"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, W. L. (1923). The birds of California: A complete, scientific and popular account of the 580 species and subspecies of birds found in the state. Vol. 2. South Moulton, San Diego, CA. Close 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, E. (1823). Account of an Expedition from Pittsburgh to the Rocky Mountains, performed in the years 1819 and 1820, under the command of Major Steven H. Long. Philadelphia: H.C. Carey and I. Lea. Close 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, E. (1899). Very early record of the Cliff Swallow. Auk no. 16:359. Close Coues 1899). This species is known to the public for its mythical return to Mission San Juan Capistrano on 19 March each year ( Bruton, L. (1975). The swallows of San Juan Capistrano. Orange, CA: Paragon Agency. Close 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, C. R. (1984a). Laying eggs in a neighbor's nest: benefit and cost of colonial nesting in swallows. Science no. 224:518-519. Close Brown 1984a, Brown, C. R. and M. B. Brown. (1988a). A new form of reproductive parasitism in cliff swallows. Nature no. 331:66-68. Close 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, M. D., P. K. Stoddard and P. Loesche. (1985). Recognition of parents' voices by young Cliff Swallows. Auk no. 102:600-605. Close Beecher et al. 1985, Medvin, M. B., P. K. Stoddard and M. D. Beecher. (1992). Signals for parent-offspring recognition: strong sib-sib call similarity in Cliff Swallows but not Barn Swallows. Ethology no. 90:17-28. Close Medvin et al. 1992, Medvin, M. B., P. K. Stoddard and M. D. Beecher. (1993). Signals for parent-offspring recognition: a comparative analysis of the begging calls of Cliff Swallows and Barn Swallows. Anim. Behav. no. 45:841-850. Close Medvin et al. 1993); and they observe each other’s foraging success and learn from other colony residents the locations of food ( Brown, C. R. (1986a). Cliff Swallow colonies as information centers. Science no. 234:83-85. Close 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, C. R. and M. B. Brown. (1996). Coloniality in the Cliff Swallow: the effect of group size on social behavior. Chicago: Univ. of Chicago Press. Close Brown and Brown 1996, Brown, C. R. and M. B. Brown. (2000a). Heritable basis for choice of group size in a colonial bird. Proceedings of the National Academy of Sciences of the United States of America no. 97 (26):14825-14830. Close Brown and Brown 2000a, Brown, C. R., M. B. Brown, E. A. Roche, V. A. O'Brien and C. E. Page. (2016). Fluctuating survival selection explains variation in avian group size. Proceedings of the National Academy of Sciences of the United States of America no. 113 (18):5113-5118. Close 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, C. R. and M. B. Brown. (1998b). Intense natural selection on body size and wing and tail asymmetry in Cliff Swallows during severe weather. Evolution no. 52 (5):1461-1475. Close Brown and Brown 1998b, Brown, M. B. and C. R. Brown. (2011). Intense natural selection on morphology of Cliff Swallows (Petrochelidon pyrrhonota) a decade later: Did the population move between adaptive peaks? Auk no. 128 (1):69-77. Close Brown and Brown 2011, Brown, C. R. and M. B. Brown. (2013). Where has all the road kill gone? Current Biology no. 23 (6):R233-R234. Close Brown and Brown 2013, Brown, C. R. and M. B. Brown. (2014). Breeding time in a migratory songbird is predicted by drought severity and group size. Ecology no. 95 (10):2736-2744. Close Brown and Brown 2014) and the dynamics of fluctuating selection in the wild ( Brown, C. R., M. B. Brown and E. A. Roche. (2013c). Fluctuating viability selection on morphology of Cliff Swallows is driven by climate. Journal of Evolutionary Biology no. 26 (5):1129-1142. Close Brown et al. 2013c, Brown, C. R., M. B. Brown, E. A. Roche, V. A. O'Brien and C. E. Page. (2016). Fluctuating survival selection explains variation in avian group size. Proceedings of the National Academy of Sciences of the United States of America no. 113 (18):5113-5118. Close 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, C. R., A. Padhi, A. T. Moore, M. B. Brown, J. E. Foster, M. Pfeffer, V. A. O'Brien and N. Komar. (2009c). Ecological divergence of two sympatric lineages of Buggy Creek virus, an arbovirus associated with birds. Ecology no. 90 (11):3168-3179. Close Brown et al. 2009c, Brown, C. R., A. T. Moore and V. A. O'Brien. (2012). Prevalence of Buggy Creek virus (Togaviridae: Alphavirus) in insect vectors increases over time in the presence of an invasive avian host. Vector-Borne and Zoonotic Diseases no. 12 (1):34-41. Close Brown et al. 2012, O'Brien, V. A., A. T. Moore, G. R. Young, N. Komar, W. K. Reisen and C. R. Brown. (2011). An enzootic vector-borne virus is amplified at epizootic levels by an invasive avian host. Proceedings of the Royal Society B-Biological Sciences no. 278 (1703):239-246. Close O'Brien et al. 2011).