The big, beautiful mute swan inspired Tchaikovsky’s “Swan Lake.” They are associated with the postcard-perfect images of the River Thames and Lake Geneva.

But some wildlife biologists worry that the native of Eurasia doesn’t make such a perfect match with the Chesapeake Bay.

Recent surveys show their population is skyrocketing. Biologists fear the mute swan is competing for food and habitat with native waterfowl, such as the tundra swan, and that large flocks of mutes are scaring other birds away.

In the mid-1990s, a large molting flock of more than 600 mute swans overran a colony of least terns and black skimmers on Barren Island. Both birds, listed as threatened species in Maryland, fled.

“A mute swan’s foot is about the size of your hand, so if you’ve got a few hundred of them standing around on a tern colony — and terns are only about the size of the palm of your hand — they’re all going to get stepped on,” said Doug Forsell, a biologist with the U.S. Fish and Wildlife Service’s Chesapeake Bay Field Office.

He and others fear conflicts could escalate if the population continues to grow. The latest Chesapeake Bay waterfowl survey, conducted last fall, counted 3,955 mute swans in Maryland’s portion of the Bay. That was a 46 percent increase over the 2,700 swans seen during the last survey, completed in 1996.

Virginia’s numbers are lower — but growing — partly because of swans moving in from Maryland. Last year’s survey, conducted by state biologists and the U.S. Fish and Wildlife Service, counted 488 mute swans, up from 419.

The problem, said Larry Hindman, waterfowl project manager with the Maryland Department of Natural Resources, is that mute swans did not evolve in the Bay and could have unpredictable impacts as their numbers grow. “We don’t know,” he said, “what conflicts we are going to have with native wildlife if the population is allowed to continue to increase and expand their range, which they obviously are doing.”

Recognizing the growing threat such exotic species pose for natives, the Bay Program in its draft Chesapeake 2000 agreement — expected to be signed this summer — calls for identifying exotic species that pose a threat to the Chesapeake by 2002, and developing management plans for them by 2004.

But agreeing to control a big, beautiful bird with no natural predator is not easy. “You can’t just go out and blatantly destroy mute swans out there,” said Gary Costanza, a waterfowl biologist with the Virginia Department of Game and Inland Fisheries. “You are going to raise the ire of people.”

When Maryland approved killing some of the Barren Island swans for the sake of the terns and skimmers — it was the only skimmer nesting site in Maryland’s portion of the Bay — it touched off a furor. The governor ordered a moratorium against killing any mute swan. A task force is studying what, if anything to do about them.

Indeed, mute swans are in the Bay because people like them. They were brought to North America in the late 1800s to adorn ponds or other private waters. Over time, they escaped and began breeding in the wild. In Maryland, five escaped captivity in 1962. Those five are mostly responsible for today’s Bay population, although numbers may be bolstered by other escaped swans.

Similar populations have turned up in parts of New York, New England and the Great Lakes. But the mute swan population seems to be growing faster here, possibly because of the milder winters.

Right now Maryland, Virginia and federal agencies “addle” mute swan eggs on lands they manage to limit reproduction. Addling usually consists of either shaking the eggs or coating them with vegetable oil, which prevents them from hatching.

Even that has drawn criticism from some. And, officials acknowledge, it has not halted the overall growth of the population. Even in Virginia, where mute swans are listed as a nuisance bird, hunters hardly shoot any. There is no agreement on what more to do.

William Sladen, an internationally recognized swan expert, said Maryland officials erred when they occasionally allowed permits to kill mute swans in the past.

“Though the alien mute swan is becoming a serious problem, they should not be killed; there are better methods of dealing with them,” said Sladen, who heads the nonprofit Environmental Studies at Airlie.

Sladen argues that wild swans should be caught by state-licensed dealers, rendered flightless, then sold in same-sex pairs to people who want swans for ponds. That would prevent breeding and remove swans from the wild while still allowing people to have the birds, which he calls “elegant ambassadors for wetland conservation.”

Sladen suggests dealers should be prevented from breeding mute swans, and anyone holding a swan should need a permit. He has experimented with the procedure in Virginia and says it works well.

Other biologists are not totally convinced. They worry that promoting the establishment of mute swans on inland ponds may ultimately increase demand for the birds. “When people see them, they want them for their own ponds,” Costanza said. Most of the calls he gets about the swans, are not complaints, but from people wanting advice on breeding more of them. “We recommend that they render them flightless and sterilize them,” Costanza said, “but that normally doesn’t happen.”

Mute swans are a potential threat to other species — especially tundra swans — because they have a big competitive advantage: They don’t migrate.

The tundra swan, by contrast, makes the longest migration of any Chesapeake waterfowl, traveling more than 4,000 miles each spring and fall to and from western Alaska nesting grounds.

Without facing the rigors of migration, the mute swan population can outpace the tundra swan population. Also, mute swans are, on average, about a third larger than the tundra swans, growing up to 29 pounds. In some instances, they appear to be territorial and aggressive, chasing tundra swans from prime loafing areas. “I’ve seen several occasions where a handful of mutes can keep hundreds of tundras out of their preferred area,” said Dan Day, a scientist with the U.S. Geological Survey’s Patuxent Wildlife Research Center. “They tend to push them around a little bit.”

But, such behavior seems to be confined to small areas such as coves or ponds, he added. Along open coastlines, the birds seem to be content within a few yards of each other.

Also, because they don’t migrate, biologists are concerned about their impact on underwater grasses — restoration of which is a major Bay Program goal. Mute swans consume underwater grasses all year — not just during the winter like most other waterfowl. Using rough calculations, biologists say the birds just in Maryland could eat about 11 million pounds of grasses a year.

Many species of waterfowl have already deserted the Bay as it lost 90 percent of its historic grass beds. The mute swans could make the situation worse, especially during the winter when grasses — already in short supply — are needed to support migratory ducks and tundra swans that use the Bay as a winter haven.

“We complain that birds are declining in the Chesapeake Bay and yet we have more and more exotics eating up the food resources,” said Forsell, who chairs the Bay Program’s Waterfowl and Wetland Bird Workgroup. “You just can’t have it all. We’re going to have to decide, do we want native birds, or exotic birds?”

He and others acknowledge, though, that critical questions are unanswered. Biologists acknowledge the potential threats to tundra swans are based more on anecdote than research. The same is true for concerns about impacts on grasses.

The swans are poorly studied. Because they don’t migrate, they are not covered under the federal Migratory Bird Treaty Act, so federal agencies don’t manage them. Because they are exotic species, states have been reluctant to fund studies.

The lack of research makes quantifying the swans’ impact difficult. And, because few advocate eradicating the big birds from the Bay, no one can say what number the Chesapeake could support without harming native birds. Or, even whether the Bay might be able to support them all. But with the rapid rise of the swan population, all agree such answers will be needed soon.

“They are beautiful birds; I thoroughly enjoy watching them,” Day said. “If it turns out that they are not a problem, I think they are a wonderful addition to the Bay. But if they have a negative ecological impact on the Bay, considering their rapid population growth, the situation is potentially getting out of hand.”