WASHINGTON - Birds that once flourished in suburban skies have been devastated since West Nile virus emerged in the United States seven years ago.
Populations of robins, bluebirds, crows and four other species have dramatically declined across the continent, scientists reported today in the most comprehensive study of its kind. The research, published in the journal Nature, compared 26 years of bird-breeding surveys to quantify what had been known anecdotally.
"We're seeing a serious impact," said study coauthor Marm Kilpatrick, a senior research scientist at the Consortium of Conservation Medicine in New York.
West Nile virus, which is spread by mosquito bites, has infected at least 23,974 people since 1999, killing 962, according to the Centers for Disease Control and Prevention.
But the disease has been far costlier for birds. The death toll for crows and jays is at minimum in the hundreds of thousands, based on extrapolations from the number of dead birds found, Kilpatrick said.
It hit the seven species - American crow, blue jay, tufted titmouse, American robin, house wren, chickadee and Eastern bluebird - hard.
Only the blue jay and house wren bounced back, in 2005.
The hardest-hit species is the American crow. Nationwide, about one-third of crows have been killed by West Nile, said study lead author Shannon LaDeau, a research scientist at the Smithsonian Migratory Bird Center in Washington. The species was on the rise until 1999.
In some places, such as Maryland, crow loss was at 45 percent, LaDeau said. Around Baltimore and Washington, 90 percent were gone.
While crows are scavengers and often disliked, they play a key role in nature by cleaning up animal carcasses, LaDeau noted. Researchers will next look into what species benefit from the crows' disappearance.
The deaths came in patches, scientists said, with many in some places and none in others. Maryland appeared to be the epicenter of bird deaths, though LaDeau said that was partly because the data were not as good from New York, where the virus first hit in 1999.
In 2005, chickadees, Eastern bluebirds and robins in Maryland were found at levels 68 percent, 52 percent and 32 percent of what had been expected. Tufted titmouse populations in Illinois were one-third of what had been expected.
The die-off "tends to be more suburban areas," LaDeau said. "Some of the common backyard species including the blue jays, the robins, the chickadees have suffered significant declines. That heavily packed urban corridor is a bad place to be a bird. The reason for that is that the mosquito prefers human landscape. They do very well in suburbia."
The researchers looked at 20 species that were regularly counted each breeding season. Thirteen of them were not down because of West Nile; seven were.
Biologists have reported deaths in other species, including owls, hawks, sage grouse and yellow-billed magpies, but there are no breeding surveys to quantify the problem.
Although entire small clusters of crows were "wiped out by West Nile virus in a single season," Greg Butcher, director of bird conservation at the National Audubon Society, remained hopeful.
"All of those have the capacity to rebound," he said.