Narrator: This is Science Today. Recent reports have indicated that the West Nile Virus has taken a worse-than-expected toll on seven species of North American birds, including robins, bluejays, and house wrens. The hardest hit species has been the American crow – with up to 45 percent of their population cut since the West Nile virus first emerged in the United States in 1999. Research entomologist Bill Reisen of the University of California, Davis, describes how the virus enters into areas where there are communal roosts of birds such as the American crow.

Reisen: These birds will have up to ten billion particles of virus in their blood by the time they die per millimeter of blood and so they're just essentially a sack of virus. They'll get ill and land in people's yards and then infect any mosquito that feeds on them with so much virus. So, these birds then amplify the virus and when they go out to forage from the communal roosts and then become ill, they don't return, so they can move the virus.

Narrator: For Science Today, I'm Larissa Branin.