A bird navigates a cave by clicking

The oilbird nests in caves and finds its way in the dark by echolocation. The clicks are slow enough to hear. The system is coarser than a bat's, and it works.

Oilbirds roost and nest deep in caves, where there is not enough light to fly by. They make short clicks, a few milliseconds each, in pairs, and they dodge the walls. The clicks sit in the range of human hearing, unlike the ultrasonic calls of most bats, which is why a cave full of oilbirds sounds like a cave full of typewriters. Experiments in the dark — birds flying an obstacle course, clicks recorded, hearing measured — showed that the clicks are the instrument and the ears are tuned to them. Masakazu Konishi and Eric Knudsen put the hearing and the click together. The resolution is metres, not the millimetres a bat manages while hunting a moth.

The coarseness is the honest part, and it is still astonishing. The bird does not need to track an insect in flight. It needs to not hit a stalactite, and to find a ledge it already knows, in a black room. For that, an audible click and an ear are enough. Out in the forest at night, feeding on fruit, the same bird uses its eyes and its nose. The sonar is for the cave. One animal, two ways of knowing where the stone is.