Giant straw coloured fruit bats inhabit the great forests of the Congo and in late October they set off on a long journey across the forest canopy. Flocks of hundreds become thousands, and tens of thousands become hundreds of thousands. With wings measuring nearly a metre across, they are fast, powerful fliers that can travel over 1,000 miles in just a few nights. How they know when and where to travel is a mystery, but they all end up in one place, a remote swamp in Zambia called Kasanka. In a patch of forest no bigger than two or three football pitches, 10 million bats crowd together to rest and relax for a few weeks after the long journey. This is the largest fruit bat roost on Earth. It's so crowded, complete strangers rub shoulders with one another. As night falls, the reason for their lengthy journey becomes clear. The bats fly out to take advantage of the glut of mango fruits produced by the forest at this time of year. Each bat feeds on 2 kilos of fruit a night, and after six weeks the trees have been stripped of around a billion fruits. With the glut at an end, the vast aerial armada takes flight again and each bat returns to its particular patch of the vast Congo forest.