As if right on cue for Halloween, this is the time of year when millions of straw-colored fruit bats begin to descend on Zambia"s Kasanka National Park. They come in such vast numbers that it"s thought to be one of the largest concentrations of mammals in the world. From the end of October to mid-December, these large fruit-eating bats pack into a small area in the forest to roost. But when the sun begins to set, they start to swarm, darkening the skies as they head out to gorge on fruit in the Mushitu swamp forest. Experts believe their migration starts about 2,000 miles north, in Congo.