reed bunting females increase the viability of their offspring and thus fitness through extra-pair mating with genetically dissimilar males.

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No abstract

Females of many socially monogamous bird species accept or even actively seek copulations outside the social pair bond. It has been shown that females profit from extrapair fertilization by increased offspring quality, but extrapair mating may also induce costs to females. We measured parental food provisioning and paternity in the reed bunting, Emberiza schoeniclus, a species with biparental brood care and high levels of extrapair paternity (EPP). We found a negative relationship between the proportion of EPP in broods and paternal care across the local population. Individual males adjusted food provisioning to the amount of EPP between sequential broods. Females did compensate for low male food provisioning. The cost of extrapair fertilizations to females is increased nestling mortality due to the social partner's reduction of brood care. If the mixed mating system of the reed bunting is at an evolutionary equilibrium, we expect that genetic fitness for males as well as for females is maximized. Any costs incurred by extrapair mating should therefore not exceed the benefits for both sexes.