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In Montana, as in most of the Lower 48, American Robins are present year-round. So I had long assumed that the cheerful songbird hopping around my yard in the fall had nested nearby that summer. Imagine my surprise, then, to learn it may have spent those months a thousand miles or more to the north. Where these common birds go, and what drives their decision to migrate long or short distances—or simply stay put—are mysteries that scientists are just starting to solve.

“They’re a truly cosmopolitan bird,” says Emily Williams. “They may be the most widespread songbird in North America, and yet there’s so much we don’t know about them.”

Williams, a former biologist at Alaska’s Denali National Park, is taking part in the first-ever study to track the movements of ­individual robins across seasons. Scientists have a solid grasp on the behavior and physiology of the birds during the breeding season but a poor handle on their experiences the rest of the year. Four birds that Williams tagged in Denali, for example, traveled more than 5,500 miles roundtrip each, winging it as far south as Texas for the winter. Meanwhile, two robins one of her collaborators tagged in Washington, D.C., spent the entire year within four miles of where they were captured.

If scientists can crack the mystery of robin migration, the species could one day be sentinels of environmental shifts at a continental scale. “They’re so plastic, so adaptable,” says Williams. “If we find that they can’t deal with the challenges of climate change or other environmental changes that are affecting them, then that spells pretty dire straits for other birds that are much more specialized in what they need.”

Finding out where the birds go is just the first step. Since 2017 collaborators have put GPS tags on 96 robins in five states, and they plan to add more sites, in Ontario, next year. When they catch each bird, and later recapture them to retrieve the data stored in the tag, they quickly gauge the health of their temporary captives with a physical exam to assess their weight, fat stores, and feather condition. They also take a blood sample for later analysis.

If year-round birds are doing poorly in one area, that could clue scientists to investigate a local threat, such as pollution or declines in the fruits or insects that sustain the omnivores. For migrants, researchers might instead search for threats along the birds’ flight paths or at sites where they spend the winter.

“Learning where they move helps to provide an important link to environmental health,” says Melanie Smith, program director of ���Ϲ���’s Migratory Bird Conservation Platform, who spent two days recapturing birds with Williams in Denali in July.

The research could also have implications for human health. In a 2015 study in California, researchers reported that robins, as well as other common birds, are reservoirs for Lyme disease—a bacterial infection transmitted to humans by tick bites, largely in the Northeast, mid-Atlantic, and upper Midwest regions. It’s well known that mice and deer spread the disease to ticks in new areas. The discovery that robins carry infected ticks, too, raises concerns that they’ll accelerate Lyme’s spread, says , an Indiana University evolutionary ecologist who leads the robin project. “Because they’re migratory, a robin could potentially carry this bacteria and infect ticks across counties, across states, across the continent,” he says, “dispersing it much more ­effectively than a deer could.”

The team check each robin they capture for ticks and send any they find for testing. Ultimately, Jahn aims to combine new insights about robins with the Lyme disease findings, along with information about weather, climate, and land transformation, to develop a forecasting system that could detect disease outbreaks and other threats.

Crucial to that effort will be collecting more data, a seemingly simple task that can prove difficult. Robins generally return to the same breeding grounds each year, but not the exact site. To get them in hand a second time requires setting up mist nets along several miles of habitat over several weeks, and there’s still no guarantee of catching a tagged bird. When Smith joined Williams in the field, for instance, multiple Swainson’s Thrushes and one moose got tangled in the net, among other animals, but not one of the 13 robins Williams tagged last year. In fact, Williams didn’t recapture any of those birds this past summer, though she got two tagged in 2018.

“I know to expect the disappointment, but it’s still so crushing,” says Williams, who will continue to work on the project as a graduate student at Georgetown University. “I have to hope we’ll get at least some back next year.”

As soon as this fall, the scientists will receive cutting-edge technology that will dramatically increase the scale of data they collect. Mini-transmitters supported by the ICARUS Project will be able to relay the birds’ locations in real time through an antenna on the International Space Station. This will make recapturing birds far easier and provide other invaluable information, such as whether individuals seek out new breeding sites, or if they die.

Meanwhile, in Montana, as the green leaves of the elms, ashes, and maples around my home transform into the fiery hues of fall, I’ll be paying newfound attention to birds I might otherwise overlook, wondering where in the world they’ve been.

This story originally ran in the Fall 2020 issue as “Hey, Stranger.”​ To receive our print magazine, become a member by .