West Nile virus hits bird populations

At least five common birds, including robins and bluebirds, suffered sustained, large-scale population declines because of West Nile virus, a new study suggests.

The mosquitoborne virus, already known to sicken and kill wildlife as well as people, was first documented in the United States in 1999. Now, birders’ records give the first look at what the disease has done to whole bird populations, say researchers from the Smithsonian Migratory Bird Center in Washington, D.C., and the Consortium for Conservation Medicine in New York City.

Shannon L. LaDeau of the Smithsonian turned to records from the annual nationwide Breeding Bird Survey. Each June, volunteer surveyors drive predetermined 25-mile routes along secondary roads and stop every half mile for 3 minutes to count birds.

LaDeau and her colleagues studied 20 species commonly reported in these surveys. The researchers selected a range of species that earlier experiments had suggested would have different sensitivities. Lab studies had exposed various species to the virus and tracked the percentages of deaths. Field studies had identified the bird species that disease-carrying mosquitoes prefer to bite.

After developing statistical approaches to cope with the irregular data from a huge corps of birders, LaDeau plotted trends in reports from 1980 to 2005 along 228 routes in 10 states. Once the virus hit a particular state, she used the older data to predict what the birds’ numbers would have been without the disease.

For seven species, numbers dropped significantly below these predicted levels. House wrens and blue jays dipped but rebounded by 2005. Declining and not rebounding were American crows, American robins, Eastern bluebirds, both black-capped and Carolina chickadees, and tufted titmice, she and her colleagues report in the June 7 Nature.