Hundreds of emaciated seabirds have washed up dead on beaches from Marin County to Monterey Bay, and wildlife officials said they suspect that a dip in the birds' food supply may be killing them.

The black, iridescent Brandt's cormorants began dying in mid-April, puzzling scientists who have seen the species thrive in recent years on the Farallon, Alcatraz and Año Nuevo islands.

Dead birds have been found in more than a dozen locations in the Bay Area, including Ocean and Baker beaches in San Francisco; Montara and Pescadero beaches on the San Mateo County coast; Rodeo and Muir beaches in Marin County; and on the new east span of the Bay Bridge in Oakland.

Biologists from the U.S. Fish and Wildlife Service and the state Department of Fish and Game are asking beachgoers not to move dead birds. Volunteers from the Gulf of the Farallones and the Monterey Bay national marine sanctuaries are marking dead birds by clipping their wings or toes to help get a valid bird count.

About three dozen of the surviving cormorants have been sent to the International Bird Rescue Research Center in Cordelia, along with some western grebes. Ten birds have been released to the wild.

'Emaciated and starving'

"Most of the birds brought in are emaciated and starving," said Gerry McChesney, acting manager of the Farallon Islands National Wildlife Refuge. "They are having trouble finding food and are responding well to feeding."

Seabird experts believe that the birds are suffering from a lack of food as ocean conditions have changed, triggering a crash in anchovy numbers in 2008. The birds, which had reached their highest numbers in years on the Central California coast, don't appear to be ailing from viruses and bacteria, they say. Results of tests are expected Monday.

The Brandt's cormorant population reached about 40,000 in the region in 2007, according to scientists. But the birds didn't reproduce in normal numbers in 2008.

"Usually by now, the breeding colonies are well along," said McChesney. "The birds should be into the egg-laying period. But the colonies are almost empty. The birds don't seem to be around in very high numbers, and the birds that are around have been starving to death."

Ocean conditions are generally good this year with vigorous upwelling of the nutrients that feed plankton and the small fish on which the seabirds feed. They like rockfish, flatfish and anchovies.

Last year, some species of small fish were scarce, according to trawl information gathered by the National Marine Fisheries Service. Northern anchovies - popular food for Brandt's cormorants - hit their lowest numbers since 1990 as the bulk of the fish moved to Southern California, where they normally spawn in years of cooler waters.

Steve Ralston, research fisheries biologist at the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in Santa Cruz, said it took 10 trawls to catch one anchovy last year. In a typical year, more than 600 anchovies are caught in a trawl, he said.

"The ocean can wax and wane between warmer and cooler states, and when it's warmer, anchovies and sardines are more plentiful in our survey. Some of the cooler species, like calamari squid and juvenile rockfish, are less abundant," Ralston said.

Bird and fish populations are closely linked, Ralston said.

Because young rockfish were scarcer in the ocean during the warmer years of 2005, 2006 and 2007, species such as seabirds and humpback whales may have switched to anchovies, said biologist Bill Sydeman, director of the Farallon Institute of Advanced Ecosystem Research in Petaluma.

When anchovy numbers dropped around the Farallones last year, the birds may have been unable to find food, which would have triggered reproductive failures of the Brandt's cormorants, Sydeman said.

Other possibilities

There could be other reasons, such as their population reaching the maximum number that the ecosystem can support, he said. Or perhaps more cormorants are being found dead because there are more of them, he said.

Jan Roletto, research coordinator for the Gulf of the Farallones sanctuary, said the disappearance of anchovies is a valid explanation. But as she waits for results of pathogen tests, she is looking at other environmental factors.

"We're just speculating right now," she said.