ON LAKE CHAMPLAIN --Young Island once was prime duck-nesting habitat, a green gem floating off Grand Isle. The small state-owned island is now more like a war zone.

All the trees are dead and their leaves and twigs have been plucked off. The island is dotted with huge, white splotches of bird guano. Thistle, yellow rocket and sheep's quarters - weeds of waste places - are the only green plants that can survive in this inhospitable place.

The culprits responsible for this Jekyll-and-Hyde transformation are the 3,200 double-crested cormorants and more than 20,000 ring-billed gulls that nest on the island. Over the past two decades, biologists have watched with a mix of fascination and horror as the numbers of both species, especially cormorants, have skyrocketed.

Although gulls outnumber the cormorants, scientists worry about the cormorants because they are so much bigger and more destructive of fish and the trees they defoliate to build their nests. As their numbers grow, the birds not only destroy an island's ecosystem, they prevent others from nesting, particularly ducks favored by hunters.

Cormorants started nesting on Lake Champlain in 1982. Researchers estimate as many as 3,000 nesting pairs and 1,500 juveniles that are too young to breed are living on six islands on the lake. Researchers from the University of Vermont are studying the birds to understand how much fish they take from the lake and what makes their populations grow so rapidly.

In a separate effort, state biologists are trying to come up with a plan for slowing down the rapid expansion of cormorants to other Lake Champlain islands. Last week, the state and the federal Animal Damage Control Program worked together on privately owned Bixby Island, just off Young Island, to stop cormorants from nesting there.

In the fall, the state plans hearings to ask residents what they think should be done - if anything - to prevent the birds from colonizing more of the lake's 75 islands. "Cormorants are part of Lake Champlain’s ecology,'' said Larry Garland, district wildlife biologist for the state, ''but we want to protect the biodiversity of the islands on Lake Champlain.''

Not for the squeamish

Young Island is one of the few places on the lake where yellow perch rain out of the sky. These are not yellow perch you'd want to eat; they've already been eaten. Twice. Once by an adult cormorant, and again by a cormorant chick.

But when visitors walk underneath or near the 1,540 cormorant nests on the island, the baby birds defend themselves with the only ammunition they have. They cough up their breakfast.

While the birds' habit might be disgusting to humans, it's a useful way for UVM graduate student Margaret Fowle to find out what cormorant parents feed their babies.

Now in her second summer of research, Fowle says adult cormorants eat about a pound of fish a day - almost exclusively yellow perch. That's contrary to the impressions of many anglers, she says, who believe cormorants eat stocked trout and salmon. ''I've been following stocking trucks all spring, and I've never seen anything close to a flock of cormorants feeding,'' she says.

Fowle's work also has given researchers clues as to why cormorants are doing so well. Cormorants are extremely good parents, says David Capen, Fowle's supervisor at UVM. ''They never take time out during the breeding season,'' Capen says.

Another factor: Lake Champlain cormorants spend the winter in the Gulf states, where they feast on farm-raised catfish and survive in record numbers, Capen said. Cormorants are a coastal and mid-continent species that has been expanding into Vermont.

Toxic guano

Bixby Island is just 300 yards north of Young Island, but the two could be a thousand miles apart. Young Island is defoliated; Bixby Island is a haven of green.

This year, for the first time, biologists found cormorants nesting on Bixby - about 150 pairs, up from none last year - and biologists have seen new ways in which cormorants can change an island. It's well known that the birds' guano around their nests like huge piles of dried plaster of Paris, kills trees outright, but cormorants on Bixby Island were speeding up the process this spring by literally plucking leaves off the trees, Capen said.

That might be because the birds need sticks to build their big, messy nests. Cormorants prefer to nest in trees, but will build on the ground, sometimes building the nests as tall as 2 feet. Nests have to be strong, since the adults and young birds combined can weigh a total of 12 pounds.

Bixby Island owner and real estate developer Jeff Davis said he was concerned about the cormorants' taking over and displacing the mallards, gadwalls and widgeons that nest on the island.

''What is at stake is that in two or three years, we will look like Young Island,'' Davis said. ''I don't think that is healthy, environmentally, economically or otherwise.''

Davis gave the state and the U.S. Department of Agriculture's Animal Damage Control Program permission to remove the nests from the island. The action requires a federal permit, which the state has.

Late last month, the South Hero Fire Department participated in the most unusual call it's ever had. Using the fire department's pumps, biologists blew the nests out of trees with high-pressure water. The nests contained eggs but no live birds, Garland said.

The effort was somewhat successful, but wasn't able to reach nests higher than about 50 feet. Garland says he thinks removing the nests won't discourage the birds from returning to Bixby's inviting green trees, which is why the Fish and Wildlife Department will ask Vermonters for their opinions.

''We're not trying to eliminate cormorants from Lake Champlain,'' Garland said, ''but there should be a balance.''