The spatial heterogeneity of West Nile virus (WNV) activity in Sutter County, CA, as measured by mosquito infection rates, was associated with spatial variation in the prevalence of Culex blood feeding on competent passeriform hosts. Overall, 42 vertebrate host species (31 avian, 11 mammal) were identified from 601 blood-fed Culex tarsalis Coquillett and 151 blood-fed Culex pipiens L. complex females using sequences of the cytochrome c oxidase I gene and the Barcode of Life Data Systems database. WNV infection rates were low at sites where the primary vector, Cx. tarsalis, fed frequently on domestic cattle or incompetent galliform birds and high when females fed frequently on American Robins, American Crows, and Yellow-billed Magpies. Opportunistic host selection by Cx. tarsalis in combination with spatial variation in the presence of highly competent corvid hosts appeared to determine the distribution of WNV activity in rural Sutter County, CA.