EASTERN SHORE, Maryland (CNN) -- They're beautiful and graceful -- yet aggressive and destructive. Mute swans have taken over some waterways in the Northeast, especially in Maryland, where they are accused of disrupting natural habitats and even harassing children and pets.

The swans were imported from Europe for gardens and zoos. In places where they've escaped into the wild, they're blamed for throwing entire ecosystems out of balance.

Biologists on Maryland's Eastern Shore say mute swans are crowding out native bird species, such as the Black Skimmer and the Least Tern. They dot the rivers that feed into the Chesapeake Bay, a key habitat for American birds.

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The swans can weigh more than 40 pounds, and their gigantic web feet often destroy other birds' nests, including eggs.

"Their feet are pretty big, probably as large, if not bigger, than my hand when the web is outstretched, so they can cover a good chunk of ground with just one footstep," said Keith Weaver of the Blackwater National Wildlife Refuge.

Mute swans also have enormous appetites; each bird consumes up to 8 pounds of aquatic grasses a day.

"These grasses are key to fish survival. There are crab nurseries. It's an important source of food for native waterfowl and migrating waterfowl here in the winter," Weaver said.

Maryland's mute swan problem began in the 1960s when five birds escaped from captivity.

Today, there are at least 3,000 mute swans in Maryland and more than 10,000 along the Atlantic seaboard. Biologists are fighting back with birth control. Tactics include shaking swan eggs or coating them with oil so adult birds waste breeding time on eggs that will never hatch.

Despite all the problems they cause, mute swans are considered wetland game birds, making it illegal to kill them under Maryland state law.

"We're not going to eliminate mute swans here ... what we're trying to do is manage them at a level so that they can coexist with native wildlife and man," said Larry Hindman of the Maryland Department of Natural Resources.

But some Maryland legislators say birth control is not enough. They want to make it legal for residents to kill mute swans. Critics say killing the beautiful, yet destructive, birds would be a public relations nightmare.