Genes guide cliff swallows to select the size of colony in which they live. Charles and Mary Brown, researchers at the University of Tulsa, USA, who studied the swallows, may have scored one for heredity -- at least when it comes to cliff swallows. They say this is the first report of heredity influencing an animal's choice of a social system. "We clearly found that individuals have a genetic basis as to where they choose to live," says Charles Brown. Charles and Mary selected nearly 2,000 hatchlings to study. They placed identification tags, moved those hatched in nests in big colonies to nests in small colonies and vice versa. Mature swallows raised the foster hatchlings along with the other chicks. The two biologists discovered that when these birds returned, about nine months later, to settle into their own nests and start their families, they chose the same colony size as that in which they were born. The study suggests that there is a genetic difference between birds that choose to live in large groups versus those who choose to live in small colonies.