Thursday, November 02, 2006

Crow Trends in West Virginia

In a recent post about corvids, John of A DC Birding Blog commented about the susceptibility of American Crows to West Nile Virus (WNV). Using information from the Christmas Bird Count, he demonstrated a decline of about 50 percent in early-winter abundance in the Mid-Atlantic States of Delaware, Maryland, and Virginia, from about 14 birds/party-hour in 1992 to about 7 birds/party-hour in 2005.

In striking contrast, early-winter populations of American Crows in West Virginia (as indicated by the number of birds counted/party-hour) have increased dramatically over the same time period, especially since 2002, and an apparent 5-fold increase since the mid-1990s (left-click on the graph below to enlarge). Also, crows appear to be relatively more abundant in West Virgina by an order of magnitude.

The same general trend is also evident when we graph the actual number of birds counted (below).

The reason(s) for the dramatic differences in population trends between the two adjacent regions is not obvious, but would certainly seem to merit additional investigation. Are the birds wintering in these regions from different breeding grounds in which susceptibility to WNV differs? Have crows shifted their wintering grounds westward for reasons unrelated to WNV? Is WNV less prevalent in West Virginia than in the Mid-Atlantic States? As is so often the case, this comparison raises many more questions than answers.



Blogger John B. said...

That is an interesting result. I wonder if urbanization has an impact on the prevalence of WNV.

November 02, 2006 4:54 PM  
Blogger John L. Trapp said...

I wondered the same thing, John. Although I haven't had time to research it, I believe it to be true that WNV is more prevalent in urban areas because the mosquitoes that transmit the virus primarily breed in containers, not natural wetlands.

November 02, 2006 6:17 PM  
Anonymous Anonymous said...

Look for recent closing and opening of regional garbage dumps for the mysteries of crow demographics.

November 03, 2006 9:16 AM  
Blogger yojimbot said...

My guess is that the WNV disease vector was first started on the east coast somewhere. By now it has run its course and the crow populations in these affected areas have already rebounded. Using New York as an example, crows have been virtually absent from the landscape since '98. However their numbers are now returing to the levels prior to infection. WNV is still just getting started in other areas of the Oregon, Washington, Canada and Alaska. I would expect that crows are now either dying or moving out of these areas and into places where the infection has already run its course.

November 03, 2006 1:40 PM  
Blogger Bill Pulliam said...

I'd suspect the old standby of land use changes, even in such a short time frame. Garbage dumps, as mentioned above; or mine reclamation areas, perhaps? The next step I would take is to look at individual counts and see if it is really statewide or being driven by changes at only a few spots. 500 crows/party hour is a phenomenal detection rate: one every 7 seconds. There must be major congregations contributing to this.

November 03, 2006 11:11 PM  

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