Spread by mosquitoes, the West Nile virus has sickened more than 23,000 people in the United States since its introduction around 1999, killing more than 930 of them. Indeed, most news accounts of the disease's rapid spread have focused on West Nile's infection risk to people. However, birds have also proved highly vulnerable, as a new study indicates.
Walter D. Koenig of the University of California Berkeley and his colleagues charted changes in the breeding populations of birds throughout California between two counting seasons—the mid-1990s and the 2004-05 season. The latter period was selected because it is the first year that West Nile virus had been detected throughout the state.
The federally organized North American Breeding Bird Survey collects population data each year at the height of a species' breeding season. Such data were used for the new study only when the same person was responsible for bird counts in both periods, owing to large individual variations in counting techniques and reliability.
The scientists also pored over data on West Nile virus testing of 13,000 dead birds in the year leading up to the later breeding-bird census. The researchers restricted their estimates of West Nile's likely influence to just the breeding populations of the 29 passerines—perching birds or songbirds—for which virus prevalence information existed. Birds in this collection included the raven and crow, American goldfinch, song and house sparrows, barn swallow, robin, Western bluebird, and northern mockingbird.
In the just published March EcoHealth, the researchers report finding that species that had the highest virus prevalence rates, based on dead-bird testing, also experienced the greatest population declines between the two survey periods. The correlations proved especially strong for four species: the American crow, yellow-billed magpie, Western scrub jay and Steller's jay. When the researchers examined breeding-population data for these species in the 10 years leading up to the latest census, they saw no similar trend. In other words, it emerged only in the 2004-05 season, when the virus became widespread throughout California.
Other species also appeared to show some West Nile losses, but not to the same extent as these resident—nonmigratory—species. One bird that appeared fairly resistant to the virus: the raven.
Koenig's team concludes that West Nile infections have "had significant negative impacts on avian populations at a large geographic scale."
