CHARLOTTE -- Thousands of water birds that normally spread out across Lake Champlain are seeking refuge in the channels left by two ferry routes that carry passengers between Vermont and New York during this bitterly cold winter.

Bird watchers have been drawn to the Essex, N.Y., landing of the ferry from Charlotte in hopes of catching a glimpse of sometimes-rare birds that are usually scattered across the length of the 120-mile lake. During a winter of below-zero temperatures, the ducks, bald eagles and other birds have been forced to scour the open water of the channels for food.

"They are surviving the winter in a lake that's over 100 miles long that right now is down to five puddles," said Ian Worley, a retired University of Vermont environmental studies professor who goes birding along the lake two to three times a week.

It's the first time the lake has frozen since 2007 and it's created a paradise for birders, who peer through the eyepiece of a scope to watch birds foraging for the zebra mussels, fish, plants or other animals they need to survive.

"The lake -- as it ices over and pulls the birds into this little isolated place -- also pulls the possibility of uncommon or rare or really rare species right to you as well," Worley said.

Birders on the New York side of the ferry crossing are eager to spot the single tufted duck, which is common in Europe and Asia but exceedingly rare in the eastern United States. The duck is spending this winter in the lake among the more-familiar mallards, black ducks and common goldeneyes.

"We like things that are hard to find, but we can find," Worley said. "The birds are really, really important in telling us what's going on in the world. If we know that a bird is beginning to occupy a new place or disappearing from an old place, we want to know why."

The birds can also be found along the route of the Grand Isle-Cumberland Head, N.Y., ferry, about 30 miles north and in a narrow stretch of water between Vermont's South Hero and North Hero island in Lake Champlain.

The birders have been a common sight for the crew of the Grand Isle, the Lake Champlain Transportation Company ferry that makes its hourly run as long as it's able to keep the channel open, said ferry captain Bill Pinney.