Tests are being carried out on dead seabirds found coated with a mysterious substance that looks and feels like dirty rubber cement.
About 200 dead birds have been found along the San Francisco Bay Area’s shorelines, said Andrew Hughan, spokesman for the California Department of Fish and Wildlife.
The goo covers their feathers, and the birds lose their ability to insulate themselves. They eventually freeze to death.
Hughan said the wildlife agency's preliminary tests show the gray gunk is not a petroleum-based substance or an organic product like vegetable or fish oil. Final results might not come until later this week.
"You can't rush science," Hughan said. "It will be what it is, when it is."
Officials say the substance is not a public health or safety risk to humans.
They are investigating whether it could be polyisobutylene, a sticky, odorless and largely colorless material that killed thousands of seabirds in the United Kingdom in 2013.
Even when the pollutant is identified, it could be longer before officials pinpoint its source.
Hughan said 315 live birds have been taken to the International Bird Rescue to be cleaned.
Workers at the Fairfield center use baking soda and vinegar to loosen the sticky substance before washing it off with dish soap.
The birds - surf scoters, buffleheads and horned grebes - began turning up on a beach Friday.
Since then, volunteers have combed shorelines looking for and trying to catch living birds.
The dead ones were found along the shorelines in Alameda, Foster City, San Leandro and Hayward, east of San Francisco, Hughan said.
"We don't expect to find any more live birds," he said.