At the turn of the 20th century, observers estimated the brown pelican population in Louisiana at close to 50,000. But by 1961, no nesting pair could be spotted along the state’s entire coast, according to LaCoast, a Coast Guard Web site. Like another subspecies of the brown pelican found in California, the local birds had been hard hit by DDT and other pesticides, which acted to thin the shells of their eggs. The eggs were crushed when the adults sat on them. (DDT was banned in the United States in 1972.)

In 1968, Louisiana took birds from a surviving Florida colony and reintroduced them along the state’s southern coast in three spots. One was Queen Bess Island, which had been the site of one of the last breeding pairs before extinction, said Kerry St. Pé, program director of the nearby Barataria-Terrebonne National Estuary Program.

Still, the birds struggled, threatened this time by the loss of their habitat. The local wetlands, hurt by levees in the Mississippi that blocked sediment from flowing downstream and by canals cut by oil companies looking to lay pipe, were sinking into the gulf at an astonishing rate. Queen Bess was going under as well until 1990, when a coastal restoration project financed a rock barrier around the island, which stabilized it. The pelican colony began to flourish and the birds’ offspring helped repopulate the coastline, Mr. St. Pé said.

Last year, the birds were officially taken off the endangered species list. But the oil spill, experts said, could change that. Like all birds, pelicans are very sensitive to oil, said Melanie Driscoll, director of bird conservation for the National Audubon Society’s Louisiana Coastal Initiative. It prevents them from regulating their body temperature when it gets on their feathers, she said, and in Louisiana the pelicans are subject to overheating. The oil can also poison the fish the pelicans feed on and seep through the shells of pelican eggs, killing the embryos.

The potential for damage was frighteningly apparent at the rescue center set up here by the International Bird Rescue Research Center with BP and federal and state officials. All day Thursday, oiled birds, including the 29 brown pelicans, arrived at the makeshift veterinary emergency room built in a hangar on a former military base. They were carried from Coast Guard helicopters in dog kennels and cardboard boxes with air holes punched in them.

Image A laughing gull in the surf was coated in heavy oil on Friday. Credit... Win Mcnamee/Getty Images

Most of the birds were so thoroughly coated in crude that they could not stand up. Some were stuck to the floor of their cages. Workers wiped off thick globs of oil with towels, then gave them fluids and fed them a fish slurry.