PATTERNS AND CONSEQUENCES OF EGG DESTRUCTION AMONG JOINT-NESTING ACORN WOODPECKERS

Reproductive competition among joint-nesting female acorn woodpeckers, Melanerpes formicivorus, culminates in the removal and subsequent des truction of eggs laid by co-breeders. Females destroying eggs gain bot h by synchronizing egg laying and by having more of their own eggs inc ubated in the final clutch. Here results are summarized based on obser vations of 19 joint nests. Egg destruction accounted for the loss of 3 8% of all eggs laid. Either or both joint-nesting females destroyed eg gs at the same nest, and a female that destroyed eggs during one nesti ng attempt sometimes had her own eggs destroyed during the next. Egg d estruction was not moderated by the genetic relatedness of joint-nesti ng females and was apparently unrelated to behavioural dominance, but substantially lowered per female success of three- compared with two-f emale nests. Females did not discriminate between eggs; thus egg destr uction ended when both females laid normal eggs simultaneously in the nest cavity. However, egg destruction ended most frequently when a fem ale did not destroy an egg laid by her co-breeder 1 day prior to layin g her own first egg. Such failures to destroy eggs entail short-term f itness costs that may be compensated by the potential for reciprocity across breeding attempts. Abnormally small 'runt' eggs, rarely found i n other species of birds, were commonly laid as the first egg of the c lutches of one or both joint-nesting females. The laying of runt eggs may allow females to destroy normal eggs laid simultaneously by their co-breeders. Egg destruction is a major constraint on the degree to wh ich females nest jointly. More generally, egg destruction and its asso ciated behaviour patterns demonstrate that reproductive interference c an be as complex and destructive within groups of close relatives as b etween unrelated individuals. (C) 1995 The Association for the Study o f Animal Behaviour