A migratory bird that environmentalists say is threatened because of horseshoe crab fishing along the mid-Atlantic coast should be listed under the Endangered Species Act, a coalition of environmental groups said in a letter to federal officials.

The letter was sent by the American Bird Conservancy, American Littoral Society, Citizens Campaign for the Environment, Defenders of Wildlife, Delaware Audubon, Delaware Nature Society, Delaware Riverkeeper Network, National Audubon Society, and New Jersey Audubon Society. The groups ask Interior Secretary Dirk Kempthorne and U.S. Fish and Wildlife Service Director Dale Hall to list two subspecies of red knot under the act.

"The science was clear years ago that the red knot faces imminent extinction yet the U.S. Fish and Wildlife Service failed to list this bird. The causes of the red knot's decline have only gotten worse in the two years since that decision. The most recent information leaves no doubt that the service should list it immediately," said Jason Rylander, staff attorney for Washington, D.C.-based Defenders of Wildlife.

The medium-size shorebird flocks to the Delaware Bay each spring after flying nonstop from South America. They feast on horseshoe crab eggs to nearly double their body weight in less than two weeks before flying to their breeding grounds in the Arctic.

Environmentalists say the harvesting of horseshoe crabs for bait is threatening the birds' survival. Because of those concerns, horseshoe crab harvests have been reduced along the coast, including Maryland and Virginia. But fishermen say other factors could be affecting the birds-such as habitat destruction or global warming-and that a more limited harvest or a harvest in which only male crabs are collected would not hurt the birds.

In New Jersey, some state lawmakers have proposed a bill that would ban the harvesting of horseshoe crabs, the red knot's prime food. Earlier in February, New Jersey's Marine Fisheries Council rejected a proposed moratorium on the harvest of horseshoe crabs, setting the red knot on what bird-lovers say is a path to certain doom.

The letter cites a new report that found smaller populations and lower body weights of both of the bird's subspecies.