Scientists have lately taken to snatching feathers from osprey nests around the Bay. And, in some cases, they've set up cameras to spy on everything going on in eagle nests.

And when it comes to cormorants, they wade straight into their colonies.

"The adults flush, but the young stay and regurgitate whatever is in their stomach," said Adam Duerr, a biologist with the Center for Conservation Biology, a research center operated by the College of William and Mary and Virginia Commonwealth University.

The object, of course, is not to wade through bird vomit but to learn what's been on the menu of fish-eating birds around the Bay.

Unlike fisheries scientists, who simply cut fish open to see what's inside, those studying birds need to be more creative. "You can't go out and kill hundreds of osprey to see what they've been eating," said Greg Garman, director of the Virginia Commonwealth University's Center for Environmental Studies.

Instead, they can get some diet information by analyzing osprey feathers, or watching what species birds bring back to nests to feed their young. In the case of cormorants, Duerr said, biologists "can go around the colony and see whatever they have spit up, then count, measure and identify the fish they were consuming."

Knowing what birds eat is important because populations of birds that eat fish-eagles, osprey, cormorants, brown pelicans and great blue herons-have soared in recent decades.

After DDT nearly eliminated them in the 1970s, thousands of eagles and ospreys now nest along the Bay and its tidal tributaries. Brown pelicans and double-crested cormorants, which were not previously present, have moved into the Bay, and their numbers have increased dramatically.

That, in turn, has resulted in a huge potential demand for fish.

Using crude estimates, the scientists say birds may have consumed about 4.5 million pounds of fish when populations bottomed out in 1975. By 2005, avian predators around the Bay needed about 38 million pounds of fish-and scientists expect that number to increase for at least another decade.

In some lakes, cormorants have been shown to have significant impacts on fish populations.

In the Bay, birds appear to consume a relatively small, but still significant, number of fish compared with other predators-mainly other fish and humans.

Nonetheless, the increase could affect management. Fishery managers set catch limits based on models that estimate how many fish are in the population. Those models assume that "natural mortality"-all sources of death except fishing-remains constant over time.

Those models don't account for the fact that birds are eating eight times as many fish as 30 years ago, and that number could increase over the next decade.

"In the world of single-species assessments, these things are all constants," said Jim Uphoff, a fisheries biologist with the Maryland Department of Natural Resources, who is cooperating with the study. "We don't typically do assessments with that thought in mind."

The research is being funded by the National Oceanic and Atmospheric Administration's Chesapeake Bay Office in the hope that it will provide information for future models that will better reflect predator-prey relationships, and therefore better inform management decisions.

To that end, the scientists-who are midway through a four-year project-are trying to learn what types and amounts of fish the birds are eating to better refine their estimates.

The greatest impacts, though, may not be how birds are affecting fish, but rather how changes in fish populations may be affecting birds.

For instance, many believe the Bay's menhaden population is in decline, possibly depriving striped bass and other predators of food. The research suggests effects could reach into bird nests as well.

Several studies in the last three decades have examined osprey diets in Mobjack Bay, located between the mouths of the Rappahannock and York rivers in Virginia.

In the mid-1980s, those studies found that 75 percent of the diet of nesting osprey was menhaden. By 2005, only a quarter of their diet was menhaden. The switch from menhaden, an oily, energy-rich food, to other species appears to have dramatically affected osprey.

Production of young osprey in Mobjack Bay today is as poor as it was during the DDT era. The difference, said Bryan Watts, director of the Center for Conservation Biology, is that during the DDT era, when the pesticide caused thinning of egg shells, only 30 percent of the eggs even hatched. Today, more than 90 percent of the eggs hatch, but the young birds die.

"The chicks just are not being fed enough and die at a young age," Watts said. "I believe that because menhaden are so energy-rich, they are not a replaceable component in the diet."

As a result, the osprey population in Mobjack Bay has stagnated.

A similar pattern is emerging around the Chesapeake. With somewhere between 6,000 and 8,000 active nests, the Bay region holds the largest osprey population in the world, but the number of nesting osprey in high-salinity areas has leveled off.

"If menhaden are a critical component of osprey diet and the rug is essentially pulled out from under them, it is possible we could see a population collapse," Watts said. "But if menhaden came back, they would likely recover."

Meanwhile, osprey populations in tidal-fresh areas-those near the upper limit of the Bay's tidal influence-continue to grow exponentially.

Tidal freshwater areas contain a unique mix of marine and freshwater species. Osprey can still find other clupeoids-a fish group that includes menhaden, herrings and shads-to eat as migratory hickory shad and nonmigratory gizzard shad abound. Thriving populations of exotic predators, such as blue catfish, flathead catfish and others, are present in huge numbers.

The booming populations of fish in tidal-fresh areas may help bald eagles, as well. Their nesting was once timed to coincide with the spring migration of shad and herring into Bay tributaries. Nesting would occur in winter so the hatching of hungry young birds occurred as vast numbers of fish, filled with energy-rich eggs and sperm, began migrating up the rivers.

Populations of herring and shad have been decimated by loss of habitat from dam construction, pollution and overfishing. But like the osprey in tidal fresh areas, bald eagles have found substitutes, including blue catfish, an introduced species and voracious predator that seems to make up a good portion of the eagles' diet.

"The only good thing that I can say about blue catfish is they've probably been pretty important in the recovery of bald eagle and osprey, and it explains why the greatest concentration of both of these bird species has been in the tidal fresh waters," Garman said.

The change in diet does have a downside. Some areas, such as the tidal fresh portion of the James, have fish consumption advisories because of elevated levels of PCBs and other toxins in blue catfish.

"In a few years, are we going to see some sort of toxic effects from the contaminants in the blue catfish?" Garman asked. "It is a reasonable expectation that these birds might be affected."

Meanwhile, cormorants and pelicans, which are generalists in what they eat, appear to be doing well around the Bay as their populations continue to expand. Work is continuing this year to analyze their diets.

Understanding the blue heron diet is the most problematic. They are widely dispersed around the Bay and its tributaries, so their diets may vary considerably from place to place. But with more than 18,000 pairs estimated to be around the Bay, they are also the most numerous avian predator.

"The great blues are the real gorilla of those species here, the population is huge now, and they are here most of the year," Watts said.

Scientists say getting fisheries biologists and ornithologists to work together to understand a significant part of the Bay food web has been as important as their findings.

"It's a great opportunity for fisheries people to work with the bird people, and for the first time begin to see what some of the relationships are," Watts said. "I think as we begin to look at some of the fisheries regulations, considering some of the other consumers in the equation would be a great thing."

That has begun to happen in some places, as fishery regulations along the mid-Atlantic coast work to conserve horseshoe crabs because their eggs are essential food for migrating red knots, which stop each spring to eat.

"If you're going to start looking at things on an ecosystem basis, these things are important," Uphoff said. "If you don't consider it, you're living in a fool's paradise.

Feathers offer glimpse into ospreys' diet

Ospreys-or at least their feathers-may soon become a key tool to monitor menhaden populations around the Bay.

By examining stable isotopes in their feathers, scientists already can determine how much of an osprey's diet in the previous few weeks or months came from freshwater fish or marine species.

The scientists, who are studying fish predation by birds with a grant from the National Oceanic and Atmospheric Administration's Chesapeake Office, have used the technique to study historical changes in bald eagle diets.

They obtained a feather from every eagle in the Smithsonian collected around the Bay since the mid-1800s. The feather analysis showed their diet overwhelmingly originated from marine environments, as they ate shad and herring returning from the ocean to spawn, until the 1970s, when shad stocks collapsed around the Bay. After that, freshwater fish became the mainstay of their diet.

Now, Stephen Macko, at the University of Virginia, is trying to use isotope analysis to identify an individual species-menhaden. This spring, scientists are replacing some of the food in several osprey nests with menhaden, and comparing isotopes in their feathers with those from nests without menhaden.

If they can determine a specific "marker" for menhaden, they will eventually be able to determine the relative size of the menhaden stock by collecting feathers from osprey around the Bay and seeing what portion of their diet consisted of menhaden in the previous weeks.

The osprey are ideal for such monitoring because they seem to prefer menhaden, unlike other birds which are more general in their feeding, and they are widely distributed around the Bay.

"You could get a snapshot of different places around the Bay at the same time," said Greg Garman, director of the Virginia Commonwealth University's Center for Environmental Studies. "Over time, you could track the stock more effectively than you can just from landings alone."

Cap on menhaden catch may be extended until research is complete

The catch limit for menhaden in the Chesapeake may be capped for an additional three years as research aimed at determining the health their population in the Bay continues.

The existing annual cap of 109,020 metric tons of menhaden from the Chesapeake Bay is set to expire after next year.

In 2006, the Atlantic States Marine Fisheries Commission adopted a five-year cap based on the average landings from 2000 through 2005. The intent was to allow for research to determine whether the Bay is suffering from "localized depletion" of menhaden.

While ASMFC stock assessments show the coastwide menhaden stock is healthy, sportsmen and some conservation groups have charged that the Bay has too few of the small, oily fish to support striped bass and other predators because of fishing pressure in the Bay. The menhaden fishing fleet is based in Reedville, VA.

The three-year extension was proposed because much of the research will not be completed and analyzed before the cap expires.

Unlike other fisheries, Virginia's menhaden catch is regulated by the General Assembly, rather than the Virginia Marine Fisheries Commission. To extend the cap past 2010, the General Assembly would have to approve the change when it meets next winter.

To keep that timetable, the ASMFC may need to approve a draft amendment to its management plan when it meets in August, and then submit the plan for public comment. Final action could happen when the commission meets this fall.

But an extension may face opposition unless the cap is changed.

Ken Hinman, president of the National Coalition for Marine Conservation, said he was disappointed that no other other extension options were proposed, such as resetting the cap to reflect the most recent five years of catch data.

That would certainly result in a lower limit as almost all recent catches in the Bay have been less than 100,000 metric tons.

He also said that recent studies raise "red flags" about menhaden. The studies show increased striped bass mortality in the Bay, while menhaden reproduction remains low. Overall menhaden landings along the East Coast continue to decline.

"There are a lot of reasons to want to take more precautionary actions as opposed to a cap that is not really constraining the fishery at all," he said. "We probably need to do more than we've been doing, so we're not in favor of just extending the current cap."