The Impact of West Nile Virus on Birds in California’s Hardwood Rangelands

West Nile virus has undergone an unprecedented rate of infection in North America, sweeping from New York to California in four years. It attacked an exceptionally broad range of hosts/vectors over a broad geographic area. In 2004, the California Department of Health Services received approximately 98,000 reports of dead birds, representing tens of millions of birds killed by West Nile virus in California. The rates of dead bird reports in California Hardwood Rangelands varied from 54 + 11 birds/km2 in the upper Sacramento River Valley to 3.7 + 0.8 birds/km2 in the southern coastal foothills. Overall, about 80 percent of hardwood rangelands were in areas of moderate to high rates of West Nile outbreak. Areas suffering the highest rates of mortality had primarily blue oak and valley oak woodlands, and bird species in blue oak woodlands suffered the greatest potential exposure to West Nile virus.