David Attenborough visits a colony of oilbirds deep in a cave. Disturbed by his light, they make loud alarm calls so David switches to a red filter. Oilbirds are about the size of pigeons and are related to the nightjar. Their nests are compiled from their droppings and bits of regurgitated food. When their alarm calls subside one can hear the clicks by which they navigate. The calls are of a much lower frequency than the sonar calls of bats and are therefore less accurate. In fact, they can not detect an object less than about a foot across, but it is good enough to stop them crashing into the cave walls or one another. Their favourite food is the fruit of a jungle tree and the cave floor is covered by a soggy carpet of seeds. Many of them germinate although it is too dark for them to develop chlorophyll here so they soon die.