An interesting study in PLoS ONE: Collision Mortality Has No Discernible Effect on Population Trends of North American Birds. The authors essentially conclude that, while
"...millions of North American birds are killed annually by collisions with manmade structures, this source of mortality has no discernible effect on populations.
Naturally, I wanted to specify where wind-turbines fit into this picture, so I emailed one of the authors, Robert M. Zink from the Bell Museum of Natural History. He was kind enough to respond last evening, explaining that turbines constituted a relatively small component of man-induced avian mortality, in fact a trivial component when considered next to the effects of windows or even house-cats.
Mind you, this isn't an excuse not to make structures as bird-proof as possible. The City of Toronto, for example has guidelines re how to do that here. Zink's study is also fairly broad-brushed, and I don't know that it accounts for possible effects upon species at risk such as the Bobolinks of Grand Valley, where Grand Valley Wind Farms wants to build nine turbines. In such instances, there might well be reason to oppose the developer's efforts.