THE COST OF EXTRA-PAIR FERTILIZATIONS TO FEMALE RED-WINGED BLACKBIRDS

It has recently become apparent that females of many species of birds often copulate with, and produce young sired by, males other than thei r social mates. Understanding the adaptive significance of this behavi our requires knowing whether extra-pair matings entail any cost to fem ales. We investigated nest success relative to paternity in red-winged blackbirds (Agelaius phoeniceus) over 5 years and found that nest suc cess declined as the proportion of nestlings that was sired by extra-p air males increased. Nest defence against potential predators is the p rincipal form of paternal care in this population. Males defended nest s less vigorously when they contained nestlings sired by other males, suggesting that the lower success of those nests was a consequence of reduced paternal care. Nests with extra-pair young were more successfu l when the true father was a local resident, suggesting that resident males may defend other males' nests if they have sired some of the you ng. Our results indicate that females must realize substantial improve ment in offspring fitness if they are to compensate for the cost of ex tra-pair fertilizations.