More information on this subject, from a 2005 US Government
Accountability Office report. This is the basic summary:

"The impact of wind power facilities on wildlife varies by region and
by species. Specifically, studies show that wind power facilities in
northern California and in Pennsylvania and West Virginia have killed
large numbers of raptors and bats, respectively. Studies in other
parts of the country show comparatively lower levels of mortality,
although most facilities have killed at least some birds. However,
many wind power facilities in the United States have not been studied,
and, therefore, scientists cannot draw definitive conclusions about
the threat that wind power poses to wildlife in
general. Further, much is still unknown about migratory bird flyways
and overall species population levels, making it difficult to
determine the cumulative impact that the wind power industry has on
wildlife species. Notably, only a few studies exist concerning ways in
which to reduce wildlife fatalities at wind power facilities."

The entire report (64 pages) is available here:
http://www.gao.gov/new.items/d05906.pdf

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