Many animal species live and breed in colonies. Although biologists have documented numerous costs (increased competition for limited resources) and benefits (more pairs of eyes to watch for predators) of group living, they often still do not agree on why coloniality evolved in the first place. Drawing on their 12-year study of a population of cliff swallows in Nebraska, USA, the authors investigate 26 social and ecological costs and benefits of coloniality. They explore how these costs and benefits are reflected in reproductive success and survivorship, and speculate on the evolution of cliff swallow coloniality. This study of vertebrate coloniality should be of interest to all who study social animals, including behavioural ecologists, population biologists, ornithologists and parasitologists. Its focus on the evolution of coloniality should also appeal to evolutionary biologists and to psychologists studying decision making in animals. The authors' research on swallows was the subject of an award-winning exhibition at the Peabody Museum of Natural History in New Haven, Connecticut, and was included in the BBC television production, "The Trials of Life".
