Variable helper effects, ecological conditions, and the evolution of cooperative breeding in the acorn woodpecker

The ecological conditions leading to delayed dispersal and helping behavior are generally thought to follow one of two contrasting scenarios: that conditions are stable and predictable resulting in young being ecologically forced to remain as helpers (extrinsic constraints and the \"habitat saturation\" hypothesis), or that conditions are highly variable and unpredictable leading to the need for helpers to raise young, at least when conditions are poor (intrinsic constraints and the \"hard life\" hypothesis). We investigated how variability in ecological conditions influences the degree to which helpers augment breeder fitness in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus), a species in which the acorn crop, territory quality, and prior breeding experience all vary in ways that have important effects on fitness. We found that the relationship between ecological conditions and the probability that birds would remain as helpers was variable, but that helpers generall...