Tiny, light-sensitive devices strapped to the backs of purple martins and wood thrushes in their Pennsylvania breeding grounds have yielded the first clear picture of songbird-migration routes and over-wintering areas, information that could help pull songbird populations out of a long tailspin.

The migration study, reported in the journal Science today and funded in part by the National Geographic Society, marks the first successful tracking of songbird-migration routes and also shows that scientists have significantly underestimated their flying ability.

The data show the birds flew more than 311 miles in one day, compared to previous studies that estimated their flight performance at 93 miles a day. The study also revealed that prolonged stopovers were common on the fall migration to the South and their migration was two to six times more rapid in the spring than in the fall.

Two purple martins tracked from their breeding grounds around Edinboro Lake in Erie County stopped in the Yucatan Peninsula of Mexico for three or four weeks before continuing on to their wintering-over grounds in Brazil. One of those birds took 43 days to fly to the Sao Paulo area of Brazil but only 13 days to return.

"We were flabbergasted by the birds' spring return times. To have a bird leave Brazil on April 12 and be home by the end of the month was just astounding," said study author Bridget Stutchbury, a professor of biology at York University in Toronto, Ontario. "No one had any idea these little songbirds would be able to fly that far that fast. That was the gee-whiz part."

The study also uncovered evidence that wood thrushes from a single breeding population in Hemlock Woods, near Cambridge Springs, Crawford County, did not scatter over tropical wintering grounds but stayed in a narrow band of tropical forest in eastern Honduras and Nicaragua.

That region is clearly important to the overall conservation of wood thrushes, a species that, like the overall songbird population, has declined by 30 percent since the mid-1960s, Ms. Stutchbury said. The tracking data research is important, she said, not only to protect at-risk species of songbirds but also to assess environmental and habitat loss related to deforestation, pesticide use and climate change.

"There are many songbird species showing significant, chronic, long-term declines and in order to stop that we need to understand if the wintering grounds are causing or contributing to that," Ms. Stutchbury said. "Until now our hands have been tied in many ways because we didn't know where the birds were going when they left in the fall. They would just disappear and come back in the spring. It's wonderful to now have a window into their journey."

That window was made possible by technological advances that minimized the size and weight of a device called a geolocator. Unlike larger birds, songbirds are too small to be tracked by satellite or radio transmitters.

The geolocators, weighing just half a gram and measuring a half-inch long and a quarter-inch wide, detect light, allowing researchers to estimate the birds' latitude and longitude by recording sunrise and sunset times. The plastic-covered devices are mounted on the birds' backs by thin teflon straps looped around their legs.

The geolocators can be small because they don't transmit. They only collect data. The birds must be trapped when they return to their breeding grounds to allow researchers to remove the devices and retrieve the data.

Fourteen wood thrushes were outfitted with the geolocators in the summer of 2007, and seven came back to the Cambridge Springs breeding grounds, though only five were trapped. Of 20 purple martins outfitted, only two returned.

Wood thrushes and purple martins were used because they represent two songbird extremes -- the thrush is a woodland bird and flies at night while the martin favors grasslands and is a day flier. Both are also among the largest Eastern songbirds and have long-established study colonies in northwestern Pennsylvania.

The wood thrush, one of the most common woodland birds of the East, is 7 to 8 inches long and weighs about 1.8 ounces, about 50 grams.

Purple martins, measuring nearly 71/2 inches from beak to tail, are the largest North American member of the swallow family and weigh about 1.9 ounces, or 55 grams. Unlike most songbirds, its population overall is stable, although declines have been noted in New England, along the West Coast and in parts of Canada.

"It was good and natural to do this study with Stutchbury and at an established colony like ours, because the catch is you have to recapture the bird and the device before you can download the data," said John Tautin, executive director of the Purple Martin Conservation Association, headquartered in the Tom Ridge Conservation Center on Presque Isle State Park in Erie. The 22-year-old organization bands more than 1,200 martins a year. The study was continued last summer with geolocators attached to 20 purple martins and 30 wood thrushes. The birds are expected back this spring.

"We're not sure why we only got two back last year," Mr. Tautin said. "Even with only two returning, it's significant because now we have information between when the birds were banded and when they return. I think the scientific community will jump on this and there will be more demand to use the geolocators to track other species." Researchers are already using the devices in new studies to track migration of bobolinks, Bicknell's thrushes and veeries.