The function of extrapair fertilization in passerine birds

In more than two thirds of all bird species with a socially monogamous mating system, females engage in copulations outside their pair bond. As a consequence, extrapair fertilizations and mixed-paternity broods are common in many of these species. The sel ective advantage for female extrapair fertilization is poorly understood. Recent evidence from our research on bluethroats indicates that females seek fertilizations with males that have a compatible genotype with respect to genes that encode immunocompet ence. A standardized immunological test of bluethroat nestlings revealed a higher T-cell mediated immune respone by young sired by extrapair males than by their maternal half-sibs sired by the male at the nest. The d ifference can only be attributed to paternal genes since the half-sib groups share the same rearing environment and genes from the mother. The current project aims at testing whether a similar genetic benefit of female extrapair copulation exists in other passerine birds with an extensive extrapair mating system. The projects will also try to identify possible fitness effects (in terms of post-fledging survival) associated with the immune responsiveness of nestlings.