Nesting swiftlet, Deer Cave, Borneo. These birds belong to the Apodidae family. They roost at night and breed in dark caves, using echolocation to detect objects. The nests are collected for the chinese delicacy Bird's nest soup. Deer Cave in Borneo is home to millions of bats. Their droppings collect on the cave floor and build up to huge heights. The droppings support a whole ecosystem of animals, described as troglobites, that live exclusively in the darkness of the cave.