An oil spill stained Tacoma's Commencement Bay (search) and spread to beaches on nearby islands Thursday, polluting an ecologically rich area where grebes, ducks and other birds spend their winters.

State Department of Ecology investigators were unsure Thursday night how much thick, black oil had spilled or where it came from, agency spokesman Larry Altose said.

The oil, first reported about 1 a.m. Thursday, spread over five or six miles around the southern tip of Vashon Island (search) north of Tacoma, and tainted nearby Maury Island. The Coast Guard said it appeared to be a heavy-grade industrial oil that can coat beaches, form tar balls and does not easily evaporate.

Federal and state wildlife agencies received reports that at least two birds were found coated in oil, said Joan Jewett, spokeswoman for the U.S. Fish and Wildlife Service (search). Experts from a bird rescue research center were called in to aid the cleanup.

Mary Nicholson, 47, lives two doors from the ferry dock on Vashon Island and said she awoke to the smell of oil. She said she found a rock coated in thick, dark goo during a midmorning beach walk.

"I picked up this rock and this globby stuff was dropping off of it in clumps," Nicholson said. "It was really bad."

Cleanup operations began around midday Thursday with three boats skimming oil from the surface. Altose said two more were sent to aid cleanup efforts, and the Coast Guard said protective booms would be installed in some areas.

Along a beach on southern Vashon Island, Altose said crews raked oily sand and small stones above the tideline so they wouldn't be washed out.

Although the spill had been reported several hours before the cleanup began, Altose said, "There's really very little that can be done in the dark."

Thick morning fog further complicated efforts, but Kathy Fletcher, executive director of the environmental group People for Puget Sound (search), said the time lag was a big disappointment.

"We know quite a lot about currents and tides in Puget Sound. And so even though it's dark ... knowing the approximate location of a spill could alert the agencies to the possibility of oil coming to shore in certain areas," Fletcher said.

The Coast Guard was reviewing a list of ships that had been in the area, trying to narrow the possible sources of the spill, Petty Officer Adam Eggers said. The agency did not immediately release that list or the number of ships on it.

Wintering seabirds flock to the area about 30 miles south of Seattle because of a plentiful supply of herring and other forage fish that thrive in eelgrass beds along the shorelines, Fletcher said.

"One of the few fortunate things is the wintering seabirds have not come in their greatest numbers yet," Fletcher said. "When the birds do come in force, if the stuff they eat has been poisoned, that's going to be a bad situation."