Their greatest impact may be on the few species of bats that migrate. Bat experts say that the problem, which peaks during migration season from July to late September, may be worse than we know—but there’s cautious optimism, too. Proposed solutions include installing speakers that blast ultrasound to drive bats away and selectively shutting off windmills when bats are most active.

The first hint that wind farms were killing bats came from the Mountaineer Wind Energy Center on Backbone Mountain on the ridge of the Alleghenies near Thomas, West Virginia (map). Biologists looking for birds killed by the windmills in 2003 found nearly 400 dead hoary bats and eastern red-tailed bats. They soon concluded that the West Virginia site