Rescue workers found more dead birds Wednesday — bringing the death toll to 200 since a mysterious goo appeared in eastern San Francisco Bay.

Some 315 birds have been rescued and are under care — a small increase from the previous day, the state Department of Fish and Wildlife reported in a tweet.

Meanwhile, workers at a state fish and wildlife lab are attempting to identify the sticky substance that has coated seabirds and prevented their ability to fly and stay warm.

State lab workers on Wednesday ruled out a shipping fuel additive called polyisobutylene. They already had ruled out oil.

Since Friday, coated birds have been found in the bay or on its shoreline at or near the San Leandro Marina, Hayward Regional Shoreline and the Harbor Bay Isle area of Alameda.

On Tuesday, three surf scoters coated in the substance were picked up near the Foster City shoreline and brought to the International Bird Rescue Center in Fairfield for treatment.

The discovery is not proof that the goo has spread to the west side of San Francisco, said Barbara Callahan, the rescue center’s interim executive director. The birds may have been slimed by the substance in the East Bay and then flown across the water to the Foster City shoreline, she said.

The nonprofit wildlife center is absorbing the $8,000-a-day cost of caring for coated birds because no responsible party for the spill has been identified, Callahan added.

State fish and wildlife wardens patrolled the bay by boat Wednesday and found one live seabird and three dead ones, the agency reported.

Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff.