Study Links West Nile Virus to Declines in Crows in California

Jays and magpies also affected

Dead birds tell tales. A study comparing data collected from dead birds and from bird population surveys reveals new information about the impact of West Nile virus. Since the virus began its rapid westward march from New York City in 1999, sharp declines in localized crow populations have been documented across the country.

A study by Walter Koenig of the University of California at Berkeley and colleagues, including Janis Dickinson, director of Citizen Science at the Cornell Lab of Ornithology, shows a direct link between exposure to the virus and bird population declines on a much larger geographical scale than before, and for multiple species (EcoHealth, February 2007).

Black-billed Magpie
by Pedro Fernandes

Koenig and colleagues looked at data for 29 species tested by the California Dead Bird Surveillance Program between 2004 and 2005, when West Nile virus spread to 95 percent of the state. Blood tests on the dead birds showed whether or not they had been exposed to the virus. Koenig then looked at population changes for the same species in California Breeding Bird Surveys conducted during nine years preceding the arrival of the virus, continuing through 2005.

"We did indeed find that species showing a higher rate of exposure to the disease experienced population declines on a statewide scale," says Koenig. "Declines were generally most pronounced for crows, jays, and magpies—the majority of species in the corvid family, with the exception of ravens—even though the West Nile virus is known to affect more than 200 species of North American birds."

American Crow
by Pedro Fernandes

"I think it's fascinating that ravens turned out to be no more affected by the virus than your average perching bird, even though they belong to a family that is, for the most part, highly susceptible to the disease," Koenig says.

Koenig says other questions deserving study include whether or not there is a direct correlation between temperature and population declines, since more disease-carrying mosquitoes breed in warmer weather. He says he'd also like to know if populations of affected species bounce back, and how the sudden decline of a species because of West Nile virus may affect an entire ecosystem.

—Pat Leonard

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu