Jan Dubuisson heads up the least-tern sanctuary for an Audubon Society chapter in Gulfport, Miss. She's been working with the migratory birds for the past 30 years—her chapter formed to help imperiled springtime breeding colonies there in 1976.

The smallish birds have an unsavory habit: they dive-bomb intruders and defecate on them as a defense. So Dubuisson is used to coming home filthy from the field.

But when she returned home from her monitoring efforts about a week after the oil spill began, she saw stains on her clothes she'd never seen before. They were dark, she says, and "when I washed my hat and shirts, the spots did not come out, which makes me wonder if they're not oil-based."

So far, Dubuisson's suspicions have not been confirmed. As of Saturday, just 24 oiled birds had been rescued from Gulf of Mexico waters around oil-spill containment ships or on the beaches, where volunteers and experts are on the lookout. No terns have been among them.

But it's entirely possible that terns could be exposed to an oily sheen as they dive for fish. Chuck Hunter, an ornithologist with the U.S. Fish and Wildlife Service in Atlanta, says royal terns and Sandwich terns (named for the town of Sandwich, along England's southeast coast), along with brown pelicans, black skimmers and laughing gulls, are among the birds most likely to come into contact with oil as the spill comes closer to shore.

"Those are on the front lines of being most heavily impacted. They tend to forage out into the Gulf, where the oil is dominating the seascape," he says.

View a slideshow of Gulf Coast birds

Breeding season is underway for most of the world’s birds, with hatchling season close behind. Those that nest around the Gulf’s shores are right now raising their chicks on oil-free beaches. Marshes, too, are holding their own, aided by many miles of various styles of floating boom placed outside vital habitat. Boom most often looks like huge, floating sandbags strung together in a line, and either absorbs oil or deflects it toward a collection point. 

But a single storm could change everything, pushing oil over or under the boom. Once there, oil could hammer marsh dwellers, nesting shorebirds, migratory shorebirds on heroic migrations from South America traveling to as far north as the Arctic, and some fishing birds whose main populations thrive in the Gulf.

For example, the southeastern United States supports about 90 percent of all the brown pelicans in the U.S., with the Gulf Coast from Louisiana to the Florida panhandle comprising around 40 percent of that habitat, Hunter says.

And the danger could linger for a year or more, adds Greg Butcher, director of conservation for the Audubon Society.

"It might peak after storms, certainly all through this year, maybe into next year," he says. "You could have multiple peaks of oiled birds. This is not a short-term phenomenon, especially as the volume of oil increases."

Passing through
The birds rescued so far include northern gannets, pelicans, a sora, a magnificent frigatebird and laughing gulls, along with a handful of unexpected species like rock doves and a cattle egret that are not generally expected to come into contact with the oil.

The list represents a variety of avian lifestyles—and a mix of reasons why oil would reach the birds.

Northern gannets, for example, are seabirds rarely seen on shore. They breed in the Maritime Provinces of eastern Canada and winter along the Atlantic coast. In the coldest months, they make a trip around Florida and into the Gulf, where they feed by diving from great heights to catch fish.

"Most of them have departed, but we always have some that hang around into May," Hunter says. And several of those stragglers have already ended up among the small numbers of oiled birds.

Magnificent frigatebirds, on the other hand, move into the Gulf in warm months. Called kleptoparasites, those birds watch other birds catch fish and then harass them in mid-air so they lose their catches. When they get a tern or pelican to drop a fish, they plunge down and grab it off the surface of the water, which may have been how this bird got oiled.

Shorebirds like sandpipers and plovers, which travel from South America to, in some cases, the Arctic to breed, are at their peak migration right now, which makes them vulnerable, Hunter says.

"We're talking about the potential for really high proportions of North American shorebirds being exposed to oil," he says. "If this persists, they're going to be right back in it going south again in July and August."

Nowhere to go
Familiar, non-migrating locals like brown pelicans are also a special worry, Hunter added, because their numbers in the Gulf have only recently been stabilizing after habitat loss to erosion on the east side of the Mississippi Delta, combined with hard hits in 2005 by hurricanes Katrina and Rita. Part of the birds' recovery strategy has been to expand their nesting sites to both sides of the Mississippi Delta—but now both sides are in the path of the spill.

It remains to be seen whether some birds will wise up to the oil and find ways to avoid it such as foraging farther afield, Hunter says.

"We're going to probably have a big experiment on that," he says. 

Secretive marsh birds like clapper rails, king rails and soras—one of which has already been found oiled—frequent brackish and salt marsh habitat. Clapper rails in particular are highly dependent on the marshes—including fiddler crabs, a dietary staple—and they'll have nowhere to go if the oil arrives.

For all the Gulf birds and their habitats, conservationists are keeping their fingers crossed for continued fair weather that allows the protective boom to do its job.

But Hunter isn't overly optimistic. "One big storm," he says, "and it's just going to go right over it."