West Nile virus depletes bird species from coast to coast

By Anita Manning, USA TODAY

West Nile virus has dramatically reduced populations of several common bird species, including robins, chickadees and other backyard visitors, says the first national assessment of the virus's effect on wild birds.

HUMAN ELEMENT: CDC maps and figures on West Nile cases

None of the declines has reached the point at which species are threatened, "but the impact is extensive. Almost 50% of crows have declined in some regions," says Marm Kilpatrick, senior research scientist with the New York-based Consortium for Conservation Medicine at the Wildlife Trust, a co-author of the paper in today's issue of the journal Nature.

Researchers examined 20 species, but "there are another 300-400 species of birds in North America. It would be foolish to think these are the only birds that have been affected."

West Nile, a mosquito-borne virus, first appeared on this continent in New York in the summer of 1999. During the next five years, it spread across the country and is now considered established here. It infects birds, humans, horses and many other animals, usually through the bite of an infected mosquito. There is a vaccine for horses, but not for humans.

Last year, according to the Centers for Disease Control and Prevention, it infected 4,268 people and killed 177.

Kilpatrick and researchers at the Smithsonian Migratory Bird Center used 26 years of data from North American Breeding Bird Survey sites across 10 states to predict the size of populations of common bird species. They compared the predictions with what was seen in the bird survey in the years after West Nile arrived and found that once the virus appeared in a region, there was a "steep and sometimes progressive" decline in populations of crows, robins, chickadees and eastern bluebirds, all of which had been increasing before the virus arrived. Declines also were seen in populations of tufted titmice, house wrens and blue jays.

Of the 20 species studied, 13 hit 10-year population lows after the West Nile epidemic of 2003, the worst year so far in the USA, when the CDC reported 9,862 people were infected and 264 died.

"We show a wave of bird decline from the East Coast to the West Coast that mirrors what we know about the dispersal of West Nile virus," says lead author Shannon LaDeau. But the decline is not uniform, even within regions. Crows, among the hardest-hit species overall, show declines up to 45% in the mid-Atlantic, she said, but "some sites in Maryland had declines closer to 85% and some had none at all." Researchers are trying to understand the variations.

Though the declines have been significant, the news is not all bad, says Erik Hofmeister of the U.S. Geological Survey. The survey shows an increase in some populations in the last year counted, he said, suggesting "there may be a rebound in some species." The report notes that blue jays and house wrens, which suffered declines, had returned to their pre-West Nile virus populations by 2005.

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