Migration, the coming and going of fish and fowl, suits the Chesapeake Bay well. The bay itself has, in a sense, been "coming" for several thousand years, since the influence of the ice age last waned. High seas fed by melted polar ice flooded the narrow, ancient valleys cut by the Susquehanna and James rivers, forming our present-day broad estuary. Such warming periods are brief, geologically speaking, and last for only tens of thousands of years. It will not be long, on planetary time scales, before the glaciers advance, and this bay, too, ebbs surely as the tide.

On a smaller, daily scale, coming and going is the bay's nature. Tides surge up and down the estuary, rising and falling every six hours or so like the respiration of a great sea creature.

So it seems especially fitting that our bay is embroidered fantastically with nature's cycles of migration. We tend to celebrate the bay for the quantity and diversity of life that is here; yet fully as enriching are the near-constant leavings and arrivals: The passage of wild geese in autumn and the appearance of soft crabs in spring reassure us and reset our clocks, if only briefly, to a vaster frame of reference.

There is scarcely a month or a week on the Chesapeake Bay when something is not moving in or out, or preparing to do so.

FEBRUARY

The great blue herons come early to nest, bringing welcome reassurance of spring. Once on a February day, I was hiking a bay marsh on one of the nastiest, windiest, sleetiest days I can recall. The winter had been hard, and looked as if it had staying power. Then overhead, I heard a guttural croaking. A pair of great blues circled on the storm, heading for a clump of tall trees where last summer's nests were piled high with snow and ice. What powerful urges must lure them from the balmier Carolinas and Florida at such times. They are certain, even as winter grips hardest, that warmth and food aplenty will soon emerge to ensure the survival of another generation of Chesapeake-born chicks.

MARCH

By St. Patrick's Day, the ospreys have begun to show. Like the herons, they do not come and go in the spectacular masses of the waterfowl; but they are nonetheless great travelers. These large fish hawks, whose nests crown thousands of bay channel markers and buoys, commonly live for 15 years. In that time they may travel more than 100,000 miles. Eastern Brazil is the commonest wintering destination for Chesapeake ospreys, but some go as far south as Uruguay and Chile.

APRIL

The rivers start filling with traffic. Most of the anadromous fishes -- those that run from the sea up rivers to spawn -- are now schooling in the river mouths. White perch, yellow perch, hickory and American shad are all pushing to get on with it.

My personal pick is the common river herring, Alosa pseudoharengus, a silver, 10- to 12-inch specimen, too bony for anything but pickling. The little alosa thrashes its way to the outer ends of the bay's tributaries to lay its eggs. I can't think of a better place to be in April than camped beside a herring stream when squadrons of alosa begin finning across the sun-spangled sand and gravel shallows.

The alosas' appearance represents the end of a journey that began with the herring somehow homing from hundreds of miles out to sea onto the precise little stream where they were born.

MAY

Busy, busy, busy, but I'll pick the rockfish anytime as May's migrant. A rockfish can weigh more than 75 pounds and attain lengths of several feet. They are, except for the rare stray shark, the biggest sea-goer that journeys up the bay's tributaries. It is an awesome experience to be afloat in a canoe on a quiet, dark reach of a river and hear the silence of a May evening shattered by the rolling of a huge cow striper, attended by a dozen smaller, leaping males.

Pollution and overfishing in the bay have caused a serious decline in anadromous species over the years, but the worst of the fishing excesses have been stopped: The rock are recovering nicely, the herring are hanging in there (though at much-reduced levels), and the shad may be mounting a slow comeback. There is even an experimental effort to bring back the nearly exterminated sturgeon by releasing baby sturgeon into the wild from hatcheries.

SUMMER

Throughout the months of June, July and August, some of the bay's most important migrations take place in a few days or hours. Summertime seems reserved for the short-trippers, whose navigational efforts are no less remarkable for not involving great distances.

Diamondback terrapins, a species spread throughout the Chesapeake and its rivers, appear to live and die within a small radius of where they were born. Maybe it is one reason they live so long -- more than 40 years in many cases.

Once on Smith Island I was swimming off a remote bay beach when I noticed a long line of terrapin heads moving purposefully southward down the bay, following the shoreline. It was the first time I had ever seen such a group movement. I never knew where they were headed, but in light of recent research, they were likely females homing in on a very specific and close-by nesting beach.

If a terrapin's "home" patch of beach has been developed or altered by human presence, it appears that it simply won't come ashore to reproduce. Even when the coast looks clear, nesting terrapins take no chances. Scientists have spent hours hunkered down in blinds, watching female diamondbacks bob in the water just off their chosen beach, checking for any sign of disturbance before emerging to lay as many as 20 leathery-soft eggs in the warm sand. The slightest movement or irregularity ashore -- the barking of a dog -- can send them back offshore for hours.

Another summer migrant is the larval Chesapeake oyster. After being spawned anytime from June through August, the oyster makes the only migration of its life. A microscopic, free-floating larva at this point, it has only two to three weeks to find a suitable place on the bay bottom to attach itself to and form a permanent home. If it fails, it dies.

AUTUMN

The great spectacles begin to fill the air. Waterfowl from across ,, the hemisphere's northern latitudes begin to arrive, eventually converging more than a million strong on the marshes and

waters of the bay by late autumn. From eastern Canada's remote Ungava Peninsula come the Canada geese. Redheads, canvasbacks, wigeon, pintail and dozens of other species arrive from prairie potholes and river deltas across Alberta, Manitoba and Saskatchewan, Canada. Last in, but perhaps most spectacular of all, are the tundra swans, which nest across Alaska's North Slope and into Siberia. These giants, with wingspreads of nearly 7 feet, mass in late November near the Canada-U.S. border in North Dakota. There they wait to catch the coattails of a blustery cold front, riding it south to the bay in one incredible non-stop flight that may be up to 1,200 miles.

Amid all these autumnal comings there is a massive departure that scarcely gets noticed, though it occurs throughout virtually every thread of water across the six-state watershed.

Usually moving at night, eels begin coursing down every stream of the bay drainage. They are the bay's only true catadromous species, running downriver to the ocean to spawn. Along with eels from all over North America, Europe and the Mediterranean region, they are drawn by forces we know nothing of toward a single destination: a super-saline, stagnant region of the north Atlantic known as the Sargasso Sea, northeast of the West Indies. Here, after swimming thousands of miles without eating, their bodies badly deteriorated except for reproductive organs, the eels mix their sperm and eggs and sink slowly into the cold depths to die.

Months and even years later, great, slow oceanic currents will deliver some of the billions of baby eels thus produced to the mouth of the Chesapeake. From there they will navigate into every corner of the watershed and settle down to a residence that can last well over a decade. Then, like their ancestors, they hear the Sargasso's call and begin their long journey.

DECEMBER, JANUARY

Migration winds down. In the deep channels, sooks -- as female crabs are called -- are still scuttling toward Virginia to bury themselves for the winter, but soon, even they stop moving. For perhaps as long as a few weeks, the bay is absolutely without migration. But even in coldest January the days have begun to lengthen. Far out to sea, the longer days have triggered the herring to begin moving coastward. On Georgia bayous a few great blue herons must be fidgeting and facing north more frequently. And somewhere between Bermuda and Cape Charles, Va., larval eels revolve lazily in the grip of a current pushing them bayward. In the vast watershed of the Chesapeake, something is always coming or going.

Free-lance writer Tom Horton and free-lance photographer David Harp roam the Chesapeake Bay region in search of moments great and small. Their collaboration, forged during their years as colleagues at The Baltimore Sun, has yielded a new book of essays and photos to be published April 1: "Water's Way: Life Along the Chesapeake" (Elliot & Clark Publishing, Washington, D.C., $36). The text and photos used here today were excerpted from it, with permission from the publisher.