our reporter jurgen schneider visited zambia's kasanka national park to witness the world's greatest mammal migration. jurgen: it gets busy in october in zambia's kasanka national park. that's when the fruit bats arrive. >> the bats come from the drc, but also other colonies in zambia. they come here because this time of the year the fruits are ready and they're ripened. mainly they're feeding on local masuku fruits and waterberries, but also other fruit they just have in the villages like mango and bananas -- any fruit they can find. every time i see it, it's amazing. i mean, it's funny. at the start of the bat season when the bats first started arriving, we decided to come down to see how many there were. and even just coming down right at the start, when there are maybe only 10,000 or 20,000, we were blown away. jurgen: the bats' arrival marks the beginning of high season for dion scott. he's the park manager. and kasanka is the destination of the world's largest mammal migration. no one knows exactly why the fruit that grows here annually draws in some ten million of the animals,