DURING THEIR fall and spring migrations, vast flocks of waterfowl pass over the Chesapeake Bay area, pausing briefly in their flight at several nearby wildlife refuges. But even in winter, large numbers of birds--Canada and snow geese, whistling swans and a variety of duck species--can readily be seen on an afternoon's drive or hike through the refuges.

At the 14,263-acre Blackwater National Wildlife Refuge, about 12 miles south of Cambridge on Maryland's Eastern Shore, as many as 25,000 Canada geese and 2,000 snow geese customarily winter over between November and April, though a heavy freeze might send them farther south at any time. The brackish tidal marshland and thickets are also home to the great blue heron, about 30 bald eagles and sometimes one or two golden eagles, which have been spotted there.

On the 2,285-acre marsh and timbered island that is the site of the Eastern Neck National Wildlife Refuge (easily visible from the eastern end of the Chesapeake Bay Bridge, but a 60-mile roundabout drive to get there), a winter visitor should be able to pick out whistling swans as well as canvasback, mallard, black and other ducks. Both refuges also are among the few remaining habitats of the rare and endangered Delmarva red fox squirrel.

The two refuges in Maryland are part of a nationwide system of 413 reservations administered by the U.S. Fish and Wildlife Service, which has just published a visitor's guide and map to them.

The refuges are listed by state, with a chart that indicates the best season for wildlife viewing, what facilities (such as visitor's center) are available, and whether hiking, camping, fishing, swimming or other activities are permitted. The guide also lists the address of each refuge, where more specific information can be obtained.

At Blackwater, for example, a 4 1/2-mile paved roadway makes a loop through the marshland for a self-guided auto tour. Along the way, visitors can take a couple of short hikes along marked nature trails or climb the 40-foot observation tower. The initial view is often of masses of geese and ducks floating on the numerous ponds, but after a few minutes the visitor begins identifying individual species. The visitor's center is open daily from 7:30 a.m. to 4 p.m. (except summer, when it is closed on weekends).

Hiking trails take the visitor across the fields and marshes and around the coves and ponds of Eastern Neck, where the peak viewing seasons are during spring and fall migrations. Birdwatchers can pick up a brochure listing more than 225 species of birds that have been identified on the island. Crabbing and fishing are also possible, and boat-launching facilities are available. There is no visitor's center, but brochures and other information can be obtained weekdays from 7:30 a.m. to 4 p.m. at the refuge office or at the trail head.

In the Chesapeake and coastal areas near Washington, the guide lists six refuges in Virginia--Back Bay, Mackay Island, Chincoteague, Great Dismal Swamp, Mason Neck and Presquile--and two in Delaware--Bombay Hook and Prime Hook. Numerous others are located within a day's drive north and south along the Atlantic Coast.

To obtain the "Visitor's Guide to the National Wildlife Refuges," send $2.25 to the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. Specify Stock No. 024-010-00529-7.

TWO--WHEELER'S CHINA: As if a trip to China is not sufficiently exotic, a tourist can add even more adventure by seeing the countryside on a 10-speed bicycle. After all, millions of Chinese rely on bicycles to transport themselves. Two-wheelers, not autos, clog the roadways.

For the third year, American Youth Hostels is offering several three-week cycling tours, focusing primarily on the countryside around Canton in the subtropical province of Kwangtung. Typically, travelers fly to Hong Kong for a one-day stay, cruise up the Pearl River overnight to Canton for a visit of a couple of days and then spend the next two weeks on a loop tour outside Canton. At the conclusion, the tour flies to Peking for three days before returning to Canton and Hong Kong.

The route tends to be fairly flat, says a spokeswoman, and cyclists cover 50 to 60 miles per day. A "sag wagon" carries luggage or a cyclist whose legs are too weary to keep pedaling. Along the way, stops are made at communes, hospitals, schools and factories. Accommodations are in two-person rooms in tourist hotels. Groups generally are kept to between 10 and 20 cyclists, and the trips are open to adults over 18 (teen-agers, if accompanied by a parent). Travelers in their 60s, and even older, have made the trip.

Upcoming departure dates: Jan. 31, April 16, July 9, Aug. 13 and Oct. l. The land costs (except Hong Kong meals) are $1,699 for the January departure, $1,999 for July (two days longer) and $1,799 for the others (air fare extra). A Chinese guide and a translator from Hong Kong accompany each group.

It is a way, says one woman who made the trip, to escape from behind the tour bus window and come to know intimately "the sights, sounds and smells of the villages and country lanes."

For more information: American Youth Hostels, Metropolitan New York Council, 132 Spring St., New York, N.Y. 10012.

TICKETS ABROAD: Add Copenhagen to the short list of European cities (London, Vienna among them) where it has become easier to book theater tickets in advance. Scandinavian Airlines has announced it will make reservations for its passengers to many opera and ballet performances at Copenhagen's Royal Theatre.

For visitors in the Denmark capital between Jan. 28 and Feb. 5, the airline also can arrange backstage tours at selected performances when tickets are purchased through SAS.

OLD COUNTRY AUDITION: It's not the Grand Tour of Europe, but the theatrically inclined have a chance at six months in The Old Country of Busch Gardens in Williamsburg, Va.

Talent scouts from the theme park are visiting 13 cities in the East seeking singers, dancers, musicians, actors, variety artists and technicians (age 18 and over) to fill 200 full-time jobs for the six-month 1983 season.

Auditions are scheduled in Washington on Saturday and Sunday, Jan. 22-23, from 1 to 5 p.m. at the Kennedy Center, Rehearsal Room No. 1.

WINTER RIDE: Vermont, which for years has lured downhill and cross-country skiers, is making a bid for snowmobile enthusiasts. The state boasts 1,800 miles of groomed snowmobile trails, which are maintained by the 15,000-member Vermont Association of Snow Travelers.

One volunteer group, the Mountain Tamers Snowmobile Club, which is responsible for about 90 miles of trails in central Vermont, is offering midweek tours in February (snow conditions permitting) to beginning and experienced snowmobilers, especially those from outside the state. The day-long outings can be tailored to the group, but most will cover the club's 54-mile loop through the wooded countryside near Calais and Woodbury.

Two guides, one to lead and one to follow, accompany each tour, and a campfire lunch can be arranged along the route. The tentative cost is $20 per person, with proceeds going to the club's fund for trail-grooming equipment.

"We ride the ridge tops. You can see for miles and miles," says organizer Milan Cobb Lawson, district engineer for the state highway system, a snowmobiler for almost 10 years. One catch: You have to provide your own snowmobile.

For more information: Vermont Agency of Development and Community Affairs, Montpelier, Vt. 05602, (802) 828-3236 or the Mountain Tamers' tour committee, (802) 223-7061.

SNOW COUNTRY: Closer to home, Pennsylvania claims 3,000 miles of designated snowmobiling trails, including several marked trails that are more than 100 miles long.

And for those who prefer a quieter means of exploring the winter landscape, the state lists 22 commercial cross-country ski centers, 23 alpine areas with touring facilities and 600 miles of designated trails in 62 state parks and forests.

Information about these sports facilities can be obtained from several state sources:

* For a copy of the Pennsylvania Snowmobile Trail Directory, write: Snowmobile Unit, Department of Environmental Resources, P.O. Box 1467, Harrisburg, Pa. 17120. The hot line for snowmobile trail conditions is (717) 787-5651.

* For a list of cross-country centers, write: Winter Sports Brochure, Pennsylvania Bureau of Travel Development, Dept. KCC, 416 Forum Building, Harrisburg, Pa. 17120.

* A guide to trails in Pennsylvania park lands is available by writing: Cross-Country Skiing in State Parks and Forests, Press Office, Department of Environmental Resources, P.O. Box 2063, Harrisburg, Pa. 17120.