How the Owl Tracks Its Prey
Experiments with trained barn owls reveal how their acute sense of hearing enables them to catch prey in the dark
Locating Artificial Sounds
If the owl can be trained to locate electronically generated sounds, the cue effectiveness of various acoustic parameters can be analyzed under rigorously controlled conditions. I trained three owls to strike in the dark protected loudspeakers emitting various sounds of known physical properties. This I did in the following manner.
Since the hand-reared owls used in this work had had no experience catching mice in the dark, the training was done in several steps. The owls were first allowed to catch live mice under dim illumination and then in the dark. When pure tones and noises were broadcast through an earphone placed next to a dead mouse in dim light, the owls quickly learned to associate the artificial sounds and the mouse. After this step, they struck in the dark the earphone emitting those sounds.
Since the owls would stop striking the target as soon as they had eaten two mice, I constructed a device to dispense small pieces of meat as rewards for accurate location of the target. The owls learned to eat from the feeder within a few days. In the final stage of the training, the owls struck protected loudspeakers in the dark, then moved to the feeder for reward under dim illumination, and returned to the perch to wait for the next signal.
The accuracy of location was measured by an electronic device that registered the position of the owl as it struck the floor. It consisted of 100 square masonite plates, 10 centimeters x 10 centimeters and 20 centimeters x 20 centimeters, laid out like a chessboard covering the part of the floor where the owls were trained to land. The smaller plates surrounded the speakers, and the larger ones filled the remaining space. These plates were padded with foam rubber so that the owl could strike them hard without damaging the talons.
When the owl struck the plates, the microswitches installed beneath them closed and turned on small neon lamps which projected the chessboard pattern in a reduced size onto a panel. Six loudspeakers were installed under the chessboard floor. Short rubber tubing led sound from the loudspeakers to fixed intersections on the surface of the chessboard. The distance between the owl’s position and any one of the speakers could be read immediately on the lamp panel.
The resolution of this measuring system was satisfactory for the purpose of the work. It was adjusted to the owl’s talon spread, which covered an area slightly larger than one 10 centimeters x 10 centimeters plate. When only one plate was struck, the midpoint between the owl’s feet was always close to the center of the plate. A slight deviation from the center would cause the owl to step on an adjoining plate, which means that little would be gained by using plates smaller than 10 centimeters x 10 centimeters. All training and tests were done in a soundproof, anechoic room 5 meters in length and 3 meters in width and height. The general layout of the room is shown in Figure 5. (Other technical details and the statistical treatments of the results are partly covered in Konishi 1973)
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