Changes in abundances of American crows, blue jays and robins in Northwest Ohio following arrival of West Nile Virus

Background/Question/Methods

West Nile Virus (WNV) arrived in Toledo, Ohio, in 2001/2002. It is an avian disease that causes high rates of mortality in corvids, such as the American crow (Corvus brachyrhynchos) and blue jay (Cyanocitta cristata). We examined both the local Audubon Christmas Bird Counts (CBC) and Breeding Bird Counts (BBC) from 1990 through 2010 for changes in crow, jay and robin (Turdus migratorius) abundances. Robins were included for comparison because they are much less susceptible to WNV. All three species are abundant in the region. Climate data were also examined for potential changes in temperature and precipitation patterns over this period. 

Results/Conclusions

One-way ANOVA (all N=21) revealed a significant 87% increase in CBC for robins (P=0.02), a nearly significant 17% increase in jays (P=0.08) and a nearly significant 16% decrease in crows (P=0.08) after WNV. Linear regressions showed consistent trends in CBC increase for robins (R2=0.405, P<0.01) and jays (R2=0.238, P=0.02) over this 21-year period, but no overall pattern in crows. In contrast, crow CBC fell 59% from 2001 to 2002, and showed a significant linear increase over 2002-2010 (R2=0.691, P<0.01). Regressions of BBC over time showed significant declines from 1990 though 2010 for all three species (all P<0.05), but one-way ANOVA found that only crows had a significant (40%) reduction in mean population in the years following WNV. Climate data showed no significant differences in temperature or precipitation before and after WNV or any consistent relationships to either BBC or CBC for any species. These results suggest a significant impact of WNV on both BBC and CBC for crows in the Toledo area.