Hundreds of emaciated seabirds have washed up dead along the south-eastern coast of America, alarming scientists who fear changes in the ocean could have affected the fish that the birds normally eat.

More than a thousand shearwaters, large gull-like birds that spend most of their lives far out to sea, have been found dead over the past two weeks on beaches stretching from the Bahamas to the Carolinas, say wildlife biologists.

Officials are not certain what is causing the casualties but say that the only common factor is that they appeared to have died of malnutrition and dehydration during migration.

"It's got a lot of folks talking and wondering. Is this a canary in the coal mine issue?", said Jennifer Koches of the US Fish and Wildlife Service.

"Is there something serious going on out in the ocean that it should be causing us serious alarm?"

The birds, which feed on small fish and squid, nest on islands off southern Africa and then migrate 7,500 miles north during the summer to the seas off northern Canada and Greenland.

Typically, the birds pass by America far offshore, flocking together in groups of 50 to 100 and flying closer to the water than most migrating birds.

Most of the dead birds are juveniles that were born this year. In Florida, where the casualties were first spotted, animal hospitals have been inundated with dying shearwaters brought in by members of the public.

Preliminary tests in Georgia and Florida have indicated that no disease or pathogen, such as West Nile virus or bird flu, is involved in the deaths. A similar but less severe shearwater death toll was reported in 2005.

Craig Watson, a wildlife biologist with the Fish and Wildlife Service, said the birds were extremely malnourished and appeared to have simply starved to death.

The winds on the ocean could be pushing the birds off course where they find less to eat, he said.

"The other thing is the forage fish they rely on may be unavailable to them for some reason," he said. "Is it because there is less out there? We don't know. We are hearing that off the coast of South Carolina it could be one of the worst years on record for forage fish."

"Whatever the reason, [the deaths] appear to be pretty significant. We just don't know how significant yet", he said.

Al Segars, a vet at South Carolina's Department of Natural Resources, said that dehydration was also a factor because seabirds get much of the water they need from the fish they eat.

Local birdwatcher internet chat sites have been buzzing with reports of sheerwaters flying south - the opposite of their usual flight pattern - in what observers believe is a desperate hunt for food.

Ms Koches said: "It's not uncommon for some of these birds to die off during migration, but the numbers here are significant enough to cause alarm with natural-resources people internationally. No bird can sustain this type of die-off year after year."