Extrapair copulations in western bluebirds (Sialia mexicana): female receptivity favors older males

Extrapair paternity has been the focus of numerous studies of socially mono gamous birds, but the dynamics of extrapair male-female interactions have b een explored in relatively few. In western bluebirds (Sialia mexicana), a s pecies with 20% extrapair paternity, two to four observers monitored extrap air copulations (EPCs) occurring on the female's home territory while her s ocial mate was detained in a cage near the nest. Detention elevated the fre quency and visibility of EPCs, so we could score receptivity of females to identifiable extrapair males without the interference of intermale aggressi on. Most intruding extrapair males were neighboring breeders visiting outsi de their own mate's fertile period. Local male relatives of the detained ma le attempted nonincestuous EPCs, but neighboring sons did not attempt to co pulate with their mothers and, in some cases, chased away intruders. Female receptivity was not influenced by whether the detained mate was visually p resent or hidden behind cloth and did not increase with the persistence or relative size of the extrapair male. Female receptivity increased with the age difference between the extrapair male and the female's social mate, sug gesting that females prefer older males as extrapair partners. This study s uggests that costs of resistance are relatively unimportant in determining female receptivity in western bluebirds, but identifies preferential recept ivity to older males as a potential factor in the outcome of inter- and int rasexual conflict over paternity.