A surf scoter is treated at the International Bird Rescue Research Center in Cordelia, Calif. Thousands of birds have been found dead or blackened by sludge since the spill, but no species has been hit harder than the surf scoter. ((AP Photo/ Michael Macor))

An oil spill in San Francisco Bay two weeks ago killed and oiled thousands of birds, with a Canadian sea duck among the largest casualties.

Nearly 227,124 litres of oil spilled into the bay on Nov. 8 when a cargo ship hit the San Francisco-Oakland Bay Bridge. The area is a key wintering ground for shorebirds, seabirds and waterfowl.

Since the accident, at least 1,365 birds from 27species have been found dead and more than 1,000 oil-slicked birds have been taken to a wildlife care centre for cleaning.

Oil on birds' feathers impairs their ability to keep dry and warm, which forces them to shore and away from the food supply. It also can make birds sick if they ingest it while cleaning their feathers.

The surf scoter accounts for nearly 40 per cent of the captured birds and more than 25 per cent of the dead ones, said Michael Ziccardi, head of the Oiled Wildlife Care Network, which is leading the rescue effort.

Scientists say the population has suffered so much from the spill because they spend almost all their time in the hardest hit areas.

The sea ducks breed in lakes and wetlands in Canada's northern boreal forests and winter along the Pacific Coast, with a smaller population migrating to the Atlantic Coast. The birds are notlisted as a threatened or endangered species, but their population has declined by 50 to 70 per cent over the past 40years, experts say.

Biologists said they fear the spill could decrease future populations, since the injured birds were mostly healthy adults.

"It definitely could have an effect on overall populations," said wildlife biologist John Takekawa of the U.S. Geological Survey, who helped track scoters from the bay to their Canadian breeding grounds. "There could be a lasting effect over time."

With files from the Associated Press