Once a year, a small forest in Zambia becomes the site of one of the world’s greatest natural spectacles. In November, straw-colored fruit bats migrate from across the African continent to a patch of trees in Kasanka National Park. For reasons not yet known, the bats converge for three months in a small area of the park, forming the largest colony of bats anywhere in Africa. The exact number of bats in this colony, however, has never been known. Estimates range anywhere from 1 to 10 million. A…