They're back. Local skies are filling with migrating birds beginning their annual fly-in and short-term stopover in the Chesapeake Bay.

The bay is the Atlantic Coast's most important migration and wintering area for waterfowl. The Chesapeake and its 19 major tributaries -- including the Patuxent and Potomac rivers -- provide a winter habitat for 24 species of ducks as well as Canada geese, greater snow geese and tundra swans.

As recently as 1950, half of the continent's population of canvasback ducks -- about 500,000 -- wintered in Chesapeake Bay, relying on aquatic grasses as their primary food source.

Now, about 50,000 canvasbacks come every year. "Long-term worsening of the bay's water quality and loss of wetland habitat have contributed to declines in wintering waterfowl populations here," explains Chris Swarth, director of the Jug Bay Wetlands Sanctuary on the Patuxent River and coordinator of an annual bird survey there.

According to Chesapeake Bay Foundation estimates, only about 65,000 acres of submerged grasses remain in the bay, down from historic levels of more than 600,000 acres. Canvasbacks and other ducks dependent on these grasses have had to switch their foraging efforts to small clams found on the shallow bottom of the Chesapeake and its tributaries.

Annual bird counts, Swarth says, "may tell us how much declines in SAV [submerged aquatic vegetation] have affected wintering ducks."

Half a century ago, 4 to 5 million birds spent some time in the Chesapeake Bay during the winter. Now, the number is about 1 million a year.

Nonetheless, says Judy Burke, who is a naturalist at the Jug Bay park and conducts the counts with Swarth, "for now, we should be thankful because we live near one of the best places on Earth to see waterfowl in winter and as they migrate in and out during fall and spring."

On the Move

Most bird species in North America migrate, but "perhaps because waterfowl are more visible than other species in migration, they epitomize this phenomenon to most people," writes Frank Bellrose in Ducks, Geese and Swans of North America.

Only a few waterfowl species, such as Florida ducks, are nonmigratory. And in the southern part of their ranges, wood ducks and hooded mergansers are resident throughout the year. Some groups of mallards, Canada geese and common eiders migrate only short distances or not at all when conditions for staying put are favorable.

On the whole, however, waterfowl make "tremendously long migratory flights," Bellrose writes.

Lesser snow geese and pintails that breed in Siberia spend the winter in the Central Valley of California. Black brant that nest along Queen Maud Gulf in the Canadian Arctic migrate to Baja California. Pintails and American wigeons, most of which come from Alaska, regularly winter in the Hawaiian Islands.

Migrating waterfowl fly continuously unless forced to land by exhaustion or bad weather. Some species make nonstop flights of as far as 3,000 miles. Most waterfowl migrate at speeds of 40 to 60 mph. A flight of 2,000 miles at 50 mph would take an individual duck about 40 hours.

"We humans can't even keep a plane in the air that long," Swarth says. "Waterfowl in migration perform quite a feat."

Migrating waterfowl fly at altitudes ranging from a few feet above the ocean to more than 20,000 feet. The lowest migratory flights, according to Bellrose, are those of sea ducks over the ocean.

But other waterfowl species fly high enough to clear the lofty peaks of the Rocky Mountains, some of which exceed 14,000 feet. In general, the longer the flight, the higher the altitude. Airplanes work the same way.

Where Are We?

Why do waterfowl migrate, and how do they find their way? Most wouldn't survive if they didn't migrate because northern water areas where they breed freeze, making food impossible to reach. However, ducks migrate as short a distance as possible to find open water and food. These conditions change annually, so migrants alter their wintering areas accordingly.

If nature provides a closer alternative, waterfowl are quick to notice. When hurricanes opened densely vegetated coastal marshes in Louisiana, tens of thousands of ducks responded by wintering there rather than continuing across the Gulf of Mexico to Yucatan.

Like many other birds, waterfowl can return to the same breeding, migration and wintering areas they visited in the previous year or years.

"In acquiring their navigational abilities," Bellrose marvels writes, "each species of waterfowl has evolved different usages of cues and different degrees of dependency upon them. Mallards that migrate short distances show a different use of sun and star cues than do blue-winged teal, for example, which migrate much farther."

Waterfowl depend heavily on landscape as they navigate across the continent, which works fine during daylight hours. But scientists have noted cases in which ducks migrating at night have "overshot" their intended stopping point, only to have to retrace their wingbeats miles backward the next day.

Waterfowl, however, also migrate across trackless regions where the landscape provides few clues. And they migrate on nights so dark with heavy, low clouds that places where land meets water are nearly invisible. Landscape alone, therefore, can't be the only way in which waterfowl successfully find their way southward or northward.

Although scientists still don't know all of the answers, they believe that waterfowl and other birds have a sort of internal clock enabling them to adjust for the changing position of the sun in the sky and that they can follow star patterns to find direction.

Beyond that, some species can navigate at night even within cloud layers, without references to landscape or celestial clues. Evidence suggests that they use Earth's magnetic field when other sources are unavailable.

Most waterfowl migrate along corridors, the well-known "flyways." Four major routes pass through the United States:

The Pacific Flyway, which runs north-south along the West Coast; the Mississippi Flyway, which leads from the bays of northern Canada and the High Arctic to the Gulf of Mexico; the Central Flyway, which leads from northwestern Canada along Central America to the Yucatan Peninsula; and the Atlantic Flyway, which funnels waterfowl from central and eastern Canada along the Atlantic Coast to Florida.

Chesapeake Bay is a major destination point along the Atlantic flyway, particularly for wintering ducks.

Bird Nursery

Many of the Chesapeake's wintering ducks began life in what's known as the prairie pothole region, which extends from the midwestern northern tier states into Canada. There, about half of North America's ducklings are hatched.

When the Wisconsin ice sheet of the last glacial period started retreating northward about 15,000 thousand years ago, tens of thousands of landlocked icebergs were left in its wake, writes Michael Furtman in On the Wings of a North Wind: The Waterfowl and Wetlands of North America's Inland Flyways.

These small icebergs melted into the soil. As they faded, "they became the foundation of the prairie pothole region. An estimated 10 million glacially carved depressions once pocked the landscape of the prairie-pothole region of the United States and Canada." As the climate warmed, the potholes evolved into a habitat so enticing that more than 130 bird species have used a single pothole system in a given year.

Ducks were likely among the first bird residents. With millions of potholes from which to choose, waterfowl had plenty of room to find nesting sites.

"The diversity of potholes, ranging from small spring ponds to large permanent wetlands, provided ducks with the various habitats necessary for each specific stage in their breeding and brood-rearing cycles," Furtman writes.

As land in this region gave way to agriculture, however, the number of potholes decreased over the last 40 years. In North Dakota's pothole region, where as many as 100 wetland basins per square mile once existed, "60 percent of the original 5 million acres of wetlands have been lost," Furtman says. "Ninety-five percent of that loss is attributable to agriculture."

Hot Times?

If increasing agriculture isn't enough of a problem for waterfowl during their nesting season, some researchers think that rising global temperatures may result in more frequent and severe drought conditions in the prairie pothole region, with a devastating impact on breeding ducks.

Lisa Sorenson, a biologist at Boston University, predicts that breeding duck populations may decline by about one-third in the U.S. portion of the prairie pothole region if carbon dioxide concentrations in the atmosphere rise to 50 percent above pre-industrial levels.

These levels currently are about 360 parts per million by volume, compared with about 280 ppmv in the early 18th century, a 35 percent increase.

According to estimates by the U.S. Fish and Wildlife Service, fewer ducks than usual flew south last fall. The group's estimate was that about 84 million ducks would make the ages-old journey from northern breeding and nesting grounds to wintering areas in the southern United States, Mexico and South America, down from 92 million in 1997.

The lower numbers were attributed to a dry winter of 1997-98 and a warm, early spring last year on prairies of the north-central United States and south-central Canada, where the majority of ducks breed.

During the drought years of the 1980s, U.S. duck populations plummeted to an all-time low of 27 million. In more recent years, some of which have had above-average precipitation, duck numbers have rebounded to about 35 million.

"As long as there is adequate habitat in place "waterfowl are able to make up for these losses during wet years," says Bruce Batt, chief biologist of Ducks Unlimited, a waterfowl conservation organization headquartered in Memphis.

But that's not what it looks like to Jim Holland, who lives along the Patuxent River.

"Forty or 50 years ago," the longtime resident of Huntingtown, Md., recalls, "this whole area was just full of canvasbacks. But no more. That's why I support this effort to find out what's happening to `our' ducks here on the river and out in the Bay.

"I'd like to see the days come back when the red heads of canvasbacks stood out in the sun as far across the river as you could look. In the dead of winter, it's the ducks that really brighten things up out here."

Canvasbacks, ruddy ducks, scaup, mergansers and the many other waterfowl that winter in the Chesapeake region have come a long way to get here, Holland says.

"The least we can do is to show 'em some hospitality by making sure they've got enough to eat and that their environment is a healthy one," he says. "Come spring, they'll have to face a different set of problems up north. Winter should be a time of rest -- for people and for ducks."

Cheryl Lyn Dybas is a Washington-area science writer specializing in aquatic biology.