WEST NILE VIRUS IMPACTS ON BLACK-BILLED MAGPIE POPULATIONS

Abstract

West Nile Virus (WNV) is an epizootic virus known to effectively kill Black-billed Magpies (Pica hudsonia). After WNV infection of humans appeared in the western United States, Black-billed Magpies birds per party-hours on Christmas Bird Counts (CBC) declined significantly. Trends for both CBC and Breeding Bird Surveys (BBS) shifted from positive before WNV infections of humans to negative during WNV infections, then rebounded to positive after WNV infection declined. Black-billed Magpies in Canada had no significant differences on CBC before or after WNV infections of humans. Canadian magpies had positive trends both before and after WNV infections on both CBC and BBS, but with negative trends during WNV infections. WNV invasion varied temporally across the continent, and Black-billed Magpies consistently showed significant and often dramatic declines soon after WNV invasion of a region. The southern Prairie region was severely hit, with Prairie states such as Kansas and Nebraska showing significant declines on CBC after WNV invasion, shifting from positive trends on CBC and BBS before invasion to negative trends during and after WNV invasion. WNV has potentially triggered or accelerated a retraction of Black-billed Magpie range in this region. The northern part of the Prairie region was less affected. Most southern and central Rocky Mountain, Intermountain, and Pacific region Black-billed Magpie populations showed significant negative effects from WNV infections. States in these regions with higher densities of Black-billed Magpies such as Colorado, Nevada, and Idaho showed significant dramatic declines during WNV invasion, but positive trends after WNV infections declined, a sign of recovering populations. California populations significantly declined with WNV invasion and have not recovered. Populations in northern and higher-elevation regions had little or no impact from WNV and populations were often stable or increasing.