Behavioural and genetic mechanisms of extrapair fertilization in bluethroats

Most birds are classified as socially monogamous, yet they frequently engage in extrapair copulations. In the bluethroat, females copulate frequently with neighbouring males with the result that around 30% of offspring are sired extrapair. The bluethroat is one of the few bird species in the world in which a genetic benefit to female extra-pair copulation has been identified, viz. an enhanced cellular immunocompetence of extrapair young. The benefit seems to arise as females seek fertilizations with male sthat have a genotype compatible to their own, rather than with males that have a high quality genotype per se. How females choose compatible males is not known. The current project aims at explaining how females choose extrapair sires and the extent to w hich female choice is influenced by male advertisement behaviour. The similarity in genotypes between females and their social and extrapair mates will be examine d for MHC genes that play an important role in the immune defense against pathogens. Experim ents will be performed to reveal whether there is a behavioral or a postcopulatory mechanism, which allows females to select genetically compatible extrapair sires . The project will also manipulate male advertisement behaviour to assess its influence on male fertilization success.