Although the fight against the West Nile virus has focused mainly on the risk to humans, state officials and wildlife experts said the disease could ultimately take its biggest toll on California's bird population, where estimates put the deaths at 50,000 and rising.
As West Nile moved into Southern California this summer, dead crows and other birds found lying in parks and backyards were among the first signs. Now officials are trying to develop an accurate tally of the losses while also making sure that endangered birds, including the California condor, are vaccinated against the virus.
"When the virus first hit, we would get reports from people who were literally seeing birds falling from the sky, dead," said epidemiologist Brit Oiulfstad, who works with the Los Angeles County Department of Health. "Now the calls we get are from people asking, 'Where are all the birds?' "
The virus has been moving west across the United States since 1999, spread in part by migrating birds. Infected mosquitoes pass along the West Nile virus to both birds and humans. California's bird population is particularly vulnerable to the virus because the state is a major migratory hub for millions.
"Waterfowl, songbirds, hawks, millions upon millions of birds fly through California," said Walter Boyce, a veterinarian and executive director of the Wildlife Health Center at UC Davis. "It's called the Pacific Flyway. It's one of the biggest concerns; as birds move through California, we're seeing West Nile spread with them."
One of the biggest questions experts are trying to answer is whether the deaths of the birds will have any long-term effect on the ecosystem. Because many of the birds eat rats and snakes, some question whether the loss of predators will cause those populations to rise. There is little evidence of a major shift now, but officials say they won't know for sure until West Nile has run its course.
"That's the $64,000 question that everybody is asking and nobody has the answer to," said Patricia Bright, an official with the American Bird Conservancy, based in Washington, D.C. "There is a potential for a big effect. We really don't know how many birds we've lost."
Birds belonging to the corvid family -- which include crows, magpies and jays -- seem especially susceptible to the disease, Boyce and others said. Officials have found fewer birds from other families that have been killed by West Nile, but aren't sure why.
Scientists are puzzled, for example, about why pigeons -- one of the most common birds in urban areas -- do not appear to have died in larger numbers.
"But we don't really know about other species," Boyce said. "It could just be that corvids tend to live alongside humans and are easier to find and test."
Figuring out exactly how many birds have died also has proven difficult.
When birds die in the wild, 80% of carcasses are scavenged within 24 hours, said Bright. And sick birds probably would seek the safety of a secluded spot, making them even harder to find.
Until recently, health officials kept careful track of West Nile bird deaths as a way of helping chart the disease's spread around the state. But this summer, as West Nile began infecting humans, some agencies stopped collecting bird carcasses and testing them.
The California Department of Health Services has confirmed the deaths of 1,922 birds because of West Nile -- mostly crows, jays and ravens in Los Angeles and San Bernardino counties.
But at a recent conference of wildlife experts and scientists, vector control officials said they have received reports from the public of 60,000 dead birds this year. How many of those birds died from the virus remains unclear, but after reviewing lab tests performed on some birds and analyzing the virus' toll in other states, the scientists concluded that roughly 50,000 birds in California have been killed by West Nile.
The U.S. Centers for Disease Control and Prevention estimate that the actual death toll of birds is often 10 to 100 times higher than the number of birds officials test.
Nationally, West Nile has been detected in 234 of the roughly 750 species of birds in North America, according to the CDC, including the bald eagle, the American robin, the Baltimore oriole, the great horned owl and the white-winged dove. Most birds that become infected with the virus die of encephalitis or meningitis.
Experts are especially concerned for species already near extinction. Species with large populations and robust reproduction rates, like the American crow and the house finch, can lose large numbers and still bounce back, experts said.
But for species such as the San Clemente loggerhead shrike, which is found only in California and has dwindled to less than two dozen, or the California condor, of which only 234 survive, one outbreak of West Nile could mean the end, experts said.