Dead crows were the first clue that a new virus had began to spread across the United States 6 years ago. Now a major analysis confirms that American crows have been extremely hard hit nationwide--populations have fallen by 45% since the West Nile virus arrived--and several other common birds continue to decline as well.
West Nile virus, which was first identified in Uganda in 1937, is an indiscriminate killer that causes multiple organ failure. Spread by mosquitoes, it infects close to 200 kinds of North American birds and other vertebrates (Science, 21 February 2003). Nearly 900 people have died from West Nile virus since it was first detected in New York state in 1999. Although the crow deaths were glaringly obvious, it has been harder to gauge the impact on other birds. Three researchers set out to uncover the influence of West Nile.
The team examined 26 years of population data gathered by the North American Breeding Bird Survey, an annual census run by the U.S. Geological Survey and the Canadian Wildlife Service. Focusing on 20 species that are either known or potential hosts of West Nile virus, the researchers determined that 13 species had their lowest populations in a decade in the years after West Nile arrived, making the virus a suspect.
Some birds have been declining from other factors such as habitat fragmentation, but the trio argues that West Nile virus was the major force behind the declines of seven species, including crows and other common urban species such as the American robin and eastern bluebird. Why blame West Nile? First, the researchers note, lab experiments have shown that these species are vulnerable to infection. Second, the virus has been detected in wild birds from these species. Finally, statistical models showed that the declines cannot be entirely explained by El Niño/Southern Oscillations conditions that cause cold, snowy winters (which shrink the populations of some species, such as seed-eaters.) Overall, the study probably underestimates the effect on birds, the team says, because the breeding survey includes few of the urban areas especially plagued by mosquitoes that spread West Nile.
Not all the news is bad. Blue jays and house wrens took a beating from the virus in 2003, but bounced back by 2005, perhaps because they reproduce prolifically. And 13 other species, such as the Baltimore oriole, haven't declined, the team reports online this week in Nature. It's not known what's happening to the other more than 780 species of birds in the U.S. Based on anecdotal reports from wildlife recovery centers, a substantial fraction could be affected, says study author and ecologist Marm Kilpatrick of the Consortium for Conservation Medicine in New York City.
Ecologist Carsten Rahbek of the University of Copenhagen in Denmark notes in an accompanying commentary that the study shows that "West Nile virus could potentially change the composition of bird communities across the entire continent." That could bode ill for plants, Kilpatrick notes, as many of the birds eat caterpillars and other pests, while others disperse seeds. The crisis for birds isn't over yet, adds ecologist Kevin McGowan of Cornell University in Ithaca, as a lot of birds have not yet been exposed.