Group breeding is perplexing, because individuals incur density-dependent fitness costs to breed in groups, yet no universal benefit appears to explain its evolution. Coloniality in birds, defined as dense nesting within limited territories, is a striking example of group breeding. Adaptive explanations for coloniality are diverse, and several research approaches have emerged. Here, we test models for the evolution of coloniality with a population of Red-necked Grebes (Podiceps grisegena). We distinguish habitat models from conspecific models for explaining the adaptive evolution of coloniality. Habitat models propose that the fitness benefits of colonial breeding are gained through the characteristics or location of the colony site, whereas conspecific models posit that the benefits of colonial breeding are gained solely from nesting near conspecifics. Red-necked Grebes are waterbirds that typically nest in a dispersed fashion, only rarely nesting in colonies. Both dispersed and colonial nesting co-occurred in the study population, and aquatic nests were found in three distinct habitat types. We gathered detailed demographic data and used analyses of covariance to test whether variation in habitat type or conspecific aggregation best explained differences in breeding success across the population. We further used fitness, genetic, and spatial data to test whether kin selection, sexual selection, or conspecific reproductive-success cueing may drive colony formation. The data provided strong support for a habitat-based model and inconsistently supported conspecific models. Our results suggest that colonial individuals of this population gain selfish benefits from relatively rare, high-quality habitat rather than from proximity of conspecifics.

Evolución de la Colonialidad en las Aves: Una Evaluación de las Hipótesis en Podiceps grisegena