Demography and the Evolution of Cooperative Breeding in the Bicolored Wren, Campylorhynchus Griseus

1. Two adjacent populations of the bicolored wren differ significantly in social structure. In one population 83% of groups consist of unaided pairs. The rest contain, in addition, a single male nonreproductive helper. About 43 % of the groups in the other population consist of unaided pairs. The rest contain from one to three helpers, and helpers may be of either sex. 2. Helpers are always individuals who have failed to disperse from their natal territory. In both populations they augment group reproductive success and reduce mortality among the breeding pair, although a single helper produces a reproductive enhancement as great as that of multiple helpers. 3. Group-size-specific reproduction and adult mortality do not differ between the populations, but juvenile mortality and average territory size differ significantly. 4. The population consequences of these differences suggest that group size is held below its optimum in one habitat by high juvenile mortality, whereas habitat saturation is probably responsible for the fact that about one-third of all groups in the second habitat contain more than an optimum number of members. 5. These demographic parameters suggest that habitat saturation is not a necessary condition for the maintenance of helping behavior in either population.