The toll of dead birds found along the shores of San Francisco Bay has now topped 200. That's more than double the estimate that was offered yesterday, as efforts to locate struggling animals (as well as the source of the goo responsible) kicked into high gear.

The growing casualties suggests the number will likely continue to rise over the course of the rest of the week, though wildlife advocates are hopeful that the worst of it has passed.

Local officials and volunteers began collecting the dead birds over the weekend, after several dozen dead birds began showing up on a local beach last Friday -- all of them covered in a mysterious goo.

Several hundred more birds have been rescued alive and transported to care facilities where they're being washed and nursed back to health.

The mysterious substance found coating the sea birds -- a combination of surf scoters, buffleheads and horned grebes -- appears to be a sort of dirty rubber cement. Officials are performing a series of necropsies on deceased birds, and are running the unidentified goop through lab tests in order to put a name to the pollutant that's to blame for the wildlife casualties.

"It's some material that we nor the wildlife center has ever seen before," Andrew Hughan, a spokesman for the California Department of Fish and Wildlife, told the Los Angeles Times. "It's a real mystery."

The majority of the goo-coated birds were found along the east side of the San Francisco Bay near the the San Leandro Marina, Hayward Regional Shoreline and Alameda.

Wildlife group International Bird Rescue is helping coordinate rescue and cleanup efforts.

"The good news is that we have modified our wash protocol and it appears to be working on healthier birds," group leader Barbara Callahan said. "However, some of the birds that have recently arrived are in much poorer condition, likely because they've had this substance on their feathers for several days now."

Most experts believe the mystery goo is manmade and not a natural fish oil and algal substance. Some have suggested it is likely a synthetic rubber called polyisobutylene. In 2013, a polyisobutylene spill killed some 4,000 birds in Great Britain.