National Geographic: Windmills–clean, quiet, simple and endlessly renewable–may be the ultimate icons of green energy. But after sundown, their whirling blades have an unintended consequence that researchers are just beginning to understand: They kill bats by the thousands. 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. …