Every fall, the northern black swift pulls a disappearing act. This gunmetal gray bird with a forked tail flies from its nesting sites near mountain waterfalls in western North America and vanishes for 7 to 8 months before returning to the same area. Now, thanks to tiny tracking tags, researchers have finally figured out where the northern black swift goes—a discovery that should help scientists better understand this elusive species.
Although researchers knew little about where the northern black swift (Cypseloides niger borealis) migrated, some observers had spotted them flying in Mexico and Guatemala. Others had even seen them as far away as Colombia, so scientists suspected that the birds wintered somewhere in South America, "but beyond that they had no idea," says Carolyn Gunn, an independent Colorado wildlife biologist and one of the authors of the new study. "We spent a lot of hours around the campfire trying to find out what was going on," adds co-author Kim Potter, a wildlife biologist with the U.S. Forest Service in Golden, Colorado.
Solving the mystery hasn't been easy. Because the swifts construct their nests near, and sometimes behind, waterfalls, they're hard to find and catch, Potter says. That didn't stop Gunn from trying. At one of the Colorado colonies the team studied, she climbed a ladder propped next to a waterfall to capture the birds. "We were lucky that one of the colonies we looked at nested in a cave," Gunn says. The researchers were able to string a fine net across the entrance to catch the swifts.
But once the team had a bird in hand, they ran into another problem. Traditional methods of attaching tracking tags, by looping them around a leg, wouldn't work on the swift's delicate, stubby limbs. They just slid right off, Gunn says. "So we developed a little backpack harness that fit around the wings and crossed under the breastbone, and it worked very well." All but one of the birds they tagged returned, and the feathers in contact with the straps showed very little wear.
The team put so-called geolocation tags on four adult northern black swifts from two breeding colonies in Colorado. The tags are different from Global Positioning System (GPS) tags in that they record light levels, enabling researchers to estimate the length of each day. That data, combined with the sun's elevation angle, give rough locations of a tagged animal.
One hitch is that scientists have to recover these tags so they can download the data, unlike satellite tags that relay their information in real time. Geolocation tags, which weigh about 1.5 grams, also aren't as accurate as satellite tags. But they are smaller and lighter, which enables their use on small birds. The lightest GPS tags, which weigh about 4 grams, are still too heavy for birds like the black swift, says Peter Marra, a conservation biologist with the Smithsonian Conservation Biology Institute in Washington, D.C, who was not involved with the work.
The three birds that researchers recovered tags from arrived in Brazil between September and October 2009 and started their northward migration in May 2010, the team reports this month in The Wilson Journal of Ornithology. Because the tags recorded each bird's location within 185 kilometers, the birds most likely also spent time in Bolivia, Colombia, Peru, and Venezuela.
And when researchers overlaid vegetation maps on to the birds' locations, they were in for a surprise. "We thought they were going to be in mountainous areas with waterfalls," similar to their breeding habitats, Potter says. But they were in lowland rainforests, which Gunn speculates provided them with good areas to hunt for insects.
Although scientists shouldn't extrapolate about this species' migratory patterns from such a limited sample size, this is an important first step, says Stuart Bearhop, an animal ecologist at the University of Exeter in the United Kingdom. Past studies relied on chance resightings of birds with unique identifiers on them, he notes. But tracking tags allow researchers to follow an individual of known origin, giving them a better handle on its movements, he says.
Potter says that the team plans to extend the study to northern black swifts throughout their western North American breeding range. "If the entire North American population goes to the same small spot [in Brazil], anything that disturbs the climate or habitat in that area will affect the entire North American population."