As wildlife in the Point Reyes National Seashore slowly recovers from a blight of tarballs that hit the West Marin coast last week, state and federal authorities continue to search for the source of the pollution.

Although a few struggling seabirds mired in oil still wash ashore each day, almost no evidence remains that Limantour Beach, Drake's Beach, and Drake's Estero were inundated with sticky tar balls just last week, said John Dell'Osso, the park's chief of interpretation.

All the same, when the cleanup effort concluded last Friday, approximately 300 dead birds, mostly common murres, had been recovered from the beaches by rangers and volunteers, Dell'Osso said.

About 200 more grebes, loons, murres, gulls, and other seabirds with oily feathers are being watched for signs of deteriorating health, and 93 oiled birds have been captured and taken to a research center in Berkeley for rehabilitation. Of those birds, 14 died in captivity.

The toll on birds "was the greatest impact on the park," Dell'Osso said. "We're going to continually have people monitoring the beaches to see what's coming in, alive and deceased."

Among the most vulnerable bird population affected by the tarballs has been a colony of approximately 100 snowy plovers, a threatened species that roosts on the shore of Drake's Bay, said Gary Page, the director of coastal and estuarine research at the Point Reyes Bird Observatory.

Since the oil spill was first discovered Nov. 16, Page said, he has found 22 snowy plovers stained with oil during five visits to the National Seashore. Unlike seabirds, which swam through the oil, the plovers appear to have tread through oily debris on shore, then wiped the oil on their feathers, he said.

Because the plovers aren't heavily oiled, Page said, they will be left to recover in the wild instead of being subjected to the stress of captivity. However, he added, they will be closely watched until the oil is all gone.

"Some of it they preen off, and then they swallow it," Page said. "And that's not good."

Meanwhile, state and federal officials await the return of laboratory tests to help determine the oil's origin. Samples were taken both from the tarballs in Drake's Bay, as well as a mile-long oil slick seen Friday off the San Mateo County coast.

Spotted 13 miles out to sea during a Coast Guard flight, the slick was the first evidence of oil on the ocean's surface anywhere in the vicinity of Drake's Bay since tarballs began appearing at the National Seashore.

A Coast Guard cutter took samples of the oil slick and sent them to both the state Fish and Game Department's laboratory in Sacramento and the Coast Guard's facility in Groton, Connecticut, said Chief Petty Officer Bob Borden.

After choppier weather Friday night, a second flight searched the ocean from Point Reyes to Half Moon Bay, covering a grid 25 miles out to sea, but found no traces of the oil. "Our guess is that it broke up, dispersed or evaporated," Borden said.

Both Borden and Dell'Osso told The Light that weather patterns this past week made it unlikely the slick will eventually wash ashore on the Marin Coast in the form of tarballs.

However, one of the reasons oil tests have not yet yielded any definitive results is that there are few identifying characteristics in tarball samples, which are essentially congealed oil globs that have washed around at sea, said Coast Guard Comdr. Rob Lorrigan, who is in charge of the investigation.

"The more weathered it is, the more difficult the analysis," he added.

Furthermore, he said, there are few distinct oil "fingerprints" to compare the recent samples against.

Lorrigan told The Light that Coast Guard technicians are trying to determine whether the recent samples match the oil released during the Puerto Rican shipwreck in 1984, the Cape Mohican spill in San Francisco Bay last year, the recent spill in Humboldt Bay, or some tar balls that washed into Drake's Bay last year.

While state and federal investigators await the lab results, the Coast Guard has been tracking every large vessel that recently traveled through the Port of San Francisco, Lorrigan said.

When these ships return to port, Lorrigan said teams of seamen will board the vessels and take samples of cargo and bilge oil to determine if they are responsible for either of the recent oil slicks.

"It's a very aggressive investigation," Lorrigan said, adding that Fish and Game is being provided with all the same information to enable them to conduct their own search for the cause of the tarballs.

If a responsible party is discovered, it will be required to reimburse the federal Oil Spill Liability Trust Fund, a pool of money financed by the petroleum industry to pay for oil spill cleanups, he said.

So far, the fund has been tapped for about $300,000 to pay for the efforts of about 100 state and federal officials, private contractors, and volunteers, he estimated.

In addition, fines of up to $25,000 a day might be imposed if the party knew about the spill but did nothing to report it. And the park might also conduct a Natural Resource Damage Assessment to put a dollar value on the harm inflicted on the wildlife, Lorrigan said.