Territory Quality and Dispersal Options in the Acorn Woodpecker, and a Challenge to the Habitat-Saturation Model of Cooperative Breeding

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Abstract
The central feature of cooperative breeding systems is the presence of one or more nonbreeding birds in a social group that devote time and energy to help raise the offspring of other group members. The most widely accepted model for the evolution of this behavior is that it arises when there is some ecological constraint to independent breeding by young birds that otherwise would disperse from their natal groups. Among territorial species, this constrain is usually hypothesized to be saturation of suitable habitat by sedentary established groups. We use data collected over a 10-yr period to test this model for a population of acorn woodpeckers (Melanerpes formicivorus) in central New Mexico [USA]. Acorn woodpeckers store large quantities of mast, and territories vary greatly in the amount of storage facilities present. Storage facilities significantly affect the survival and reproductive success of groups occupying those territories. Most helpers occur in high-quality territories; low-quality territories are frequently unoccupied and thus available for colonization. We estimate lifetime direct and indirect offspring production for yearlings that help in territories of differing quality and for varying numbers of years. Birds that forgo individual reproduction for one or more years in high-quality territories but eventually breed there can have greater lifetime fitness than those that disperse at 1 yr of age and breed immediately in low-quality territories. Had we combined data from all territories and all groups, these findings would have been obscured; rather, the resulting calculations would have erroneously indicated that, for yearlings, dispersing and breeding is always the preferred option. Our analyses indicate that habitat saturation or some other constraint on the opportunity to breed successfully is not required to explain the existence of cooperative breeding in this population of woodpeckers. Instead, we suggest that competition for space may be the outcome of factors that promote philopatry, such as access to a critical resource and/or mutualistic benefits of living in a group and eventually breeding in the natal territory. If this scenario is correct, habitat limitation is a result of other factors favoring cooperative breeding rather than the basis for this kind of social system.