Several sources of data suggest that the Yellow-billed Magpie (Pica nuttalli), a corvid endemic to California, is extremely susceptible to West Nile virus (WNV) and that its abundance has decreased since the establishment of WNV throughout California in 2004. From 2004 to 2006, 12,211 Yellow-billed Magpie carcasses were reported to the California Department of Health Services. Seventy-eight percent of the 1,007 Yellow-billed Magpie carcasses tested were WNV-positive, and this was the highest proportion of WNV-positive carcasses of all California bird species with reasonable sample sizes (>20). Assuming a starting population size of 180,000 in 2003, California Department of Health Services data suggest that Yellow-billed Magpie populations may have been reduced by 49% in just two years. Breeding Bird Survey (BBS) data show a 22% decline as of 2005, and Christmas Bird Count (CBC) data show a 42% decline as of 2006. Furthermore, flock size at three traditional urban communal roosts monitored in Sacramento, California, decreased dramatically after the establishment of WNV, with two becoming vacant by summer 2005 and the third declining precipitously into 2006. Of 38 serum samples obtained from 21 Yellow-billed Magpies in Davis, California, in 2006, only one individual was found to produce WNV-specific antibodies. Range-wide monitoring is warranted to detect and track population trends of this species. Population size, genetic diversity and population structure, cause-specific mortality, and population viability should be evaluated.