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British birders simply call it THE swallow. They may have a point. Among nearly 90 species in its family, the Barn Swallow is by far the most widespread and abundant. It nests across North America, Europe, Asia, and North Africa. And in the winter, it heads south, taking up residence throughout South America, Africa, and southern Asia, all the way to northern Australia. Vagrant Barn Swallows have even reached the Antarctic. This is one of the few truly cosmopolitan birds.

Many swallows will nest on human-made structures. This is especially true of Barn Swallows, and their name reflects it. They build nests of mud and grass, plastered against vertical surfaces in sheltered places. Such spots are readily found in barns and other buildings, as the swallows discovered long ago. Peter Kalm, a Swedish naturalist who , wrote: “They build their nests in houses, and under the roofs on the outside; I likewise found their nests built on mountains and rocks whose top projected beyond the bottom . . . and this shows where the Swallows made their nests, before the Europeans settled and built houses here.” In truth, it’s likely that some Barn Swallows nested on structures built by Native Americans before the first Europeans arrived, but regardless, it's clear the bird quickly adapted to man-made structures. 

When Peter Kalm visited this continent 270 years ago it was still easy to find Barn Swallow nests in natural sites, but that’s no longer the case. In my travels, I always look for their nests, and I’ve found them on a wide variety of manmade structures: in barns, sheds, and abandoned houses, on porches, under bridges, and in culverts. The northernmost breeders I’ve seen were north of Juneau, Alaska, where they built nests under docks along the coast. In a magazine article a few years ago, I announced that I’d never seen a Barn Swallow nest in a natural site. Ironically, about the time the article was published, I saw a few pairs nesting under overhanging cliff ledges on an island in Lake Erie. Never say never!

Regardless of such holdouts, it’s obvious that artificial nesting sites are vastly more widespread across the landscape than natural sites ever could be. As a result, Barn Swallows are undoubtedly far more numerous now than they were before humans provided them with millions of unintended nesting structures.

It’s hard to appreciate just how numerous Barn Swallows are in summer, when they’re spread out among these omnipresent sites. They become more obvious when they pour southward for the winter—mostly abandoning the North Temperate Zone for the tropics and the southern continents. During the northern winter months, I’ve seen flocks of thousands over marshes in Thailand, over lakes in the Rift Valley of Kenya, and over flooded grasslands in Venezuela. I’ve seen them twittering around coastal towns in Borneo and swooping over herds of zebras in South Africa. Their vast winter range is almost entirely south of their vast breeding range, with very little overlap, so if you’re south of a certain point, you know you’re looking at non-breeding birds.

Or at least, that used to be the case.

One February in the 1980s, I was in Buenos Aires, Argentina, far south of the equator in South America, and saw a couple of Barn Swallows—not surprising, since this is well within their normal wintering range. In conversation with a local birder, I mentioned that I had seen them. “Oh,” he said, “they are nesting here now, you know.”

My first reaction was to think: No they’re not! The closest nesting Barn Swallows were in south-central Mexico, more than 4,500 miles from Buenos Aires. Barn Swallows didn’t breed anywhere in the southern hemisphere. Impossible, I thought, and I shrugged it off.

But as I learned later, the man was right. Near the coastal town of Mar Chiquita, south of Buenos Aires, six pairs of Barn Swallows had been found nesting under bridges in January 1980. Local biologists assumed it must be a fluke. But five pairs were found the next winter, and 11 pairs the winter after that.

It was no fluke. The birds continued nesting in these areas and began to spread out, throughout the 1980s and 1990s, establishing more and more nesting sites. By 2015 they had expanded their range to scattered sites hundreds of miles west and south of the original location. The Argentina breeding population now undoubtedly numbers in the thousands.

This is an astonishing development. Birds just don’t do that—they don’t suddenly start building nests and raising young in their wintering range. We have hundreds of migratory bird species in North America that migrate to the tropics or the South Temperate Zone for the winter, and we’ve never witnessed another case in which one of these migrants abruptly established a whole new breeding population on the wintering grounds. (Another swallow, the European House Martin, has been known to attempt nesting at wintering sites in southern Africa, but it hasn’t managed to gain a foothold there.)

These pioneering Barn Swallows have pulled off a remarkable leap in habits as well as geography, starting with their internal calendars. Barn Swallows in North America breed mostly from May through August, but that wouldn’t work in southern South America, where those are winter months. The Argentinian swallows nest from about November to February, during the southern summer. And then they migrate—going north for the winter, not south. have confirmed that these birds head to northern South America from about April to October, returning south in the southern hemisphere's spring. Other aspects of their lives have gone through similar shifts, including the time of year that they molt their feathers.

How did this happen? How did multiple wintering birds come into breeding condition at the wrong time of year, and then get together and start building nests and laying eggs in the wrong hemisphere? We’ll probably never know. But we do know that once it happened, they found ready-made nesting sites provided by humans. The Barn Swallows in Argentina are nesting under bridges and in culverts, in regions where they never would have found any kind of natural nest sites.

So thanks to its adaptability, the Barn Swallow now nests on five continents, not four, as its conquest of the world continues. Maybe we should take a cue from the British and start calling it THE swallow as well.  

(Kenn Kaufman's Notebook is a regular column featuring original artwork and essays by Kaufman, a field editor for Լ����Ƶ, and a world-renowned bird expert, author, and environmentalist.)