SAN FRANCISCO (CBS SF) – A mysterious goo that killed hundreds of birds along the East Bay shoreline has been determined to be a mixture of non-petroleum-based fats or oils, according to the state Department of Fish and Wildlife.

The source of the mixture and how it came to be in the Bay remains a mystery.

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Fish and Wildlife investigators have only determined it could be some sort of non-petroleum oil, including synthetic oils such as silicone fluids, tung oils or wood-derivative oils such as resin or rosin oils. It might also contain animal fats and oil, including edible and inedible seed oil from plants.

By the time it got onto nearly 500 area seabirds, it had degraded into a sticky, thick consistency that was extremely difficult to clean from the birds.

A total of 323 birds were captured alive covered in the substance. Most of those were taken to the International Bird Rescue in Fairfield to recover. Another 170 were found already dead.

Despite the bird rescue group’s efforts, another 102 birds died there, officials from that organization said. The rest stayed there to recover and were being released in stages as they were ready.

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As of last week, about 100 had already been released.

Most of the affected birds were surf scoters, horned grebes, scaups and common goldeneyes. The goo affected their buoyancy and ability to waterproof. Continued exposure led to hypothermia, starvation, drowning, predation, entrapment, suffocation and infections, fish and wildlife officials said.

The last time a contaminated bird was found was Jan. 22, so investigators think the substance is finally gone from the East Bay shores. Evidence of the gunk showed up from Fremont to Oakland and a few cases even turned up on the Peninsula.

Investigators are still working to discover the source of the substance and exactly what it was. Anyone with information has been asked to call CalTip at (888) 334-2258.

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