SAN FRANCISCO (AP) _ State and federal investigators are using satellite photographs to find out who was responsible for an oil spill that threatens some of the richest bird habitat on the West Coast.

The spilled oil _ an estimated 2,500 gallons, or about 60 barrels _ formed a slick sheen about 10 miles long around a smaller ribbon of heavy dark tar balls. It was first spotted over the weekend about 10 miles from San Francisco.

As crews began picking up tar balls Wednesday on beaches near Half Moon Bay south of San Francisco, wildlife officials feared that a change in winds could spell disaster for tens of thousands of birds. A few dozen have been killed or injured so far.

``The bird mortality is always very high in these situations,'' said Dana Michaels, spokeswoman for the state Department of Fish and Game. ``Even when we are fortunate to find them alive, they are often scared to death when we are cleaning them.''

This time of year, birds float on the water in huge ``rafts,'' teaching newborns how to feed themselves. Coast Guard crews flying over the area on Tuesday saw one raft that had as many as 40,000 birds between the oil and shore, Michaels said.

The spill is also near the Farallon Islands, spires of rock that are a national marine sanctuary and one of the most productive seabird rookeries, or breeding grounds, in the eastern Pacific.

Most likely, the oil was dumped by a cargo vessel that had left San Francisco for Los Angeles, the Panama Canal or South America, Michaels said.

Visual evidence of the spill may come from a commercial satellite under contract with the state.