In the cool of the morning at an isolated corner of the Salton Sea, Sarah Wheeler gently cradles a tiny marsh wren in her gloved hand. After some quick measurements, she crimps an ID band on the bird's leg. Then she draws a small blood sample that could help solve some big mysteries.

Wheeler is part of a UC Davis research team tracking West Nile virus in birds along the Salton Sea. Like Wheeler, scientists across the country are trying to glean answers that could solve the puzzle of this relatively new and potentially lethal mosquito-borne disease.

For example:

Why does West Nile virus seem to devastate some birds and not others? How did birds help spread the virus from coast to coast in just four years? What lessons does it hold about how emerging diseases might spread in the future? And what will happen to the crows and jays that appear to be most devastated by the disease?

As expected, West Nile virus has reappeared this spring in Salton Sea mosquitoes and American crows in Riverside, San Bernardino and as far north as Ventura County.

California's dead-bird hot line is flooded with reports of dead and dying crows, jays and other birds. Earlier this month, so many worried Southern Californians called the state's hot line that the phone system crashed.

State health officials have quit sending people to pick up birds in some communities, such as San Dimas and West Covina, where numerous infected birds already have been found.

"In those areas, they're concentrating on control, because they already know it's there," said Robert Miller, a state Department of Health Services spokesman. Still, Miller said people should continue reporting dead birds so the state can track die-offs.

Birds vulnerable

Mosquitoes spread the virus from bird to bird - and sometimes to humans and horses. A vaccine has been developed for horses but not for people.

Mosquito-control officials have been urging the public to empty neglected swimming pools and other potential breeding areas, to fix window and door screens and to wear protective clothing - long sleeves and pants - at dawn and dusk when mosquitoes are present.

In people, the virus can cause fevers, headaches, overwhelming fatigue and, in less than 1 percent of patients, a polio-like paralysis or brain inflammation that can lead to death. Most people, however, don't know they've been infected.

Crows, ravens, scrub jays and magpies are especially vulnerable. Large die-offs have been reported in other areas hit by West Nile. Hawks and owls also are sickened by the virus. Chickens, which are used to track emerging West Nile infestations, develop antibodies but don't get sick.

No one yet knows why.

"Clearly, if a mosquito bites a crow and the mosquito's infected with West Nile virus, the crow's prognosis is grim," said Aaron Brault, a researcher at the UC Davis Center for Vector-borne Diseases. In laboratory tests, every crow infected with the virus has died. No one has yet found a crow in the wild that survived a West Nile infection, Brault said.

But will the virus wipe out every crow in Inland Southern California?

"We really don't know on the population level what kind of effect the virus is going to have," Brault said.

West Nile virus was detected in North America in 1999 when New York City crows and other birds began dying mysteriously. Scientists and health officials eventually realized they had a new virus that was killing birds and making people sick.

In other parts of the country, West Nile has caused localized declines in crows, said Emi Saito, a U.S. Geological Survey scientist based in Madison, Wis. But crows haven't disappeared everywhere the virus has emerged, said Saito, the agency's West Nile virus surveillance coordinator.

In some East Coast areas, crow numbers have plummeted one year, then revived a couple years later, she said. But scientists don't know what that means for the long term, she said, because the virus is still too new.

One worry of wildlife scientists is how the virus will affect endangered birds, such as the rare California condor or the light-footed clapper rail, which numbers only about 300 pairs in Southern California and northern Baja California. Already, scientists have tried to immunize condors.

Saito said wildlife officials are especially concerned that Hawaii's native birds - many of them rare - could be devastated once West Nile reaches the islands. For example, she said, there are only about 37 Hawaiian crows - so rare that all are in captivity.

Learning experience

Patrick Redig, director of the Raptor Center at the University of Minnesota, believes hard-hit bird species will survive.

In Minnesota, more than 100 ailing red-tailed hawks, great horned owls and other raptors were brought to his center with West Nile infections in 2002, he said. By 2003, the center saw half that number of cases.

Redig believes the virus will kill a noticeable percentage of raptors and crows, but it won't kill them all. The survivors likely will be those most resistant to the virus.

"You'll probably get clobbered this year, and then it will melt down into a residual annoyance," he said.

Whatever happens, Saito believes West Nile will teach scientists a great deal about how previously unknown diseases can be spread so quickly, and how they can affect the health of animals - and people.

"It's a really good example of what happens when you introduce a new organism into an area where it's never been before," she said.

Out at the Salton Sea, Wheeler spreads her nylon mist net in a thicket of saltbush and arrow weed. As marsh wrens and yellowthroats flit from bush to bush, some are ensnared, unhurt. Wheeler and her helper, Mark Palmer, gently untangle the birds and stow them in a jury-rigged bird box until they can return to their pickup and draw blood samples.

Wheeler patrols several locations, checking bird traps for doves and quail and setting up nets to catch warblers.

So far, Wheeler said, none of the marsh wrens shows evidence of a West Nile infection, offering another mystery.

"We don't know if we're not getting positives in these birds because they're dying or they just haven't been bitten yet," Wheeler said.

Reach Douglas E. Beeman at (909) 368-9549 or dbeeman@....