Ornitho-logic

What's So Great About Seeing the Same Bird Twice?

Scientists get extremely excited when they catch a banded bird thousands of miles from where it was first tagged.

In 1803 John James Audubon became the first recorded bird bander in North America when he tied pieces of thread around the legs of Eastern Phoebe nestlings to find out whether they would return to his Pennsylvania estate the following spring. His experiment worked—two of the birds came back. A century later Paul Bartsch of the Smithsonian Institution ran the earliest systematic bird banding study in North America, on Black-crowned Night-Herons in Washington, D.C. Scientists have been banding birds ever since, and despite its low tech-ness, banding continues to provide invaluable information about migration, behavior, lifespans, survival rates, and population sizes for a considerable percentage of the Earth’s avian species.

Many birds come back to the same nesting or wintering sites year after year, as Audubon proved, so it’s not unusual for scientists to recapture individuals. Capturing the same individuals in two different places, however, is quite rare. For example, records from the U.S. Geological Survey’s Bird Banding Laboratory show that only 81 of the 346,000 songbirds that had been banded south of the U.S. border as of 2011 were recaptured in the United States during the breeding season. That’s why it was big news in the ornithology world when researchers announced that for the first time they had caught an individual Golden-winged Warbler at a migration hotspot and then caught it again at its wintering grounds.

Amber Roth, a research assistant professor at Michigan Technological University and a member of the Golden-winged Warbler Working Group, called it “a one-in-a-million long shot.” When she first trapped the bird in a mist net on January 25 at Reserva El Jaguar, a sustainable coffee farm in the central highlands of Nicaragua, she thought it was a bird she had banded herself in Wisconsin or Michigan. “That would have been really amazing,” she says, laughing.

Bird Banding Laboratory records showed, however, that it had actually been caught and banded on September 2, 2014, at the Severson Dells Forest Preserve, a small protected area in Illinois, northwest of Chicago. The bander that day, Rockford University professor James Marshall, later told Roth that it had been his best outing of the year for catching Golden-winged Warblers. 

Other scientists express a similar joy at having the birds they band turn up elsewhere. Chris Rimmer, executive director of the Vermont Center for Ecostudies, says he’s experienced three such occurrences for a bird he studies: the Bicknell’s Thrush. Two of those birds were banded by his team in Vermont and recaptured in the Dominican Republic; one was banded in the Dominican Republic and recaptured in Vermont.

“It’s always a surprise,” Rimmer says. “The first time it happened we were dumbfounded, really. What’s even more remarkable is that the same banders recovered the bird at both ends.” In a Vermont Center for Ecostudies newsletter, one of his fellow researchers described the odds of this happening as “a bit like being hit by lightning while winning the lottery.”

When banded birds turn up in another spot, it’s not just a cool coincidence. It fills in crucial details about a bird’s lifecycle. This is especially important for rapidly declining species, including the Golden-winged Warbler, whose numbers have declined dramatically in its principal range. (The warbler is down to 40 percent of its former population in the Great Lakes region and 5 percent of its former population in Appalachia.) It’s designated as “near threatened” by the International Union for Conservation of Nature and has been proposed for listing under the U.S. Endangered Species Act. For now, researchers don’t even know whether its main problems are on its wintering grounds in Central and South America, its breeding grounds, or somewhere in between. “We’re trying to get a sense of what their survival is like year to year,” Roth says.

After catching her Golden-winged Warbler in January, Roth outfitted it with a tiny geolocator, which provides latitude and longitude estimates by recording ambient light levels. In fact, she put geolocators on every male Golden-winged Warbler that found its way into her mist net. (Only in the last couple of years have these devices become light enough to be safely used on warblers, which generally weigh about one-third of an ounce, or less than two quarters.) If Roth is lucky enough to net some of these birds again next winter, she’ll be able to use the data on the trackers to determine their general migration route and where they breed.

Nonetheless, geolocators—not to mention radio and satellite transmitters—won't completely replace old-fashioned bird banding, particularly for studies on long-term survival. “You still want those birds marked,” Roth says. “I like the fact that I can re-observe a bird for seven years and not have to recatch it.” Plus, as she found out this year, there’s always a chance that someone thousands of miles away will make the same discovery. 

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