Swallows at the End of the World

Tierra del Fuego, at the bottom of South America, is a rugged, windswept island where dense forests of southern beech cleave to the blue-black slopes of the Andes Mountains. Glaciers and snow glint year-round from the peaks, and the valleys below offer a seemingly endless labyrinth of bogs. The island, the casting-off point for Antarctica, has earned the local nickname “el fin del mundo”—the end of the world.

This southerly spot, known for bad weather, even in summer, is the final destination for flocks of migrating Chilean swallows that have traveled hundreds of miles there to breed. While adult birds return to their northern wintering grounds, their young often do not survive the summer, posing an evolutionary puzzle.

“Why is this bird coming all this way, when at times there’s a 95 percent die-off of their young?” wonders biologist Justin Proctor, who has studied swallows with research group Golondrinas de las Americas. The group is made up of biologists from multiple institutions across the Americas who hope to answer this and other questions by studying and comparing nine closely related swallow species at field sites across North and South America. (Note: The author participated in some of the research.) Their mission: build a comprehensive life history of each species, which could enable biologists to strategize about how to protect birds of conservation concern, including the IUCN red listed golden and Bahama swallows (vulnerable and endangered species, respectively). In the process, the researchers have been observing just how incredibly resilient Chilean swallows are.

Chilean swallows are the only members of the swallow family found in Tierra del Fuego. In appearance, they’re not very different from their northern cousin the tree swallow.They have the same tuneful burble and a similar profile: long wings, flat head, short neck. Like tree swallows, the Chilean species is covered in brilliant iridescent blue feathers, with a white belly and wings tipped in black. Both species nest in cavities, such as those bored in trees by woodpeckers. In flight, tree and Chilean swallows are all elegance; their paths loop extravagantly, arced wings cutting sharp, dizzy silhouettes through the air.

When Cornell ornithologist David Winkler, who founded the Golondrinas program, first encountered the Chilean swallows of Tierra del Fuego in 2004, he could not have anticipated how different they could be from other swallows in terms of their life history. Typically, birds that breed during the Southern Hemisphere’s summer—from November through February—pick warm or temperate locations where the days are long, the insects abundant, and woody nesting sites available. But summer in Tierra del Fuego, while sometimes sunny and serene, is punctuated by freak cold snaps. Average temperatures hover between 40 and 60 degrees Fahrenheit, but a dip below freezing with several inches of snow is not uncommon.

The weather has an indirect consequence on the Chilean swallows’ diet. Frequent high winds make food scarce for insectivorous swallows. While mosquitoes are typically a dietary staple, researchers observe remarkably few on the island. Meanwhile, the conditions take a serious toll on the birds’ young. For example, in a single breeding season, roughly 75 percent of chicks died in two snowy days, when the temperature dropped to 32 degrees Fahrenheit, according to research conducted by Golondrinas collaborator Marcela Liljesthröm, a biologist at Argentina’s Centro Austral de Investigaciones Cientificas.

The adult swallows, by contrast, fare far better than their young, returning year after year. “They seem to adapt well to the kind of environment in which they nest,” says Liljesthröm—a fact that is both confounding and reassuring to biologists. Video footage has revealed how adults huddle together for warmth on unusually cold nights, welcoming a third bird to the nest as one extra body.

To better study these mysterious birds, biologists in the Golondrinas collaboration construct nest boxes to house them. By comparing Chilean swallows with other swallow species, the biologists hope to better understand why, in the face of such enormous odds, these birds return each year. It may be that some years on the island are much better than others, making the risky location worthwhile. It’s even possible that these swallows have developed a different strategy from other species, living longer as a trade-off for the low survivorship of their young. Whatever the case, they’ve somehow managed to persist.

“These birds are right up against their limits,” says Cornell’s Winkler. “But that’s part of why we love swallows—they’re constantly trying to push the limits.”