Space competition in the acorn woodpecker: Power struggles in a cooperative breeder

During a 5-year study of the cooperatively living acorn woodpecker Melanerpes formicivorus, I observed 26 irruptions of agonistic behaviour involving large numbers of intruders invading a territory. Evidence from several sources suggests that these invasions are usually contests among potential immigrants for the opportunity to replace a missing group member (hence, ‘power struggles’). Thus, power struggles indicate intense competition among individuals to transfer out of their natal group. Such dispersal is not necessarily excluded by the hypothesis that individuals gain directly by living in cooperative groups through either increased foraging efficiency or better predator defence. However, birds involved in power struggles expend considerable energy and incur considerable risks. These costs are likely to be an acceptable part of dispersal only if cooperative living is a result of birds being forced to remain in groups as a result of resource localization.