Here is something to consider, while it is not a bird subject these towers will affect the avian population as well.
This is from the "Smithsonian" magazine, Nov. 2008 issue, page 16:
"Mystery Deaths Solved":
"Multitudes of bats die around the world each year when they migrate through electricity-generating wind farms. U. of Calgary researchers now say the main cause of death is "barotrauma": the turbines' large revolving blades create low-pressure zones that cause the bat lungs to suddenly over-expand, tear and bleed."
Something to keep in mind as we consider energy alternatives. I would think shutting down the generators when migration was on would solve the problem.