Seasons in the Chesapeake Bay region are marked not just by colors in the landscape, but by movements in the sky.

From as far north as the cold treeless plains of the Arctic tundra, birds take wing in search of southerly grounds. By fall, they begin to appear in the skies over and around the Chesapeake Bay. There are tundra swans, Canada geese, snow geese, as well as hawks, ducks and gulls that won't be seen in the area by spring.

Some of these species are passing through as they travel the north-south route known as the Mid-Atlantic Flyway. Others linger along the Bay and its rivers for the winter. Either way, their dramatic forms in the air and the commanding presence of large flocks on rural fields are a signature element of the Chesapeake experience.

Mark Johnson, president of the Maryland Ornithological Society, said that migratory birds aren't drawn to the Bay because they are cold.

"It's all about food. It has nothing to do with birds getting cold," Johnson said.

When water freezes in their breeding grounds, the birds lose access to the fish and aquatic plants that sustain them. The marshes of the Bay have long been a favorite solution. Geese have especially come to rely on farm fields, drawn to grain left behind by mechanical pickers.

Most migratory species are creatures of habit, typically returning to the same spots to breed and spend the winter each year. But a frozen Bay or food shortage will drive birds farther south.

The return flight takes place in early spring with uncanny accuracy, triggered by the amount of daylight. "It happens within a few days every year. It's incredibly consistent," Johnson said.

Flocks of Canada geese, with their distinctive honk and long black neck, are a familiar sight during winters in the Chesapeake region and some have become year-round residents.

The white tundra swan is known both for its large size and striking beauty. Its black beak and generally straight neck contrast with the orange beak and curved neck position of the mute swan. The mute swan is a nonnative species found year-round in the Bay region that competes with native waterfowl for food and habitat.

In the early 1970s, an average of 40,000 tundra swans would winter along the Bay. A decline in underwater grasses has cut that number in half as tundra swans increasingly seek out grounds in southern Virginia and North Carolina.

Many birding enthusiasts say that fall and winter are the best time to grab binoculars and go outdoors. Along with swans and ducks, raptors like hawks and eagles are especially active and visibility can be better because less tree foliage obstructs the view. Many birders also prefer warm clothes to 90-degree weather and mosquitoes.

Curiosity is all you need to get started-but a good field guide can help, too. Get to know the Chesapeake's winter birds before spring moves in, and the birds move on.

Birding in the Bay Region

Options for fall and winter birding in the Chesapeake region range from local parks to state wildlife preserves and national refuges. Experienced birders list the sites below among their favorites.

For more choices, visit the Chesapeake Bay Gateways Network website at www.baygateways.net, the National Wildlife Refuge locator map at www.fws.gov/refuges/ or the website for the natural resources management agency in your state.

Pennsylvania

  • Middle Creek Wildlife Management Area, on the Lebanon-Lancaster county line, is possibly the best place in Pennsylvania to view dramatic numbers of migrating tundra swans and snow geese.
  • Muddy Run Recreational Area, at the Conowingo Reservoir in Lancaster County on the lower Susquehanna River, is known for viewing snow geese, tundra swans and American black ducks.
  • At Waggoner's Gap, south of Harrisburg, hawk watchers count roughly 20,000 raptors during each migration season from August through December.

Virginia

  • At Chesapeake Bay Bridge Tunnel in Norfolk, birders can (and must) obtain official permission to stop on the islands and view migrating sea ducks such as scoters, common eiders, harlequins and shorebirds like the purple sandpiper and great cormorant. Visit www.cbbt.com/birding.html
  • Dameron Marsh & Hughlett Point Natural Area Preserves, in Northumberland County on the Northern Neck, contain significant wetland habitat for marsh bird communities and large numbers of geese, ducks, shorebirds and tundra swans.
  • Kiptopeke State Park & Eastern Shore of Virginia National Wildlife Refuge, both at Cape Charles on the tip of the Eastern Shore, are known for hawks, osprey and large congregations of migrating birds.
  • Potomac River National Wildlife Refuge Complex in Woodbridge is managed especially for bald eagle habitat.

Maryland

  • Conowingo Dam, on the Susquehanna River, is known for large numbers of eagles, wintering gulls and great blue heron.
  • Blackwater National Wildlife Refuge, near Cambridge on the Eastern Shore, is one of the chief wintering areas for Canada Geese as well as ducks and tundra swans on the Mid-Atlantic Flyway. It is also a hub of activity for bald and golden eagles.
  • Deal Island Wildlife Management Area, in Somerset County on the Eastern Shore, is one of the best places in Maryland to find ducks, geese and swans, as well as the rough-legged hawk, short-eared owl, black- necked stilt and rare Eurasian wigeon.
  • Eastern Neck National Wildlife Refuge, in Rock Hall on the Eastern Shore, is a major feeding and resting place for more than 100,000 migrating and wintering waterfowl.

Christmas Bird Count

While some people count the 12 days of Christmas, others count birds.

Beginning on Dec. 14 every year, birders take to the fields and forests for the annual Christmas bird count sponsored by the National Audubon Society.

The event began in 1900 as an alternative to a traditional competitive bird hunt that took place around the same time. Now, tens of thousands of people volunteer to count birds rather than kill them.

"For many of us, it's the high point of the birding year," said Tom Saunders of the Northern Neck Audubon Society in Virginia. "Lots of people I know like to do several events and travel around a bit."

Volunteers work in defined geographic areas or "counting circles," each with a 15-mile diameter. They observe bird activity in the circle during a 24-hour period, counting every bird they see or hear and noting the species.

Volunteers often follow specified routes through the circle, but people who live within one of the circles can arrange to count birds at their feeders and submit the data they collect.

In 2010, counts took place in all 50 states, all Canadian provinces, plus 107 count circles in Latin America, the Caribbean and the Pacific Islands. Birders in Haiti participated for the first time.

Audubon's chief scientist Gary Langham calls the Christmas Bird Count a globally recognized example of "crowd-science" that produces valuable data. "Data from Audubon's Christmas Bird Count are at the heart of hundreds of peer-reviewed scientific studies," Langham said.

The data provides insight on long-term trends, including species most in need of conservation action. It has also helped to document the return of the bald eagle, the decline of wintering populations of the American black duck and the more recent impact of climate change on birds.

A special appeal of the Christmas Bird Count is the learning opportunity.

"Do you need experience? Absolutely not," said Mark Johnson, president of the Maryland Ornithological Society. "All you need is a desire to participate and some warm clothes."

Event coordinators ensure that new participants are paired with veteran birders, providing a quality check on the collected data and a fun way for experienced birders to share their knowledge with others.

The Christmas Bird Count runs through Jan. 5 of each year. There is a $5 fee for participants 19 years and older.

A searchable list of counting circles and count dates, arranged by state, is posted each fall at www.audubon.org.