Every year, half-ounce songbirds take off from the northeastern United States and points farther north in Canada. They keep flying until they temporarily land on warm Caribbean islands, before reaching their final destination in South America. Remarkably, these little birds make most of that journey over open waters.

For the first time, scientists have tracked the migration pattern of blackpoll warblers, inserting hard evidence into an ongoing debate over how these forest-dwelling birds wind up in South America every year. Researchers from the Vermont Center for Ecostudies, the University of Massachusetts at Amherst and University of Guelph in Ontario published their findings this week in the journal Biology Letters.

"It sort of defies the imagination on a number of levels," said Chris Rimmer, a study co-author and Vermont Center for Ecostudies ornithologist. "One is just the energy, the sheer energy and physiology required to propel a bird that many miles, an average of about 1,600 miles. But it's also a navigational feat, an orientation miracle in a way that the birds can strike out from land and head out over the water and reach their destination two or three days later."

The work began after researchers used a new kind of geolocator to track a different bird species. They began thinking of other ways to use the devices, which are small and lightweight and rely on light sensors rather than GPS technology. Soon, Rimmer said, they realized the geolocators could help settle the 50-year-old debate about whether blackpoll warblers "really do take this extraordinary over-ocean flight."

Coincidentally, researchers in Nova Scotia were planning the same kind of experiment, so the two groups teamed up.

For the study, researchers took geolocators about the size of a dime and weighing half a gram and strapped them to six male birds as though they were backpacks. A small light stalk protrudes above the bird's feathers, which captures information about light transitions and encodes the information on a computer chip.

Five of the six birds made their migrations with the geolocators intact. (The sixth bird's device was lost.) Researchers downloaded the data, compared it to sunset and sunrise times and were able to estimate the warblers' paths. Some of the estimates, they noted, are more precise than others.

Two birds from Canada and two from Vermont took off between Sept. 25 and Oct. 21 in 2013, flying nonstop over the Atlantic Ocean before landing in the Caribbean, with flight times ranging from 49 to 73 hours. The fifth bird likely took a shorter trip.

"Compared with previously published non-stop flight estimates for other species, our data suggest that blackpolls undergo one of the longest distance non-stop overwater flights ever recorded for a migratory songbird," the researchers wrote.

While the data sample from five birds was very small by scientific-study standards, Rimmer said the findings were very clear and compelling enough to merit publishing.

Previous research has suggested that the blackpoll warbler takes an overseas route. For instance, people had spotted the birds landing on boats and in Bermuda. Some birds also fattened up so much that they doubled their body weight, suggesting that they were preparing for long journeys during which they couldn't stop for food, Rimmer said.

Although the birds are among the most common warblers in North America, their populations are declining by about 6 percent every year and researchers don't know why -- which makes learning about their migration patterns all the more important for conservationists wanting to protect the birds. Researchers are next interested in tracking birds migrating from Alaska, and in studying female blackpoll warblers.

Most migratory songbirds are easier to track than blackpoll warblers because they travel over land -- often through Mexico and Central America -- and make stops along the way, allowing people to spot or catch them. "We have documented evidence that the routes they follow are over land, versus these guys which effectively disappear and reappear on the Caribbean islands, and then in South America," Rimmer said.

Some larger birds travel longer distances over water, Rimmer said, but the blackpoll warbler is in a class of its own.

"If you can account for the size of this bird and the distance it goes," he said, "it's arguably more remarkable, maybe the most remarkable flight of its kind."