West Nile virus has profound effect on birds
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A lot of the crows are gone from the telephone lines. And there are fewer chickadees, bluebirds and even robins at backyard bird feeders this spring, because of the devastating and still lingering, deadly effects of West Nile virus.
Seven of 20 common bird species have declined significantly across entire regions of the country due to the alien virus, according to research published yesterday in the scientific journal Nature. And only two -- blue jays and house wrens -- show signs of recovering from the intense epidemics that hit hardest in 2002 and 2003.
"The extent of these declines shows how devastating introduced pathogens can be," said Dr. Marm Kilpatrick, senior research scientist for the Consortium for Conservation Medicine, based at Wildlife Trust. "The globalization of trade and travel that brought West Nile virus to the Western Hemisphere has completely altered our bird communities and may make some of our backyard birds relatively uncommon."
West Nile virus, native to Uganda in east-central Africa, arrived in New York City in 1999, most likely in a mosquito on a ship or plane. It spread quickly, mosquito bite by bite, in geographic ripples west and south across North America over the next eight years, killing unknown numbers of birds, thousands of horses and about 1,000 people, including four in Allegheny County in 2002.
The study, funded by the National Science Foundation, the National Institute of Allergy and Infectious Disease, and the Smithsonian Institution, was undertaken because, despite the deaths of tens of thousands of individual birds in wild, zoo and pet populations, the species-level impacts of the disease were unknown. It is the first to track the impact on species year to year.
Lab experiments predicted that certain species would be hurt more than others, and as expected, American crows were hit hardest, with declines of as much as 45 percent throughout most of the nation. Eastern bluebirds, which researchers did not think would be impacted significantly, saw populations plummet by 21 percent in Illinois to as much as 52 percent in Maryland.
"Our work demonstrates the broad and potentially devastating impacts that an invasive pathogen can have on our native wildlife," said Shannon LaDeau, an avian ecologist with the Smithsonian's Migratory Bird Center at the National Zoo and a co-leader of the study. He noted that the research focused on a small subset of bird species and did not look at birds of prey or waterbirds, which may or may not also be in decline due to the virus.
In a five-state northeastern region including Pennsylvania, the study found the chickadee population dropped 53 percent, eastern bluebirds 44 percent, American crows 31 percent and robins 17 percent. The tufted titmouse had a 10-percent decline, but populations of house wrens and blue jays were unaffected, although both species experienced population declines in other areas of the United States.
Dr. Kilpatrick said the robin turned out to be a carrier of the virus because of mosquitoes' fondness for the bird: While robins make up only 10 percent of the bird population in the northeast, they make up half of the mosquitoes' "blood meals."
The biggest bird impact of West Nile virus in southwestern Pennsylvania occurred in 2002-03, according to Dr. Robert Wagner, a University of Pittsburgh veterinarian with extensive experience at the National Aviary and the Pittsburgh Zoo & PPG Aquarium.
"The first year of West Nile was really bad, then that went down to a handful of cases the second year and declined more each year," Dr. Wagner said. "I only saw one case this past summer, but it's here. It's in Pittsburgh. We're just not seeing a lot anymore."
Why that is, no one knows for sure. It could be that most of the vulnerable birds have already succumbed to the virus. It could be that some bird species developed immunities to it, Dr. Wagner said.
His observations match up well with Allegheny County Health Department's West Nile virus program statistics that show in 2002 that 162 dead birds tested positive for West Nile, as did 34 mosquito samples. That year, 22 humans caught the virus and four died.
All those numbers are in decline. In 2006, only one bird and 11 mosquito samples tested positive for the virus and no humans were sickened.
Guillermo Cole, a Health Department spokesman, said the annual West Nile reporting program will restart at the end of May. County residents are asked to report all dead birds except pigeons, chickens and waterfowl. The department might request residents bring in dead crows, hawks, owls, blue jays and falcons, depending on their condition.
Dr. Wagner said most North American raptor species are very sensitive to the virus, especially red tail hawks, owls and others that live near swamps and rivers.
Charles Bier, director of natural heritage for the Western Pennsylvania Conservancy, said Pittsburgh's peregrine falcons could be vulnerable to West Nile, but none of the falcons has shown any signs of the virus during their annual physicals.
"Where they're living, up on the 37th floor of the Gulf Tower and near the top of the Cathedral of Learning, is not a real mosquito environment," he said. "And there's no way to pick it up from the birds that they're eating."
First Published May 16, 2007 11:16 pm