Impact of West Nile virus on California corvids with emphasis on the yellow-billed magpie (Pica nuttalii) and western scrub-jay (Aphelocoma californica)

The Yellow-billed Magpie (Pica nuttalli) and Western Scrub-Jay (Aphelocoma californica) are non-migratory corvids found in the western United States; the magpie being endemic to California. Data suggest these species are extremely susceptible to West Nile virus (WNV) and that their abundance has decreased since WNV establishment in North America. Carcass data from 2004-2005 show that 82% of magpies and 70% of scrub jays tested WNV-positive, and these were the highest such proportions of all California corvids. Widespread index counts in California showed a decline in magpie detections of 42-49% from 2004-2006 (2004 being the first season of exposure to magpies).;Density was greatest in agricultural habitats (8.2 magpies km -2) and least in urban habitats (1.3 magpies km-2). Abundance was greatest in rural habitats (245,221) and least in urban habitats (5,347). While non-significant, the potentially continued decline and restricted range of this species warrants further monitoring. For scrub jays, abundance estimates increased over the study period, from a low of 24 jays km -2 (2,340,187 jays) in February 2008 to a high of 78 jays km -2 (7,483,871 jays) in November 2008. A log-transformed regression showed a significant trend of 38% (p=0.004), suggesting any impact of WNV was mitigated by juvenile recruitment, and that scrub jay populations may be developing resistance to WNV. Density was greatest in agricultural habitats (98 jays km-2) and least in rural habitats (38 jays km-2). Abundance was greatest in agricultural habitats (2,578,360) and least in urban habitats (356,575). Inclusion of covariate data in modeling the probability of detecting birds often reduced the variance of density estimates or significantly improved model results.;Because range-wide monitoring was warranted, I surveyed randomly established point transects throughout the magpie's range in 2007 and 2008 to estimate their current distribution, and estimate the abundance and temporal trend of both magpies and scrub-jay populations. The magpie's range showed contractions since historical estimates. The lowest estimates of magpie abundance coincided with warm seasons that facilitate WNV transmission and replication. Density and abundance over the study period was 6.1 magpies km -2 (13.1%CV) and 396,399 magpies, and a log-transformed regression showed a numerical, but non-significant (p=0.37), change of -4.9%.