"Doubtless the Lord—to paraphrase Lincoln's aphorism—must love the Cliff Swallows, else he would not have made so many of them" —William L. Dawson, The Birds of California ( Dawson 1923 : 524).
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 6,000 active nests. Cliff Swallows originally were birds of the western mountains, where they still nest 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 ). This species is known to the public for its mythical return to Mission San Juan Capistrano on 19 March each year ( Bruton 1975 ), with this legend serving as a metaphor for the regular occurrence of events.
The Cliff Swallow’s 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 ( Brown 1984a , Brown and Brown 1988a ); they have a sophisticated vocal system for distinguishing their own young from the offspring of many other individuals within a colony ( Beecher et al. 1985 , Medvin et al. 1992 , Medvin et al. 1993 ); and they observe each other’s foraging success and learn from other colony residents the locations of food ( Brown 1986a ). 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 , Brown and Brown 2000a , Brown et al. 2016 ). Cliff Swallows have served as a model organism for studying rapid evolution in response to natural and anthropogenic environmental change ( Brown and Brown 1998b , Brown and Brown 2011 , Brown and Brown 2013 , Brown and Brown 2014 ) and the dynamics of fluctuating selection in the wild ( Brown et al. 2013c , Brown et al. 2016 ). In addition, the species is closely associated with an endemic vector-borne virus that has led to insights into how changes in hosts drive the evolution of different pathogen strains ( Brown et al. 2009c , Brown et al. 2012 , O'Brien et al. 2011 ).