Yellow-billed Magpies (Pica nuttalli, Corvidae) are found exclusively in central California and have experienced alarming West Nile virus (WNV)–associated mortality since 2004. The first reported case of WNV in the species was reported in July 2004. Subsequently, 81% (304/374) of dead magpies submitted that year to the California Department of Health Services Dead Bird Surveillance Program were WNV positive by polymerase chain reaction (PCR) test. We studied 43 magpie carcasses collected in 2004 and observed distinctive lesions in 24 birds that tested positive for WNV. Lesions included vasculitis and necrosis, and organs affected included brain, heart, liver, and gastrointestinal tract. From the severity of lesions observed, we suspect that a rapid onset of morbidity and mortality occurs with the Yellow-billed Magpie. Examination of bird survey data indicates that Yellow-billed Magpie abundance declined coincidentally with the onset of WNV in California. The home range and habitat of the species are nested within known areas of WNV transmission. Yellow-billed Magpies may be at risk of a decline and population bottleneck. Observations and experience with the Yellow-billed Magpie and WNV may provide insights for other endangered corvids that have not yet been exposed to WNV, including the Hawaiian Crow (Corvus hawaiiensis) and Island Scrub Jay (Aphelocoma insularis).