migration (mī-grā'shən) n. The annual or seasonal movement of animals and birds from one location to another.

Triggered by changes in weather, food availability, the length of the daylight period, and other variables, the seasonal migrations of wildlife offer special opportunities to see various birds and mammals gathering, leaving, passing through, and arriving in our national parks. Here are some of the many National Park System units in the 48-state U.S. that are made even more visit-worthy during late fall and winter by wildlife migrations.

This is only a little sample -- a mere scratching of the surface of opportunities across a 393-unit National Park System that is geographically widespread and exhibits tremendous physical, ecological, and cultural diversity. Readers with a deeper interest in the seasonal rhythms of wildlife watching in the national parks will want to research the subject in greater detail.

To lend some degree of organization to this sampling, we'll cite examples of eastern parks and western parks, working generally from north to south within each category.

EASTERN NATIONAL PARKS

Gateway National Recreation Area in coastal New York/New Jersey is situated where several important arterials of the Atlantic Flyway converge , allowing the park to serve as a hub for migrating birds, butterflies, and bats. The Mid-Atlantic Coast is generally oriented north-south, while the New England’ coast is oriented roughly east-west. Long Island lies at the “turning point” or crux, a location that concentrates migrating birds and marine or estuarine species migrating along the east-west oriented New York Bight as well as the north-south oriented Hudson-Raritan Estuary. Shorebirds, wading birds, waterfowl, raptors and other land birds, bats, and even some migratory insects are further concentrated in the Jamaica Bay vicinity of Long Island where they finds open space, wetlands, shorelines, and sheltered open water.

In the Middle Atlantic/Chesapeake Bay region, Assateague Island National Seashore, Colonial National Historical Park, and George Washington Birthplace National Monument are among the parks offering good to excellent shorebird and waterfowl viewing during the fall migration.

The Appalachian Mountains function as a vital migration corridor for raptors, making the region's three major national parks -- Shenandoah National Park, Blue Ridge Parkway, and Great Smoky Mountains National Park -- great places to "hawk watch."

The Monarch butterfly eastern flyway runs along the Appalachians. Thousands of monarchs bound for their wintering grounds in Mexico pass through here.

On Florida's Atlantic Coast, Canaveral National Seashore and the adjacent Merritt Island National Wildlife Refuge are situated in a key gathering area of the Atlantic Flyway where they provide sheltered marshes, mudflats, lagoons and impoundments that serve as resting areas and wintering grounds for a great variety of shorebirds, wading birds, and waterfowl. A typical winter will bring several hundred thousand ducks and coots to the area.

Among the most interesting of the birds that winter at Canaveral/Merritt Island are white pelicans that spend their summers in the Rockies, including Yellowstone.

Just south of Miami, Biscayne National Park provides wintering grounds for some "gentle giants" -- West Indian manatees. Stopover birds are a bonus.

You'll not see them, but it's great to know that northern right whales come to Florida's Atlantic Coast from December to March to calve and rest before returning to their summer feeding grounds in the northern North Atlantic.

Nearby Everglades National Park functions as a critical wintering area or stopover for numerous migratory bird species such as the peregrine falcon, bobolink, tree swallow, ibis, anhinga, and various species of egrets and herons. Like Biscayne, Everglades has wintering manatees.

At Dry Tortugas National Park off Key West, it's a little late now to catch the large flights of raptors (sharp-shinned hawks, broad-winged hawks, merlins, and peregrine falcons ) that gather to feed on the shorebirds in September and October. Mark your calendar for next fall.

The south-flowing Mississippi River forms the core of one of North America's great avian migration routes, the Central Flyway. Since this migration corridor funnels vast numbers of waterfowl and other birds into the lower Mississippi River delta,Jean Lafitte National Historical Park and Preserve is well positioned to provide vital resting and wintering habitat for an impressive number and variety of birds. The park's Barataria Preserve unit, which is located about 20 miles south of New Orleans, has bottomland and levee forest, cypress/tupelo swamp, and freshwater marsh habitats teeming with migrants and winter residents. Canoe tours and nine miles of trails offer ready access.

WESTERN NATIONAL PARKS

Waterfowl, seabirds, and raptors moving along the Pacific Flyway draw many visitors to Point Reyes National Seashore and adjacent Golden Gate National Recreation Area, but nothing like the tens of thousands who come to view migrating whales. Gray whales headed south to their wintering and calving grounds in the Baja California lagoons can be viewed as early as mid-December, but are most numerous in mid-January. If the weather cooperates (not too much fog), many places along the headlands offer good vantage points. The observation platform of the Point Reyes Lighthouse is as good as it gets.

Cabrillo National Monument near San Diego is another good place to watch migrating gray whales. The peak is in mid-January, but you can see whales at Cabrillo from mid-December to late March. The best viewing is from the Old Point Loma Lighthouse and the heights around the park's Whale Overlook.

As birds head south on their annual migrations to Mexico, Central America, and South America, hundreds of thousands pass through parts of Arizona, New Mexico, and Texas. Even though the deserts and mountains of southern Arizona are not situated along one of the continent's four major flyways (the region being in between the Pacific and Central Flyways), it has two of the very best fall and winter birding hotspots -- Organ Pipe Cactus National Monument and Chiricahua National Monument. Migrants account for most of the enormous avian diversity in these parks. At Organ Pipe Cactus, for example, more than 230 of the 270 recorded bird species are transients that take advantage of the desert's warm winters, available food (flowers, fruits, insects), and other habitat assets.

Padre Island National Seashore, a Globally Important Bird Area of astonishing avian diversity (over 380 species recorded), provides an abundance of protected resting and wintering habitat on the Gulf Coast of Texas where the great Central Flyway funnels millions of migrating waterbirds and land birds.

Among the many other western parks with migrants are some in which the seasonal movements of mammals impact the character and quality of "watchable wildlife" recreation. Mammal migrations even pose major managerial problems in some instances.

At Yellowstone National Park, where severely cold and snowy weather can persist for months on end, the transition from fall to winter doesn't just send the bears into hibernation, it also puts the park's elk, pronghorns and bison into motion toward lower places that have more warmth and food and less snow and predator pressure.

Pronghorns, some of the bison, and most of the elk migrate outside the park, and therein lies plenty of trouble. Ranchers, who fear the spread of brucellosis to livestock, don't tolerate the bison that move north out of the park. Fences and rampant development confound the pronghorns, whose peril-ridden annual migration is longer than that of any land animal in the Lower 48. The elk migration corridors and wintering grounds must be protected.

At Glacier National Park, the places you find moose in the winter are not the same places you find them in the summer; they'll travel as much as 50 miles to suitable winter habitat. Here, as in Rocky Mountain National Park , Zion National Park, and other mountain parks of the west, the elk move to lower elevations and herd up for the winter.

Of course, seasonal migrations have not only to do with what you can see in the national parks, but also what you cannot. At Carlsbad Caverns National Park, for example, you won't see great clouds of Brazilian free-tail bats emerging from the big cave's natural entrance after late October because they've left for warmer nesting and foraging sites to the south and won't be back until March. Far to the east in Congaree National Park, you won't see those gorgeous yellow prothonotary warblers and other neotropicals now either, since they've already headed south for winter too.

Postscript: It is admittedly awkward to use the four major North American flyways for organizational purposes in this particular context. This flyway categorization, which has been in use since 1948, was created for migratory waterfowl research and management purposes, not for general application to migratory avian species. Some of North America's most ecologically important bird migrations take place outside the four major flyways. One good case in point is the movement of neotropical species through the deserts and mountains of southern Arizona. Another is the seasonal migration of raptors along the axis of the Appalachian Mountains.