Monday, Sep. 21, 1953
Guiding Clicks
Bats navigate the night by a natural system of echo sounding, much like the Navy's elaborately developed sonar (for sound navigation and ranging). They send out pulsed ultrasonic squeaks that bounce back from obstacles and tell them how they are going from point to point. Zoologist Donald R. Griffin of Harvard, an authority on bat navigation, has long suspected that birds living a batlike life (feeding at night and roosting in dark caves) may use a similar system. At the invitation of William H. Phelps Jr. of Caracas, he went to Caripe, Venezuela, to study the cave-dwelling oil birds (Steatornis caripensis), whose buttery young are boiled down by the Venezuelans for edible fat. Griffin reported his findings in the Proceedings of the National Academy -of Sciences.
The oil birds of Caripe, about as big as screech owls but with longer wings (about a yard in span) live in a pitch-black part of a deep cave more than 2,000 ft. from the entrance. When disturbed they raise such a racket that the local Indians used to believe that the birds were the ghosts of their ancestors. Undismayed by their earsplitting shrieks and whoops, Griffin and Phelps dragged high-fidelity sound recorders into the cave. The birds quieted down when they left the cave at night, but whenever they flew they made a clicking sound like a rapidly turned ratchet.
This clicking, recorded on magnetic tape and analyzed with an oscillograph, proved to be short bursts of sound about 400 to the second, with only a few waves in each burst. They were remarkably like the pulses sent out by both radar and sonic depth-finders, and they certainly appeared to be an efficient means for measuring a bird's distance from an unseen obstacle.
To make sure, Phelps netted four oil birds and took them to Caripito, where the Creole Petroleum Co. set up an improvised darkroom to make further tests. In total darkness, the birds flew around the room, their clickings and wing beats clearly audible. But when Dr. Griffin plugged the ears of the three strongest birds with cotton and Duco cement, they blundered helplessly into the walls. With the light turned on, they could fly all right by sight, but they could not fly in darkness if they could not hear their own clicks.
Dr. Griffin concludes that the oil birds navigate by echo sounding. They differ from bats chiefly in the pitch of their guiding pulses, which are in the audible rather than the ultrasonic range.
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