Story highlights "It's a full-on mystery as to what happened," an official says

Authorities say more than 300 birds were rescued and 200 are dead

An oil spill has been ruled out

(CNN) The birds lay splayed across an otherwise serene California shore.

They're covered in a viscous substance that no one can identify.

More than 300 have been rescued, but at least 200 are dead, according to the California Fish and Wildlife Department.

So what's happened?

"It's a full-on mystery," department spokesman Andrew Hughan said. "We have no idea what they are coated in and I'm not sure when we will have the answer."

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