A preference for big-city living or smalltown life may depend in part on inheritance, at least for cliff swallows. They don't rule out effects of very early development but suggest inheritance as a strong possible explanation. After 19 years of studying cliff swallows near Ogallala, Neb., Charles Brown recognizes the tradeoffs of living in colonies ranging from 2 to 3,700 nests.
Cliff swallows feed on swarms of flying insects, and the birds routinely follow successful foragers.
Also, nesting in dense arrays can cut nest-construction time, Charles Brown observes. The birds transport mud, pellet by pellet, to build up the walls of a hollow nest into a bulge about the size of a cantaloupe. Starting from scratch takes about 2 weeks. In bigger colonies, birds often share walls with neighbors. With luck, a bird pair can build the equivalent of a townhouse in several days.