The mythical arrival of cliff swallows (Hirundo pyrrhonota) at the Mission San Juan Capistrano in California every March 19 is fabled in song and story. Not as well known is that this cousin of the less social barn swallow feeds, builds nests, mates, preens, bathes, and conducts an annual migration between North and South America, traveling as far south as Argentina, in coordinated groups that may often be of truly immense size.

In fact, some cliff swallow colonies number more than 3500 nests. These bird cities are the scene of what Charles Darwin called “the dreadful, but quiet, war of organic beings.” For the last 15 years, former Yale Associate Professor Charles R. Brown and Mary Bomberger Brown have been studying hundreds of swallow colonies in Keith County in southwestern Nebraska, along the Platte River near Ogallala. The Browns have learned that cliff swallows pay a high price for sociality. Competition for nests can be fierce and fights are common. The birds routinely interfere with each other's nests, invading them to steal, to destroy, lay, or transfer eggs, or to force copulation with neighboring females. The most serious cost, severe parasitic insect infestation that increases with colony size and causes the young swallows to die, is an inescapable consequence of coloniality.

What induces cliff swallows to put up with all this? In a word, food. Breeding swallows require enormous numbers of flying insects; the larger the colony the more up-to-the-minute news it gets as to the whereabouts of insect swarms. This information is the chief trade-off for the pitfalls of living together, but it's an important one: it results in better nourishment for swallow young.

While ectoparasites constitute the greatest source of harm to cliff swallows, predators also represent a material threat. One bullsnake (Pituophis melanoleucus), like the one depicted at left raiding and isolated nest, was known to eat 100 eggs in three days in a single colony. The snakes can readily reach nests even if these are protected by overhangs — and culverts and bridges present no obstacle either.