Trapped in a chimney, Vaux’s Swifts reveal their secrets

This article is available in Spanish through a partnership with the Institute of Ecology at the National Autonomous University of Mexico. Read in Spanish >>

To us, lines on a map often contain relevant, useful information. Usually they mark political boundaries, places where responsibility and stewardship over a landscape and all the life within it changes hands from one government to another. To animals, those lines might as well be invisible.

Take the Kirtland’s Warbler, a tiny songbird that nearly went extinct a few decades ago. The warblers breed in Michigan but spend the winter basking in the warm sun shining down on the Bahamas, Turks, and Caicos. If the winter is too dry, then their return to Michigan is delayed. And when they’re delayed, the breeding season is shortened, meaning they can produce and fledge fewer offspring that year. Intensive management in the US has allowed the species to stave off extinction, but long-term recovery will probably require active engagement with the species in the Bahamas as well. If avian conservation efforts ignore wintertime habitats, then the best laid plans during the summer are but a Band-Aid on a larger problem. To the birds, they’re just doing what they’ve always done, but their continued survival will depend on help from two very different governments.

But it’s not always obvious where songbirds spend their winters. In part, that’s because these birds are so small and light that they usually can’t be equipped with GPS transmitters. Putting plastic rings on their legs can be helpful, but birds need to be captured twice to infer their movements. Chemical techniques allow birds to be captured but once. Researchers can extract a small bit of tissue and look for “stable isotopes of hydrogen,” which vary geographically. They’re like geographic fingerprints that tell researchers where that tissue was grown. That means a bird can be captured in British Columbia, and a chemical analysis of its tissues can reveal where in Mexico or Central America it spent the winter.

That’s exactly what Thompson Rivers University biologist Matthew W. Reudink set out to accomplish for Vaux’s Swifts. A small flocking species, the cylinder-shaped swifts breed throughout western North America and winter in the tropics of southern Mexico and Central America. As they travel along their migration pathway, they spend the night roosting in tree cavities or – more often – empty chimneys in urban areas, frequently in groups hundreds or even thousands.

While their summer and winter habitats are well known, what scientists do not yet know is whether groups of swifts leave a particular winter habitat together, migrate together, and arrive at their breeding sites together, or whether any particular breeding site is comprised of birds from a variety of wintering sites. It’s important information for conservation efforts. If there is strong migratory connectivity (if birds stick together throughout the year), then a single mass mortality event in one spot can disproportionately affect the species. By contrast, if the birds have weaker migratory connectivity, then they’re a bit more protected, at least from a genetic perspective, from random die-offs.

It was just that sort of random die-off that allowed Reudink and his colleagues to see whether the stable isotopes of hydrogen lurking in their tissues indicated that they’d come from a single wintering site, or from a variety of wintertime flocks.

On May 9, 2012, some 1,350 Vaux’s Swifts died from suffocation after they were trapped in a roost in Cumberland, British Columbia on Vancouver Island. In an instant, between 1.5 and 2.7% of the estimated Vaux’s Swift population simply died. The researchers randomly selected 98 individuals to analyze. After taking body measurements and identifying the sex of each bird, they snipped 2 millimeters from the birds’ central claws for isotopic analysis. Claws grow continuously, which means they can be used as a sort of time-lapse to estimate a bird’s recent travels. The claw tips allowed researchers information about the swifts’ movements over the last few weeks to months before death.

Rather than asking where these swifts came from, which is quite a tall order, the researchers instead asked whether their stable isotopes were similar enough to suggest that they came from the same place. They found evidence that the Vancouver Island swifts came together from two or possibly three overwintering grounds. The chemical makeup of their claws suggested that those in one group came from a hotter, drier wintering habitat than those in the second. Still, just two to three wintering sites isn’t quite enough to represent sufficient variation to buffer against the consequences of mass mortality events. (However, more research, particularly at their wintering sites, is needed to verify the findings. It’s also possible, for example, that the isotopic differences among the birds reflect dietary preferences rather than geographic origin, though those should also vary geographically.)

Vaux’s Swifts are in decline, like many other similar birds. Because of their odd roosting habits – selecting abandoned or unused chimneys – they are particularly susceptible to unpredictable, mass die-offs. Combined with habitat loss and climate change, it seems increasingly prudent to identify how they migrate from their wintering sites to their summertime breeding grounds in order to figure out how best to protect them throughout the year. – Jason G. Goldman | 30 October 2015

Source: Matthew W. Reudink et al. (2015). Patterns of migratory connectivity in Vaux’s Swifts at a northern migratory roost: A multi-isotope approach. The Condor: Ornithological Applications 117, 670-682. DOI: 10.1650/CONDOR-15-82.1.

Header image: Vaux’s Swifts outside a roost chimney, via J. Garner/used with permission.