National Geographic : 1965 Feb
The birds bounce the sounds off confining walls, gauging clearance by the time of return to their ears. While the chirps of bats are usu ally inaudible to man, oilbirds' clicks are well within the range of human hearing.* The guacharo is one of only two birds known to use this sonarlike echoloca tion technique; the other, more recently stud ied, is the Southeast Asian swiftlet, genus Collocalia, whose nests are prized for bird's nest soup. To prove that sound reflection is indeed the means by which oilbirds guide their flight in the dark, Donald R. Griffin, then of Cornell University, and William H. Phelps, Jr., of Caracas, Venezuela, once captured several birds from a Venezuelan cave, allowed them to fly about in a dark enclosure, and noted that they never collided with the walls. Then they plugged the birds' ears. The guacharos became completely disoriented and continually bumped against the walls. As we worked our way deeper into the cave, the squawking and screeching became almost deafening. The cavern suddenly ex panded into a huge room. The stale air moved refreshingly, stirred by the beating wings of the birds and of bats that share this under world. We tilted and turned our lamp. What * See "How Bats Hunt With Sound," by J. J. G. McCue, April, 1961, GEOGRAPHIC, and "Mystery Mammals of the Twilight," by Donald R. Griffin, July, 1946. 286 Crevice in a cliff provides a nesting site. Birds jealously guard such ledges, which are relatively scarce for the large population of the cavern.