Birds are the nomads of the animal world, ceaselessly traveling with the change in seasons. Perhaps it is the burden they carry in exchange for the gift of flight. Though some birds can remain in one area throughout the year, most are constantly moving to find adequate food, habitat and weather conditions.

After the autumnal equinox, days gradually shorten in the Northern Hemisphere. Cold Arctic air pushes farther south. The breeding grounds of northern United States and Canada grow inhospitable. The skies fill with birds migrating to more temperate climates for the winter.

These seasonal migrations allow birds to avoid harsh climate conditions and take advantage of available food sources.

In North America, birds migrate four main routes or flyways. The Atlantic, Central, Mississippi, and Pacific flyways run north to south from North America to South America.

These "rivers of birds" are not specific lines the birds follow but broad areas which the birds migrate through. The Chesapeake Bay watershed lies within the Atlantic flyway and for some, particularly waterfowl, is their winter destination.

The Bay provides a wintering site for much of the nation's waterfowl. Close to 1 million waterfowl winter on the Bay between October and April.

Swans, geese and ducks from Alaska, Canada, north central United States, and New England seek out the wetlands, shorelines and open water of the Bay for food and habitat. In winter, waterfowl may eat up to their own body weight in food each day. Migrating waterfowl also need food reserves to fuel their migrations back to northern breeding grounds in the spring.

Swans are the largest of waterfowl and the tundra swan travels the farthest, more than 4,000 miles, to winter primarily on the eastern shore of the Bay. Autumn would not be complete without the V-shape of migrating geese in the sky. Constant honking signals the arrival of the familiar Canada goose. A lesser known visitor is the snow goose. As its name implies, the snow goose is white except for black wing tips and pink feet and bills. The small, black Atlantic brant also winters here.

A long list of ducks winter on the Bay, depending on the various aquatic habitats for food. Species of wintering ducks include the mallard, black duck; American widgeon; green-winged teal; northern pintail; northern shoveler; canvasback; redhead; greater and lesser scaup; old squaw; white-winged, surf, and black scoter; red-breasted, hooded, and common mergansers; and the ruddy duck.

Midwinter Waterfowl Surveys, conducted each year in the first two weeks of January, have tracked the status and changing species composition of waterfowl that overwinter in the Chesapeake Bay watershed since 1955.

Data from these surveys have uncovered several trends. In the past 15 years, from 1981-1996, there has been a marked increase in the number of American widgeon, green-winged teal, canvasback, ring-necked duck, bufflehead, and ruddy duck.

The brant and snow goose have increased on the Bay. Cold late springs on the tundra continue to plague Canada geese with poor reproduction. Although there is a slight decline in the tundra swan, they are still above historic population levels.

Waterfowl numbers in the Chesapeake Bay are a result of available waterfowl foods, such as submerged aquatic vegetation, and the climatic and habitat conditions over the central and eastern United States and Canada, and even Alaska, where these birds breed.

The conditions of wetlands in the prairie pothole region, a major spring breeding area for many ducks, has been good for the past few years. Abundant rain has created more ponds for the birds to breed on and more habitat to raise broods.

Additional ponds also mean that predators, such as raccoons, skunks and foxes, have to search larger areas for food and take a smaller percentage of nests. While the increase in the numbers of ponds is largely a result of good rainfall, there are also a number of programs, such as the North American Waterfowl Plan, which brings together state, federal and private programs to enhance and restore millions of acres of wetlands.

Another program, Partners for Wildlife, has restored or enhanced hundreds of thousands of acres since 1987.

Nationwide, more than 80 million ducks will fly south this fall, according to preliminary estimates. Boosted by 3 years of plentiful rain and snowfall in key nesting areas, duck breeding populations climbed 5 percent this spring, the highest level since 1979.

In the late 1980s, droughts reduced the number of breeding ducks. The recent increase in ducks can be attributed to abundant precipitation and good habitat conditions on the breeding grounds. Also, hunters and other conservationists have spent the last decade restoring and conserving vital wetlands in key duck production areas. Although duck populations naturally fluctuate over time as habitat and water conditions change, this kind of recovery from drought would not have occurred if not for habitat conservation efforts.

Many of us do not live close enough to the water to enjoy the variety of waterfowl that winter here. National Wildlife Refuges provide some of the best opportunities to observe and photograph waterfowl and other wildlife.

In Delaware, visit Bombay Hook NWR in Smyrna; in Maryland stop by Blackwater NWR, in Cambridge; or Eastern Neck NWR, in Rock Hall; and in Virginia, visit Back Bay NWR, in Virginia Beach; Chincoteague NWR, in Chincoteague; Eastern Shore of Virginia NWR, in Cape Charles; Mason Neck NWR, in Woodbridge; or Presquile NWR, in Hopewell.

National Wildlife Refuge Week, Oct. 5-13, 1996, is timed to coincide with the annual fall migration, when millions of birds will fly into refuges in the lower 48 states on their way south. Birdwatching should be at its best. Other recreational events planned for the week include birdwatching walks, wildlife art displays, photography exhibits, slide shows, nature demonstrations and more.

For information on National Wildlife Refuge Week and a brochure with a complete map of the refuge system and basic visitor's information for every refuge, call 1-800-344-WILD.