SEEING WITH SOUND

摘要

Although oilbirds and swiftlets are not related, they have something extraordinary in common - both can echolocate. Unlike bats, they utter audible clicks, but the principle is the same: they use sound waves that bounce back off objects to "see" in the dark. Low-frequency sounds are less effective for echolocation than the high-pitched calls of bats, but tests with oilbirds show that they are able to avoid objects larger than about 20 centimetres across. This is sufficient to allow them to find their nesting and roosting ledges in the totally dark caves they inhabit.