In a troubling wildlife mystery, California brown pelicans are turning up sick or dead in suburban ponds, driveways and backyards - far from their ocean home.

Two of the elegant birds, emaciated and disoriented, were found in San Jose last week. Another was rescued from Searsville Dam at Stanford University's Jasper Ridge Preserve. Others have been reported at such unlikely locations as Belmont, San Bruno, Brisbane and Burlingame. One fell out of a tree in Oakland. Two were found in a San Francisco dumpster; another stopped traffic on the Golden Gate Bridge.

"Normally, they're on piers and places where they can find fish,'' said Rebecca Ryan of the Peninsula Humane Society in San Mateo, which has stabilized several sick birds. "Now they are appearing in really unusual places.''

All told, an estimated 270 dead or ailing pelicans have been found in recent weeks from Washington state to Baja California, including dozens from the Monterey Bay area and along the San Mateo County coast, according to the International Bird Rescue Research Center, which runs facilities in Fairfield and Los Angeles. About 35 pelicans are now in treatment at the Fairfield clinic; this time last year there were only two. Another 65 are being treated in Los Angeles.

The brown pelican, whose wingspan measures six feet, is beloved for its soaring flight and dramatic capture of prey, with a headfirst dive in the water from great heights.

An endangered species that has endured decades of DDT exposure, oil spills, fish hooks, gunshot and other hardships, pelican populations have rebounded in recent years. Biologists have been celebrating the resurgence of a creature that once seemed doomed for extinction.

Die-offs of young birds aren't uncommon along the coast this time of year. But bird rescuers knew something was amiss when many of the sick pelicans were adults, found far inland, bruised and starving.

A social animal, it is rarely found alone. So it was startling to find one bewildered bird in a Kmart parking lot in Lompoc, another outside a Costco in Goleta and a third on the runway at Los Angeles International Airport. One pelican was found at an elevation of 7,200 feet in the New Mexico snow.

"It is frightening,'' said Laurie Pyne, development director of International Bird Rescue Research Center in Fairfield. "We don't know what it is or how many are truly affected. We don't know the long-term effects on their populations.''

Experts are racing to identify what's behind the latest die-off. Studies of blood and tissue are under way at the University of Southern California, the Department of Fish and Game at the University of California-Davis and the U.S. Fish and Wildlife Department in Madison, Wis. Results are expected within two weeks.

"It will take some detective work,'' said Dan Anderson, an avian specialist at UC-Davis.

Because sardines and anchovies are plentiful, experts have ruled out starvation.

The initial suspect was domoic acid, a neurotoxin produced by microscopic algae. But the pattern doesn't seem to fit. For one thing, toxic outbreaks usually occur later in the year. And birds poisoned by domoic acid generally suffer seizures; these sick pelicans have not. Finally, no other animals in the marine ecosystem seem to be affected.

Lab results on Friday found that only half of the birds tested positive for domoic acid - but scientists say that is not definitive enough to prove that it is the primary cause of the widespread illness.

"We believe these results are significant but do not explain all the signs we are seeing in the pelicans,'' veterinarian Heather Nevill, who is leading the investigation at the International Bird Rescue Research Center, said in a statement. "We are seeing a number of conditions that are not typical of domoic acid toxicity or a domoic acid event. Therefore, we are continuing to collect and test samples, keeping an open mind and considering all possibilities."

Other theories have surfaced. Perhaps the birds are ingesting toxic fire retardants that washed to sea after recent fires. Maybe they're sick with a viral infection. Or perhaps the cold weather that hit the Pacific Northwest in December boosted their susceptibility to some yet-identified disease. Or there could be a confluence of factors.

The majority of discovered pelicans are either dead or must be euthanized because of weakness or injuries suffered due to disorientation, such as severe wing fractures. "Someone described watching a pelican walk unsteadily - then he just fell over and died,'' Pyne said.

But those in treatment are being nursed back to health with a diet of fish, water and vitamins, at an estimated cost of $800 to $1,000 per bird, she said. Then they will be released.

''They're recovering and eating,'' Pyne said. "They're getting stronger and seem to be coming around.''