Every year, millions of straw-coloured fruit bats descend on Zambia's little-known Kasanka National Park, creating one of Africa's most extraordinary wildlife spectacles. There's a storm on the horizon. Lightning flickers, illuminating the Central Zambezian miombo woodlands with brief flashes of silver. The sun is setting, the air smells of damp earth and somewhere ahead of us, millions of bats are waking up. We park our vehicle in front of the Musola Hide, Kasanka National Park's prime bat