Foam from an unusual algae bloom has killed thousands of birds along the Oregon and Washington coasts in recent weeks, marine biologists said.

Akashiwo sanguinea, a single-cell algae or phytoplankton, strips the birds of their natural waterproofing, said Julia Parrish, a marine biologist and professor at Washington State University.

"It's the largest mortality event of its kind on the West Coast that we know of," Parrish told The (Portland) Oregonian in a story published Friday. "We're getting counts of up to a million cells per liter of water," she said. "Think about that. That's pretty dense."

Storms have whipped the algae into a substance similar to a sticky soap, which washes off the birds' protective waterproofing oils and causes them to die of hypothermia, said Greg Schirato, a manager for the Washington Department of Fish and Wildlife.

The bloom, which poses no threat to people or pets, has killed thousands of birds since mid-September from northern Oregon to Washington's Olympic Peninsula, Schirato said.