"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, The Birds of California (1).
The most widely distributed and abundant swallow in the world, the Barn Swallow breeds throughout most of North America and Eurasia, and overwinters 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 nesting under the eaves of buildings or on other artificial structures such as bridges and culverts. In North America, this shift in nesting sites began before European settlement and was virtually complete by the mid 1900s; nowadays, the Barn Swallow is rarely observed nesting on natural substrates. As with other swallows that have shifted to nesting on human-made structures, such as Common House-Martin (Delichon urbicum), Purple Martin (Progne subis) and Cliff Swallow (Petrochelidon pyrrhonota), the Barn Swallow now frequently nests in colonies that are larger in size than probably occurred in natural settings.
The Barn Swallow has been associated with humans and their structures for more than 2,000 years in Europe (2), and this close relationship means they are well known to the public. In many parts of the world, having Barn Swallows nesting on one's property is considered a sign of good fortune or a harbinger of spring. Legend has it that Barn Swallows consoled Christ on the cross by trying to remove the crown of thorns from his head and attempting to divert those coming to arrest him in the Garden of Gethsemane. Stories are told that Barn Swallows got their forked tails because they stole fire from the gods to give to the people, and their central tail feathers were shortened when they were burnt off by a firebrand hurled by a wrathful deity (3, 4). Other legends have it that cows will give bloody milk or go dry if anything inopportune happens to the Barn Swallows nesting on a farm; lightning may even strike a house if a Barn Swallow nest is removed.
As a consequence of their world-wide distribution and habit of nesting on accessible artificial structures near people, Barn Swallows have been studied extensively and are one of the most well-known species and subspecies groups of birds in the world. Barn Swallows have figured prominently in studies of the costs and benefits of group living (5, 6, 7), and 4 of the 6 subspecies have served as a model organisms for studies on the mechanisms of sexual selection (see 8, 9, and 10 for summaries). For Barn Swallows in Europe (H. r. rustica), tail length and degree of asymmetry in the outer tail feathers have been found to be predictors of individual quality in males and females and those traits are used when selecting mates. Tail length tends to correlate with reproductive success, annual survival, longevity, testosterone levels, propensity to engage in extra-pair copulation, parental effort, ability to withstand parasites, immunocompetence, and other measures of fitness (2). Longer tail length in H. r. transitiva in Israel is related to earlier breeding and greater confidence in paternity (11, 12). These patterns with tail length are less apparent in Barn Swallows in North America (H. r. erythrogaster), suggesting that the length of the tail is subject to weaker sexual selection and greater natural selection in this subspecies (13, 14, 9), and that ventral coloration is a more important character for sexual selection (9). The red coloration of the throat and size of the white tail spots are indicative of individual quality and are used by Barn Swallows in Asia (H. r. gutturalis) when selecting mates (15, 16, 17). Sexual selection may also play a role in acoustic divergence in the species (18).
Several species very similar to the Barn Swallow are found in sub-Saharan Africa, Malaysia, and Australia. The relationships among these species, and among the various subspecies of Barn Swallow, are unclear. Subspecies in Asia (H. r. gutturalis) and North America (H. r. erythrogaster) seem to differ as much in morphology and behavior from the nominate race of Europe and western Asia as some closely related species of Hirundo from Africa. The level of differentiation found between Eurasian and North American populations suggests that more than one species may exist within the Barn Swallow complex as currently classified (19, 20, 21).
Within the Americas, the Barn Swallow is likely the only northern temperate breeder that commonly overwinters in South America and occasionally breeds there during the boreal winter; the species has nested in small numbers in Buenos Aires Province in northern Argentina since 1980 (22). The Barn Swallow was the first bird species, other than those species used in falconry, to receive legal protection. In 1496 in Milan, a decree was released that prohibited interference with nesting birds (23). A European Barn Swallow, banded with a copper ring by an imprisoned French nobleman during the French Revolution, is the first passerine known to have been banded; the bird returned to the same nesting area for 3 years (24). Finally, the Barn Swallow—not herons and egrets, as is widely believed—has the distinction of indirectly leading 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 in North America (G. Gladden in Pearson ).