Monday, September 23, 2002

The Response of Crow Populations to West Nile Virus

It is well established that crows are susceptible to the virus (i.e., WNV has been confirmed in numerous dead crows from localities across the eastern two-thirds of the U.S.), that many individual crows have died, and that many if not most of the dead crows examined have tested positive for WNV. There have also been numerous anecdotal reports this summer of reduced numbers of crows (and other species) in some parts of the eastern U.S. But is there, in fact, any quantitative evidence that crow populations have declined since the start of the WNV outbreak in 1999? To answer that question, I delved into the Christmas Bird Count database. For each of 13 States (CT, DE, MA, MD, ME, NH, NJ, NY, PA, RI, VA, VT, WV) in the northeastern U.S., plus the District of Columbia, I extracted information on the 10 highest counts of American Crows in each of three pre-WNV years (1996-1998) and three post-WNV years (1999 -2001). To reduce the confounding influence of increased numbers of observers and count effort between the two time periods (which could give the false impression that crow populations are increasing when they are actually stable), I used the number of birds reported/10 party-hours.

Counts of American Crows in the three post-WNV years do not differ significantly from counts in the three pre-WNV years:

Pre-WNV Median (1996-1998): 135 birds/10 party-hours (n = 359 counts)
Post-WNV Median (1999-2001): 144 birds/10 party-hours (n = 357 counts)
Combined Median (1996-2001): 140 birds/10 party-hours (n = 716 counts)

Thus, I conclude that there is no evidence of an overall decline in populations of the American Crow in the northeastern United States in the three years following their initial exposure to WNV. So it appears that crow mortality caused by WNV may be compensatory rather than additive, thus having little impact on overall population levels. See this explanation of compensatory and additive mortality.


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