Gannets are notable among our winter seabirds visitors. With a 6-foot wingspread and snowy white plumage, they plunge-dive from up to 100 feet to pursue fish active near the water's surface. They enter the water like a living javelin, their long, pointed bill supported by a very muscular neck and their long, tapering wings folded behind.

Their weight is that of a goose, but the wingspread is a foot greater, with the wings narrower and tapered for soaring over water. Gannets use winter coastal and offshore wind energy to support their hunt for fish, and thus they gain tremendous mobility.

In their wind-dependence and size, they are reminiscent of albatrosses in other portions of the world's oceans. But they are not closely related, and their feeding strategy and prey are different from albatrosses.

Gannets breed locally on cold water at high latitudes in portions of all four of the world's hemispheres. In eastern North America, all gannet nesting is concentrated in six very dense, isolated colonies around Newfoundland and the Gulf of St. Lawrence, Canada.

Spectacular offshore rock stacks such as Cape St. Mary's, Newfoundland, and Bonaventure Island, Quebec, are summer destinations for ecotourists. The total breeding population size is nearly 150,000 birds, and has been increasing.

Wintering and migrating gannets are widespread offshore of the eastern United States from December through early April. Gannets remain social during this nonbreeding period, and at times form a living curtain of plunging birds as they flock-feed over schooling fish such as menhaden.

They are a signature winter bird of lower Chesapeake Bay, but harsh weather and their selection of offshore open-water habitat often puts them beyond the ken of the casual observer. Gannets also winter many miles offshore on the ocean, out to the edge of the continental shelf. Despite being out of sight, gannets can potentially serve as important sentinels of human impact.

Along with common loons, they are useful as bioindicators when something goes wrong out there, because they are big enough and robust enough to swim in to shore. No seabird is more sensitive to oil spills than the loon, but gannets are also somewhat vulnerable.

As "wind birds," gannets would inevitably overlap with offshore fields of wind turbines. If damaged by turbine blades, some would doubtless survive and swim in. My limited gannet observations at the Chesapeake Bay Bridge Tunnel, though, suggest that gannets are phobic in regard to man-made structures. The 18-mile bridge-tunnel, an engineering wonder spanning the Bay's mouth, traverses powerful tidal waters where both rockfish and gannets concentrate in pursuit of menhaden, eel, and other prey moving in and out of the Bay.

To date, I have seen the gannets enter and exit the Bay exclusively over water, flying over the 2-mile-wide main channels underlain by tunnels. They avoid the trestles of the bridge as if it were a fence, although it poses no obstacle to their powerful buoyant flight. Thus, the bridge-tunnel could provide an experimental test of gannet "psychology," which may argue they will also avoid offshore fields of wind turbines.

Limited data suggest that much or most of this Chesapeake occurrence is an ecological connection with surface-active plankton-feeding menhaden, which gannets are well-adapted to catch with their plunge-dive. Thus, gannets join ospreys, brown pelicans and common loons as Bay species for which menhaden are a central component of the diet.

Gannets range well up the Bay. Last winter was relatively mild, allowing some temperature-sensitive migratory fish to remain in the Bay, and gannets were common at the mouth of the Choptank River in March. On a windless morning in early April, I have seen 25-30 becalmed gannets scattered across the still plankton-green river, floating like big white linen dinner napkins folded fancy in the spring sunshine.

On the right wind, gannets put on a magnificent flying and diving show at exposed coastal sites such as Nantucket and Martha's Vineyard, MA; Montauk Point, NY; Assateague Island and Ocean City, MD; Chincoteague, VA; the outer banks of North Carolina; and other ocean beaches farther south in the Carolinas and northeast Florida.

In March and early April, northbound migrating gannets sometimes make a fabulous stopover at the mouth of the Bay. This is a season when great numbers of fish enter the warming Bay, and thousands of gannets may aggregate to dive after them.

The restaurant stop on the bridge-tunnel can be a front-row seat for this spectacle, as they pursue fish up the Thimble Shoal Channel, plunge-diving all the while.