It spends its days in dark caves, maneuvering by means of a batlike sonar device, or echo-locator, found in its ears. The oilbird emits a clicking sound at an audible frequency of 7,000 cycles per sec, unlike the bat's cry, which is supersonic. Hence the pulsations of the oilbird can be easily detected by the human ear while the bird is in flight. As much as 13 in. They have powerful, hooked beaks surrounded by stiff, whiskerlike hairs.
Oilbirds are also the only nocturnal, fruit-eating birds.
They nest in large colonies on high, rocky cave ledges, often a good distance into the cave. The female lays two to four eggs per clutch, which hatch in about 33 days. Their naked young are fed on rich, oily fruits and become grotesquely fat, reaching twice the adult weight at their maximum size. In the past, baby oilbirds were captured, and their fat boiled down for torch oil, hence their name. Oilbirds are classified in the phylum Chordata , subphylum Vertebrata, class Aves, order Caprimulgiformes, family Steatornithidae.