Birds that once flourished in suburban skies, including robins, bluebirds and crows, have been devastated by West Nile virus, a study found.
Populations of seven species have had dramatic declines across the continent since West Nile emerged in the United States in 1999, according to a first-of-its-kind study. The research, appearing in Thursday’s issue of 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 co-author 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 23,974 people in confirmed cases since 1999, killing 962, according to the Centers for Disease Control and Prevention.
But the disease, primarily an avian virus, has been far deadlier for birds. The death toll for crows and jays is easily in the hundreds of thousands, based on the number dead bodies found and extrapolated for what wasn’t reported, Kilpatrick said.
The virus hit the seven species — American crow, blue jay, tufted titmouse, American robin, house wren, chickadee and Eastern bluebird — hard enough to be scientifically significant. Only the blue jay and house wren bounced back, in 2005.
The hardest-hit species has been 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, and around Baltimore and Washington, 90 percent was gone, LaDeau said.
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 disappearance of crows.
Researchers noted that the die-offs came in patches, with many in some places and none in others. Maryland appeared to be the epicenter of bird deaths, though that was partly because the data were not as good from New York, where the virus first hit, LaDeau said.
Chickadees, Eastern bluebirds and robins in Maryland were 68 percent, 52 percent, and 32 percent below expected levels in 2005. Tufted titmouse populations in Illinois were one-third of what they were expected to be.
“It tends to be more suburban areas. Some of the common backyard species including the blue jays, the robins, the chickadees have suffered significant declines,” LaDeau said. “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 birds act as an early warning system for humans, said Wesley Hochachka, assistant director of bird population studies at Cornell Lab of Ornithology.
“If you start seeing crows dying and dying in numbers, that means there could be a human outbreak,” said Hochachka, who was not involved with the study.
The researchers looked at 20 species that were regularly counted each breeding season and found that populations of 13 species were not down because of West Nile. Biologists say they have seen other species with many deaths, including owls, hawks, sage grouse and yellow-billed magpies, but there are no breeding surveys to quantify how bad the problem has been.
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 (bird populations) have the capacity to rebound,” he said.