Wind turbines blades batter birds, killing 20,000 to 37,000 a year in the U.S., according to a 2007 National Academy of Sciences study, "Environmental Impacts of Wind-Energy Projects." In contrast, at least 90 million birds die annually by flying into buildings, more than 130 million die in collisions with power lines, and millions more are killed by pesticides and domestic cats, according to the study.