Barn Swallow may be the most widespread and well-known passerine, with highly migratory populations that span the Americas as well as Europe, Asia, and Africa. Related species, and some Barn Swallow subspecies, are resident in Africa, the Middle East, and Australia. This remarkable variability makes Barn Swallow a great case study in migratory behavior. The varied migratory behaviors in this species globally are also seen on a smaller scale in the Americas, as this new STEM map helps illustrate.
Barn Swallow breeds in pairs or small colonies widely across the United States and southern Canada—absent or scarce only in the driest desert, highest mountains, and colder Arctic regions. It mostly requires human structures for nesting so does not extend too far into Canada as settled areas become more rare, and while it breeds in central Mexico, it does not breed in the hotter and more humid coastal regions, as well as Central America and the Caribbean. Like almost all aerial insectivores (birds that eat flying insects), it has shown marked population declines in recent years, possibly related to widespread pesticide use among other potential causes.
Like several other swallows that move to South America, fall movements already are evident by early July, with many birds spreading out in dispersive movements to prospect future breeding sites. Others are already migrating south in July, and by 25 July the animation shows their arrival in Central America, with northern South America lighting up just after. Barn Swallows continue to flow southward until late October, with many birds staging before migration in large communal roosts in wetlands and agricultural areas. These roosts tend to occur in tall grass and are located in areas with abundant flying insects that allow these birds to fatten up for a long migration. Watching at dusk as a vortex of thousands of swallows descend into tall grass for their night roost is one of the joys of birding in late summer and fall.
Barn Swallows molt in winter, after their migration, unlike certain other swallows (e.g., Tree Swallow) that have much shorter migration distances. During the winter they may occur almost anywhere south of the U.S. border as far south as northern Chile and central Argentina (Buenos Aires province). Just a few highly localized areas in the southern U.S. get wintering Barn Swallows, such as the Salton Sea area of California, which is highlighted on the winter maps (e.g., 26 December) in the animation. Wintering Barn Swallows mostly occur in large flocks, centered again on areas with abundant flying insect life such as marshes, river edges, and moist agricultural regions. Large flocks spread out across the landscape by day and gather in large roosting flocks at night, all the while replacing their body feathers and flight feathers so they will have a fresh set of feathers for their spring migration and upcoming breeding season.
Northward migration is underway early, by late January, with the first hints of arrival in southern California and Texas appearing on the 8 February map. By early March, this vanguard may already be beginning to nest in Texas and California, even as a flood of migrants from farther south begins to arrive. From March to May, Barn Swallows push into the country on a broad from from the West Coast to the Eastern Seaboard, with an especially strong push through the Midwest and Great Plains where the species can be especially common. It is not until mid to late May that the most northern breeding areas are filled in.
This map is awash with color, which obscures a lot of migratory complexity, although it may be possible to get some hints of the movements of some subpopulations. Watch to see if you can see the following, which has been determined from stable isotope analysis by Hobson et al. (2015): West Coast birds moving to-and-from Mexico; Prairie Provinces birds moving to-and-from Central America; eastern birds moving to-and-from eastern and southern South America. Within the east, southeastern birds make a comparatively short jump to northern South America while it is the more northerly birds that are the champion migrants, making a longer jump that “leapfrogs” over northern South America, crosses the equator, and winters mostly south of the Amazon even as far as Argentina. These birds likely employ an over-water route in fall and a much more westerly spring route through South America and the Yucatan, jumping across the Gulf of Mexico from there. This looped migration is common in many species (e.g., Veery, Blackpoll Warbler, Hudsonian Godwit) and takes advantage of favorable tailwinds from an ocean crossing from North America to South America, but takes care to migrate over land on the return voyage since westerlies would be a headwind that would blow them far into the Atlantic if they attempted the same route in spring.
A final interesting wrinkle in Barn Swallows is playing out in Argentina. Watch for the strong signal for the species in the Buenos Aires area from November through January. While many Barn Swallows overwinter here, much of the strength of that signal applies to breeding birds. Since the early 1980s, a few Barn Swallows have successful bred in this area of Argentina and in the past decade the population has exploded, with many thousands now breeding across the entirety of Buenos Aires province and expanding into surrounding areas, where fairly recently-built bridges and culverts seem to be providing ideal breeding areas. It was only this year that we conclusively learned (through the use of light-level geolocators; Winkler et al. 2017) the “wintering” grounds of these birds, which apparently head to northeastern South America for the austral winter. Interestingly, Barn Swallows on the other side of the Atlantic have also bred in South Africa, giving some insight as to how new migratory strategies develop.
To learn more about the life history of Barn Swallow, please consider a subscription to Birds of North America (just $42/year), where experts in the species have written a full-length species account covering all aspects of the species biology, from migration to diet to conservation status.