SPECIAL REPORT: Oil on the Beach : Surface-Diving Birds Most Vulnerable to Oil : Wildlife: Spill could also harm primary food sources for countless migrating birds and the endangered light-footed clapper rail if it enters sensitive wetlands areas.

Each fall, the distinctive black-and-white western grebe wings south from its breeding grounds in Alaska to winter off the coast of Southern California and Mexico.

Thousands of the birds bob in the waters off Orange County, where they are joined by other surface-diving sea birds: common murres and several species of scoter.

These birds are the most vulnerable to the floating oil released Wednesday by the ruptured tanker American Trader.

“Grebes and murres spend most of their time sitting on the water, so they’re highly vulnerable to oil,” said Michael Fry, a research physiologist in the avian sciences department at UC Davis. And because oil is not a natural aspect of the birds’ environment, they don’t know to avoid it. In fact, Fry said, the oil has a flattening effect on the water that may actually attract the birds as they fly in search of choice feeding locations.

Fry has seen his share of oil-soaked birds. He oversaw the pathological study of birds killed by last year’s spill off Valdez, Alaska, in which an estimated 30,000 sea birds and 127 bald eagles died.

There are two life-threatening problems that occur when a bird’s feathers are fouled with oil.

First, a bird will try to clean itself with its bill, called “preening,” thus ingesting some of the oil. “They do eat substantial quantities of the oil when they’re trying to get it off their feathers,” Fry said.

Crude oil, such as that spilled off Huntington Beach, has a certain percentage of gasoline that gradually evaporates. If swallowed, it can cause serious damage to liver and kidneys. The oil itself causes anemia in the birds, which can lose half to two-thirds of their red blood cells, which carry oxygen through the body. For diving birds, which must hold their breath to feed, that can be fatal.

The other main problem is that the oil mats the feathers, interfering with the bird’s ability to float and to insulate itself from the cold waters. The body temperature drops, and the bird can die from hypothermia, or can literally starve to death as its metabolism increases to make up for the escaping heat.

The oil spill can effect other aspects of the marine food chain, down to its most basic elements. If the spill enters any of the county’s sensitive wetlands areas, it could smother the invertebrates that live in the ground. These are the primary food source for shore birds, including the endangered light-footed clapper rail, which nests in Anaheim Bay and Upper Newport Bay, and the myriad migrating birds that depend on the wetlands as vital refueling stops on their long journeys.

“In the long term, I don’t think things are going to be disastrous” if some oil enters the marshes, said Louann Murray, a specialist in wetland invertebrates and a member of Amigos de Bolsa Chica. Microbes in the mud will eventually break down the oil that isn’t scooped out, and the marsh will eventually recover.

The short-term effects, however, could be disastrous for the migrating birds and especially for the clapper rail. Half of the remaining clapper rails in the United States live in Upper Newport Bay, and “could be wiped out by this spill,” Murray said.

If the oil hits tide pools, the results could also be devastating, according to Harry Helling of the Orange County Marine Institute in Dana Point. “There’s nothing you can do to rescue a tide pool,” Helling said.

In the ocean waters near shore, mussels and snails can ingest the oil without apparent harm. But it accumulates in their systems, and can contaminate the birds that feed on them, such as harlequin and goldeneye ducks and oyster catchers.

Eggs and young fish are vulnerable, but because this is not breeding season, adult fish won’t be severely impacted. However, if the oil settles to the bottom it could kill such bottom-dwelling fish as halibut.

In Alaska, sea otters--which depend on their fur for insulation--were the biggest victims of the spill among marine mammals. Otters no longer live off the Orange County coast, but there are numbers of California sea lions, bottle-nose and common dolphins and the migrating gray whales.

Large mammals are usually able to escape the oil, says Jacqui Michel, a South Carolina-based scientific consultant who worked the Valdez spill and is heading to Orange County. While evidence is inconclusive, the oil does not seem to stick to the skin of whales and dolphins when they swim through.

One concern is that dolphins, which accumulate toxins in their systems, may suffer long-term effects if they eat fish contaminated by the spill.

Overall though, some experts are optimistic that wildlife off the Orange County coast will not suffer major long-term consequences if the spill is kept out of wetlands and other inlets.

“You hate to say it, but in the grand scheme of things, if you had to have a spill, this was the time and the place,” said state biologist Robin Lewis, who is helping assess potential environmental damage for the state Department of Fish and Game. “A month later, a few miles farther to the south, in that rocky reach near Corona del Mar and Laguna, and it would have taken years to recover.”