On the decline

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.

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.

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.

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.

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

Fewer scavenging crows could leave more roadkill and garbage for the proliferation of rats. Fewer seed-eating birds such as chickadees could have ecological ripple effects by reducing the natural dispersal of seeds.

How can the average person help?

Reduce the number of disease-carrying mosquitoes by keeping yards clear of standing water they need for reproduction, the researchers said.