Doubtless the Lord-to paraphrase Lincoln's aphorism-must love the Cliff Swallows, else he would not have made so many of them.
William C. Dawson ( Dawson 1923 )
The Cliff Swallow is one of the most social landbirds of North America. These birds typically nest in large colonies, and a single site may contain up to 3,500 active nests. Cliff Swallows originally were birds of the western mountains, where they still nest commonly underneath horizontal rock ledges on the sides of steep canyons in the foothills and lower elevations of the Sierra Nevada and Rocky and Cascade mountains. In the past 100 to 150 years, these birds have expanded their range across the Great Plains and into eastern North America, a range expansion coincident with the widespread construction of highway culverts, bridges, and buildings that provide abundant alternative nesting sites. New colonies continue to appear each year in areas where Cliff Swallows were previously unrecorded as breeders.
The Cliff Swallow was one of the first North American birds to be described. Although its discovery in Colorado is usually credited to Thomas Say on Stephen Long's expedition to the Rocky Mountains in 1820 ( James 1823 ), the bird and its colonial breeding habits were first mentioned by the Spaniard Silvestre Velez de Escalante in September 1776 when he encountered large numbers in the Wasatch Range of Utah ( Coues 1899 ). Formerly placed in the genus Hirundo, the Cliff Swallow bears some ecological similarity to the more familiar and now congeneric Barn Swallow (Hirundo rustica). Both species build mud nests on vertical substrates, but Cliff Swallows are distinguished by their enclosed, gourd-shaped nests and much larger colonies. Their highly colonial life style has led to the evolution of some complex behavioral traits. For instance, Cliff Swallows brood-parasitize neighboring nests both by laying parasitic eggs and by moving eggs from their own nest into others; have a sophisticated vocal system for distinguishing their own young from the offspring of many other individuals within a colony; and observe each other's foraging success and learn from other colony residents the locations of food. The Cliff Swallow's social behavior during the breeding season has been studied extensively, and this species has figured prominently in our understanding of the evolution of coloniality in birds ( Brown and Brown 1996 ). However, we know little about its migration, and surprisingly, no one has studied this bird on its poorly known wintering range.