Here in California, the wind farms have lots of windmills - there must be more than a hundred at Altamont Pass (near Livermore, site of the infamous Dec 1969 rock concert). Altamont is apparently a serious hazard for raptors (see here -click). The other wind farms I know are at Pacheco Pass (between Fresno and Monterey) and Tehachapi Pass (between Bakersfield and Barstow). I'm sure there are others.

It might seem that windmills would be a negligible hazard to birds, but a concentration of windmills can do a lot of harm to a migrating flock of birds - many of the passes where we have wind farms, are on major flyways. The passes concentrate the wind currents, which would seem to make them good for both wind farms and for bird migration - a dangerous combination.

-Joe Offer-

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This article explains how Altamont is an exception, and that the usual loss of bird life may be negligible.

Also see this article.