One hardly knows what quality to admire most in . . . the Barn Swallow. All the dear associations of life at the old farm come thronging up at sight of him. You think of him somehow as part of the sacred past; yet here he is today as young and as fresh as ever, bubbling over with springtime laughter.
William L. Dawson, Dawson 1923 , The Birds of California
The Barn Swallow is the most widely distributed and abundant swallow in the world. It breeds throughout most of North America, Europe, and Asia and winters in Central and South America, southern Spain, Morocco, Egypt, sub-Saharan Africa, the Middle East, India, Indochina, Malaysia, and Australia. Originally nesting primarily in caves, the Barn Swallow has almost completely converted to breeding under the eaves of buildings or inside artificial structures such as bridges and culverts. In North America, this shift in nest sites began before European settlement and was virtually complete by the mid-twentieth century; nowadays natural nestings are rarely seen and usually reported in print if they occur. As with other swallows that have shifted to nesting on human-made structures, such as the Purple Martin (Progne subis), Barn Swallows now sometimes nest in larger colonies than probably occurred in natural settings.
The Barn Swallow's close association with human habitations means that it is well known to the public, and in some parts of the world having Barn Swallows nest on one's property is considered a harbinger of good luck. Legend has it that the Barn Swallow consoled Christ on the cross and got its forked tail because it stole fire from the gods to bring to people, losing its middle tail feathers when a wrathful deity hurled a firebrand at it ( Turner 1991 ). Another superstition is that cows will give bloody milk or go dry if anything happens to the Barn Swallows nesting on a farm. Barn Swallows have been closely associated with humans and their structures for more than 2,000 years in Europe ( Møller 1994 ).
As a consequence of both its wide distribution and its nesting on accessible artificial structures near people, the Barn Swallow has been studied extensively throughout the world and especially in Europe. More papers have been published on this species than on any other swallow, and it is one of the most thoroughly studied birds in the world. The Barn Swallow has figured prominently in studies on the costs and benefits of group living ( Snapp 1976 , Møller 1987a , Shields and Crook 1987 ), and it has served as a model organism for detailed studies on the mechanisms of sexual selection ( Møller 1994 ). Tail length and degree of asymmetry in the outer tail-streamers have been found to be reliable predictors of individual quality in both males and females, and individuals use these characteristics to select mates. Tail length tends to correlate with reproductive success, annual survival, propensity to engage in extra-pair copulation, parental effort, ability to withstand parasites, immunocompetence, and other measures of fitness. However, most of the research on sexual selection has been done on European populations, and relatively few similar studies have been done on the North American Barn Swallow.
Several species very similar to the Barn Swallow are found in sub-Saharan Africa, Malaysia, and Australia. The relationship among these forms and even the various subspecies within the Barn Swallow is unclear. The North American subspecies of Barn Swallow, H. r. erythrogaster, differs as much in morphology and behavior from the nominate race of Europe and western Asia as some closely related species of Hirundo from Africa (A. P. Møller pers. comm.). While a limited comparison of genetic variation was inconclusive, the level of differentiation found between Eurasian and North American populations suggests that more than one species may exist within the Barn Swallow as currently classified ( Zink et al. 1995 ).
The Barn Swallow has the distinction of being perhaps the only northern temperate breeder that commonly winters in South America and occasionally also breeds there during the boreal winter; Barn Swallows have been reported nesting in small numbers in northern Argentina. In addition, these swallows—not the more famous egrets—have the distinction of having indirectly led to the founding of the conservation movement in the United States: the destruction of Barn Swallows for the millinery trade apparently prompted George Bird Grinnell's 1886 editorial in Forest and Stream that led to the founding of the first Audubon Society (G. Gladden in Pearson 1923 ).