Making Sense of Contradictory Data

A news article by Sheryl De Vore contends that birds in Illinois (particularly raptors and crows) are at risk from West Nile Virus. Of particular interest is the information that 35 percent of 31 American Crows fitted with radio transmitters in east-central Illinois since May 16 have died from the effects of WNV. That’s a pretty high rate of mortality, and some cause for concern if, in fact, it is due to WNV.

But contradictory data comes from Lake Villa, Illinois, where Richard Biss has been making daily counts of crows in his yard for the past six years. His August 2002 counts were not significantly differently from those of previous August counts. In fact, a high count of 88 birds is the largest crow count he has ever had in August.

So what are we to make of all this? Are crows (and other birds) destined to decline precipitously due to the ravages of WNV? Or is it just a passing phenomenom? I subscribe to the latter explanation. While bird mortality may be relatively high when they are first exposed to WNV, I think it is likely that WNV-related mortality will taper off in subsequent years as the most susceptible individuals are eliminated from the population.