West Nile taking toll on birds

WASHINGTON — Several common species of North American birds have suffered drastic population declines since the arrival of the West Nile virus eight years ago, a new study has found.

Of the 20 species included in the study, American crows were the hardest hit, declining about 45 percent overall from 1998 to 2005. Populations of American robins, chickadees, eastern bluebirds, blue jays, tufted titmice and house wrens also dropped.

The analysis, led by researchers at the National Zoo, offers sobering evidence that even a microscopic invasive species can wreak long-term environmental disruption.

The virus is native to Uganda and is believed to have hitched to New York inside a bird or mosquito in 1999, probably on a plane or ship. It quickly spread, one mosquito bite at a time, leaving a large but unknown numbers of birds dead in its wake — along with thousands of horses and, to date, 962 people.

Experiments had predicted that certain birds might be especially vulnerable to West Nile infection, and earlier tests on birds found dead on the ground appeared to confirm that some species were suffering a significant toll. But the new analysis is the first to track actual numbers of birds, species by species and year after year at the same locations.

"These are not the rare, vulnerable populations we think of as being at risk due to introduced species. These are our everyday, backyard country birds," said study leader Shannon LaDeau, an ecologist at the Smithsonian Migratory Bird Center in Washington.

After bottoming out in 2003 and 2004, house wrens and blue jays returned to their pre-West Nile levels in 2005, though it remains unclear if they have developed immunity and whether those recoveries will last. Other species remain significantly down in numbers relative to what was projected had West Nile not arrived.

The trend "suggests that West Nile virus could potentially change the composition of bird communities across the entire continent," said Carsten Rahbek, an ecologist at the University of Copenhagen, in a commentary with the research published in today's online edition of the journal Nature.

The analysis did not seek to document every affected species, focusing instead on just 20 for which annual data were available going back many years. The data came from the North American Breeding Bird survey, conducted by volunteer bird watchers at the same 19,000 locations every year.

The study did not look at birds in Washington. West Nile has been late coming to the state, and only 18 birds — most of them crows — have been reported killed by the virus since 2001, according to state health authorities. The first three human cases were reported only last year.

"It's very premature to talk about a [significant] decrease in the bird populations in Washington. There's not enough of the virus here," said ornithologist Dennis Paulson, director emeritus of the Slater Museum of Natural History at the University of Puget Sound, Tacoma.

Seattle Times staff reporter Warren King contributed to this report.