Last July, biologists captured the bald eagle near the northern Chesapeake Bay, fitted it with a space-age backpack and sent it on its way.

The bird was in no particular hurry: It spent a leisurely two months in the bay area, fishing and flying, fishing and flying. It meandered up the Susquehanna River and visited for several weeks at a large reservoir in central Pennsylvania. Then it flew south to spend a few days in the Richmond area. After that it swooped down to a wildlife preserve in Colleton County on the southern coast of South Carolina.

There, a state wildlife worker happened to see the banding on the eagle and immediately knew that the bird had been hatched in Colleton County four years earlier. It had come home.

While the bald eagle's wanderings may not seem the stuff of breakthroughs, the ability of scientists to track it may well be.

The bird's flight was being monitored by a weather satellite receiving signals from the small solar-powered transmitter it was wearing in the backpack. The device, made from parts costing upwards of $1,000, was developed by the Applied Physics Laboratory of Johns Hopkins University for its neighbor in Laurel, the Patuxent Wildlife Research Center, an agency of the U.S. Fish and Wildlife Service. It costs about $7 a day to track the bird, officials said.

"The object is to develop a technique, a method, that allows us to keep track of migratory birds," said Mark Fuller, a research biologist at Patuxent and the Bird Borne project manager. "They're like small aircraft setting out on a continental journey. They need to refuel along the route. They need a support system."

It's been no secret, of course, that birds migrate. The fish and wildlife service is responsible for more than 500 types of migratory birds, including endangered species such as the bald eagle. But there had previously been no efficient way of knowing how the birds get from Point A to Point B, what difficulties they encounter on their route, and what they need to survive and flourish in the future, Fuller said.

In the past, conventional radio tracking of birds has been largely ineffective and costly, he said. It requires either ground personnel or specially-equipped vehicles or aircraft and is of no practical use for seafowl or birds covering long distances.

Although large transmitters weighing several pounds have been placed on polar bears in the Arctic and sea turtles in the Caribbean, "there was nothing we could put on a bird," said Fuller.

Enter the Applied Physics Lab.

At the request of the biologists, researchers there developed the transmitter "backpack," which weighs less than six ounces and is about the size of a package of cigarettes. It is waterproof and solar-charged and is attaches to a bird's chest with teflon ribbon.

As the bird flies, signals from its transmitter are picked up by the French-designed Argos doppler tracking system. The system is attached to two weather satellites operated by the National Oceanic and Atmospheric Administration.

Each day, this system monitors the location of the bird-borne transmitters at least six times. Service Argos, the space-science group in Toulouse, France, translates the information and sends it by computer/telephone linkup to NOAA in Suitland. All Fuller has to do to find out where the eagle is located on a given day is to dial the Suitland number.

Currently, the project is only in an experimental stage, Fuller said.

"If we can make it work consistently, it can be a useful technique, widely applicable," particularly for tracking small birds, he said.

Besides the eagle, only five other birds -- from a colony of giant petrels in Antarctica -- are wearing the transmitter backpacks. The petrels, rigged in January, have stayed close to their nests ever since, he said.

As for the bald eagle, it ended up wintering in the Gainesville, Fla., area. A few weeks ago, it returned once again to its South Carolina birthplace.

"This will help us learn to conserve bald eagles in general," Fuller said. "Until this tracking, we had no idea South Carolina eagles wandered around the eastern seaboard. Biologists who studied them at their nest sites knew they went somewhere else, but where? We had no idea."