2 ways to skin a bird...
From what little I've read, bird kill mostly resulted from 2 factors. First was locating wind farms in migration corridors. Now they don't do that. the second was that early towers were built with struts and beams and what not, like those big transmission towers. They're cheap, but give birds a gazillion nesting sites. Now all the towers are round, sheathed and don't have any good place to perch, much less nest. Apparently these 2 steps have reduced bird kill by something like 90%.