Relatedness and inbreeding avoidance: counterploys in the communally nesting acorn woodpecker.

Acorn woodpeckers (Melanerpes formicivorus) live in family groups within which more than one female may lay eggs communally in a single nest. Communally nesting females are usually closely related and share evenly in nesting activities. Although birds of either sex may breed in their natal territory, reproductive inhibition of offspring by the presence of their parent of the opposite sex and dispersal by unisexual sibling units ensure that inbreeding between close relatives is rare.