"Several common species of North American birds have suffered drastic population declines since the arrival of the West Nile virus eight years ago, leaving rural and suburban areas quieter than they used to be and imposing ecological stresses on a variety of other animals and plants, a new study has found.

In Maryland, for example, 2005 chickadee populations were 68 percent lower than would have been expected had West Nile not arrived, and in Virginia chickadee populations were 50 percent below that prediction.

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

West Nile virus is native to Uganda and is believed to have hitched a ride to New York inside a bird or mosquito in 1999, probably on a plane or ship. It quickly spread across the United States, one mosquito bite at a time, leaving a large but unknown number of birds dead -- along with thousands of horses and, to date, about 1,000 people.

"This is another warning that we need to reassess how we go about global trade and how we move things around on planes and through the pet trade," said ecologist and study leader Peter Marra, with the zoo's Smithsonian Migratory Bird Center in Washington.

The next shipment of exotic birds to pet stores could bring avian flu, Marra said, forcing widespread culling of commercially valuable flocks and perhaps even launching a human epidemic. "We have to ask, 'Are the risks worth it?' "

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 populations directly, species by species and year after year at the same locations.

It shows that the post-1998 declines were greatest at times and places in which the virus was especially prevalent -- as indicated by the number of human infections diagnosed. As expected, American crows were among the worst hit, suffering declines of as much as 45 percent in some regions and wipeouts of 100 percent in some smaller areas. Other species that suffered included the blue jay, the tufted titmouse, the American robin, the house wren, the chickadee and -- unexpectedly -- the American bluebird.

"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 Shannon LaDeau, an ecologist at the bird center who led the study with Marra.

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 whether they have developed immunity and whether those recoveries will last. Other species remain significantly down in numbers relative to what scientists would expect to be seeing had West Nile not arrived, based on trends over more than 25 years.

In the Northeast, for example, chickadees have dropped by 53 percent and the Eastern bluebird is down 44 percent. In Maryland, American robins took an especially large hit, with the virus apparently responsible for a 32 percent population reduction.

The trend "suggests that West Nile virus could potentially change the composition of bird communities across the entire continent," Carsten Rahbek, an ecologist at the University of Copenhagen, wrote in a commentary accompanying the research 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.

Because the survey is largely a record of bird calls heard on certain rural roads during morning outings that take place each June, the new analysis does not capture the fates of waterfowl, nocturnal birds and birds other than songbirds, including the hawks and owls that are believed to be among the hardest-hit species.

"The declines we see are probably a signal of a more serious ecosystem challenge that is having much broader effects than we're currently able to detect," LaDeau said.

Given the interconnectedness of the food web, those broadened effects are almost certainly reaching beyond birds, she added.

For example, crows are important scavengers, clearing away roadkill and keeping competing pests at bay. And their penchant for eating other birds' young suppresses a wide range of other avian species.

"American crows are often considered a nuisance, but when the crows go, do we get more rats?" LaDeau asked. "What other scavengers come in, and what happens to the bird populations that are regulated in part by crows?"

Field studies suggest that the loss of seed-eating birds such as chickadees could also have ecological ripple effects by reducing the natural dispersal of seeds.

Also of concern: bird species that are weakened but not devastated by West Nile could collapse if a second challenge arises, such as an especially hot or dry summer or the loss of a crucial food source due to disease or pollution.

Beyond working to limit the unregulated transport of animals, the best thing people can do to stem West Nile's tide of destruction is to keep their yards clear of standing water, Marra and LaDeau said.

Even a bottle cap of water can be home to hundreds of mosquito larvae, LaDeau said -- a chastening thought for backyard beer drinkers." (Rick Weiss, The Washington Post, May 17, 2007)