I wrote a post at 10,000 birds that starts to explore this question by comparing the number of birds that there are, the number of birds that seem to die off every year, and the possible number that get munched by cats, and briefly discusses predator niches and that sort of thing. I hope this is the start of an interesting discussion that will eventually get us tilting at windmills.
Have a look: Are there so many birds that cats don’t matter?
I realize that this discussion is mostly about birds, but those studying kills at wind farms in Oklahoma tell me that more bats than birds are found dead near the wind turbines and that some species of bats appear to be hatrmed more than others.
That's an interesting discussion as well.
I think you are right, bats are a bigger problem. http://www.fort.usgs.gov/BatsWindmills/
Which reminds me when I was driving in he UK recently I saw these high fishnetty things going high above highways. Apparently they are to provide bats that follow hedges a continuity from one side of the road to the other. I thought it was cool. You can see an article about them here:
I'm still betting high-rise buildings kill more birds than wind turbines do. Then there are the airplanes on takeoff and landing. My list of things to ban is getting longer: cats, skyscrapers, airplanes, helicopters, humans ...
Oh - does the dead bird count include all those chickens I get from Frank Perdue?