Every scientist who studies ospreys has a story about finding odd man-made objects in the birds' nests.

On the Patuxent River, there was a nest with a toy machine gun in it. On the South River near Annapolis, there were teddy bears. In the harbor near violence-plagued Baltimore, one bird had brought back pieces of crime scene tape.

"They see it floating in the water, they're going to put it in the nest," said Peter C. McGowan, an Annapolis-based biologist with the U.S. Fish and Wildlife Service. His favorite find: a bra, in a nest in Virginia's Elizabeth River.

But the ospreys' pack rat tendencies can pose a problem, scientists say, when they pick up one common bit of man-made debris: fishing line.

When brought back to the nest, the line can entangle osprey chicks and sometimes parents, causing serious injury or even death by starvation.

The thought of fishing line ending up in nests and threatening the newly revived osprey population was enough to make McGowan want to do something. So last year, he began a campaign to reduce the amount of fishing line in osprey nests. Because he can't change the birds, he's aiming to change local anglers by making them more aware -- through fliers, advertisements, and waterside signs -- of one of the dangers posed by discarded fishing lines.

"Most fisherman are environmentalists," McGowan said. "I don't think they would purposefully go out there and try to harm wildlife" with their fishing line.

The birds, fish-eating predators that mate for life, return from South America in the spring to prepare their nests for the arrival of chicks. The nests, often several feet across, are made mainly of twigs, with a soft center lined with sea grass where the eggs will be laid.

Within the nest structure, ospreys often stash trinkets they find in the water or on the beach: gloves, pieces of crab pots, Mardi Gras beads -- even newspapers, still in their delivery bags.

Mitchell A. Byrd, of the College of William & Mary's Center for Conservation Biology, has studied ospreys for decades and still can't explain it.

"They just pick stuff up and bring it back," said Byrd, who once found part of a bikini in a nest. "I don't know what the motivation might be."

Steve Cardano, who has studied ospreys on the Patuxent since the 1970s, has given the trinkets a name: "ornamentals." He discovered the toy machine gun in a Patuxent nest, and has found beer cans and deer antlers in others.

His theory is that the ornamentals are brought back by male ospreys to make up for a poor hunting trip.

"I've got an idea that it's the male," Cardano said. "He's supposed to be coming back with the fish; he comes back with a present for the female."

In the case of fishing line, many scientists believe ospreys mistake it for sea grass, or pick up line accidentally because it is tangled on grass or a twig that they want.

The osprey has been one of the bay's great conservation success stories over the last 20 years, as it recovered from the ravages of the pesticide DDT.

In the 1960s, the pesticide was used by farmers in the area, and it accumulated as it traveled up the food chain. At the top of the chain were ospreys, which eat primarily fish that they snatch from the top foot of the water.

The pesticide caused the shells of osprey eggs to be thin and weak, and many eggs broke before they were ready to hatch.

One survey, in 1973, showed the osprey near its low point: There were about 1,450 breeding pairs on the bay, according to McGowan.

DDT was banned in 1972. Since then, the bay's population of ospreys has grown rapidly, and there are now about 3,600 breeding pairs. Some scientists believe that is one of the densest concentrations of the birds anywhere.

The bay has its own natural advantages: It has an abundance of fish such as menhaden, perch and shad that ospreys can eat, plus clear water that allows them to spot prey swimming near the surface.

In addition, humans have provided good nesting sites. Ospreys build atop the thick posts that mark channels in many rivers, on duck blinds or on platforms erected specifically to house their nests. About 90 percent of the bay's breeding pairs are nesting on man-made structures, according to the Fish and Wildlife Service.

"They really coexist quite well with man in a lot of . . . sites, particularly in the Chesapeake Bay," said Barnett Rattner, a research scientist at the U.S. Geological Survey's Patuxent Wildlife Research Center in Laurel.

Rattner quoted an author who had written about the birds, calling the Chesapeake "the osprey garden of the world."

But in the midst of the osprey's successful recovery, McGowan said, he began to notice that fishing line was taking a toll on the birds.

He said he realized the danger during studies in 2000 and 2001, when he was checking nests in Baltimore Harbor and on the Elizabeth, Potomac and Anacostia rivers.

McGowan's aim was to examine eggs and osprey chicks to see if they were affected by chemical pollutants in the water. It turned out, McGowan said, that the chemicals weren't causing significant damage. But the fishing lines were.

He said he repeatedly noticed birds tangled up in line, sometimes in danger of losing a foot or dying of starvation.

On the Elizabeth River, "we found at least two birds wrapped up pretty significantly. One actually had a fish hook that was attached to that line," McGowan said. The scientists had to remove the hook from the bird's leg.

If a bird's leg is wrapped tightly in fishing line, McGowan said, the bird could lose the leg, a very serious injury because the bird depends on its talons to grab prey.

A Virginia researcher, Reese Lukei, recalled several grisly examples. He remembered finding a chick so tangled in line that it could move only its head. The chick was freed, but later died because its lower legs were gangrenous.

In another case, near Virginia Beach, Lukei found a mother osprey that had line tangled around her feet. She had tried to fly, could not, and wound up hanging -- dead -- from the side of the nest.

"It's a traumatic death to them, very agonizing," McGowan said.

After his survey of nests, McGowan began a campaign, using some grant money, to educate anglers about the dangers of discarding their line outdoors.

Last spring he posted about 400 signs near the water in 15 different Maryland counties around the bay. He also printed 200,000 fliers, which were sent to bait and tackle shops and distributed with sport fishing licenses in Maryland. The brochures ask anglers to "retrieve broken lines and deposit them in trash containers."

Even after those warnings, McGowan said, line has been found in many nests this year.

A survey of the Patapsco River found that about half of the nests contained fishing line, and another survey of the South River found line in about 41 percent of nests, McGowan said.

On one recent day, McGowan and a Fish and Wildlife Service intern, 20-year-old Matt Duffy, a New York college student, set out in a boat to survey the osprey nests on the Severn River near Annapolis.

This was their routine: McGowan would steer up to one of the Severn's many channel markers, steel poles that usually protrude about eight feet out of the water.

Many had osprey nests at their tops. Duffy would clamber up and examine the nests as the anxious adult ospreys -- often both the mother and father -- circled above.

From his perch above the nest, Duffy would call down the number of eggs and chicks, noting any man-made items in the nests. Those included plastic bags and thick rope, but -- for the first several nests -- no sign of fishing line.

"Those signs must be working," McGowan said.

Then, on marker number 9, McGowan noticed a green ball on one side of the nest.

"There's a fishing line, a monofilament line right there," he said.

In that case, however, the line was only on the outside of the nest. They checked inside and found an egg and a single, healthy chick.

U.S. Fish and Wildlife Service biologist Peter McGowan displays tangles of fishing line removed from osprey nests on the Patapsco River. When ospreys bring discarded line back to their nests, it can fatally entangle chicks or adults. Clockwise, from above left: Waterside signs educate anglers about the dangers of discarded fishing line to the bay's osprey population, now revived following a ban on the pesticide DDT; an osprey on a nest built atop a channel marker; an osprey chick and eggs in a nest; and a biologist uses a mirror on a pole to check a nest's contents.