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
Editor’s Note: This Classic article was first published in the July–August 1973 issue, and is reprinted as part of American Scientist’s centennial year celebration. The author is recognized for his research on prey capture auditory systems in owls and singing in songbirds. Konishi received his B.S. and M.S. in zoology from Hokkaido University in Japan before coming to the University of California at Berkeley, where he obtained his Ph.D. in zoology in 1963. After two years in Germany in postdoctoral positions at the University of Tubingen and at one of the Max-Planck Institutes, he returned to the United States, working as a professor first at University of Wisconsin, then at Princeton University, and finally at California Institute of Technology, where he spent the rest of his career. He was elected into the National Academy of Sciences in 1985. Dr. Konishi’s current address: Division of Biology, MC 156-29, California Institute of Technology, Pasadena, CA 91125; konishim at caltech dot edu.
Payne and Drury (1958) were the first to demonstrate the ability of the barn owl (Tyto alba) to locate mice acoustically in total darkness. In a series of experiments, I have replicated their observation. A barn owl in pursuit of a mouse in the dark flies about 3.6–4.0 meters per second; it will fly faster if the mouse is visible or more slowly if the identity of the target is uncertain. As the owl comes within a range of about 60 centimeters from the mouse, it brings its feet forward and spreads the talons in an oval pattern. Just before hitting the mouse, it stretches its legs forward with the face and the wings lagging behind, often closing its eyes during this last phase of the strike.
The mouse does not seem to die instantly despite the powerful impact of the strike. Soon after landing, the owl always manages to bite the back of the mouse’s neck to kill it. Should the owl miss the mouse it will remain motionless on the ground and listen to the mouse in order to strike again from the landing site. If the owl can see the mouse hide behind the wall or under the floor, it will eagerly search for and run after it like a cat.
In the infrared photograph in Figure 1 it appears as if the owl were looking at the tethered mouse as it is about to strike. This worried me a little, since I could see through three layers of infrared filters the strobe filaments glow red as they fired. Of course, the owl could catch mice without the infrared strobes. My worry was whether or not the pictures I was taking depicted the true behavior of the owl in total darkness. In order to clear this doubt I repeated a clever experiment conducted by Payne (1962).
When a mouse walked quietly on foam rubber towing a rustling piece of paper several inches behind its tail, the owl tried to strike the paper instead of the mouse. Figure 2 shows the owl preparing to land on the paper, without noticing the mouse a small distance away. Besides demonstrating that the owl cannot see the mouse, this experiment proves two other important points: the owl cannot locate the mouse either by its smell or by its body heat (infrared radiation). (See Payne 1962 and 1971 for earlier papers on prey capture by owls with methods other than passive sound location.)
The above and later experiments might give the reader the impression that the owl strikes any sources of noise indiscriminately. Quite the contrary is true; the owl will not strike sounds new to it. Also, it can learn quickly slight differences between sounds bringing reward and no reward. If the owl has associated the appearances and sounds of prey and enemy a few times, it should be able to discriminate between the two in the dark by hearing alone.
The rustling noises of the prey contain all the information needed for the owl to locate it in space. In order to design the later experiments, precise knowledge of the physical characteristics of these noises was needed. Since the vole is the main diet of the barn owl in the northeastern United States, I recorded and spectrographically analyzed the noises made by a vole moving through its subterranean hay-lined tunnel runways in a 20-gallon container within a soundproof room. The rustling noises contain a wide range of frequencies, as shown in Figure 3, but these data alone do not mean anything without knowing the auditory capability of the owl, and thus the hearing threshold of the barn owl had to be determined.
Three owls were used for this purpose. They were trained to take off from the perch for reward when they could hear a tone. Figure 4 presents the results from the owl that was most carefully tested. The owl’s hearing curve was drawn by connecting sound levels at which it responded correctly 75 percent of the time. The figure also compares the audibility curves of man, cat, and barn owl. Note that the cat and the owl have very similar auditory sensitivities up to about 7 kilohertz, beyond which the cat continues to be sensitive, while the owl’s sensitivity starts declining sharply. Both animals are much more sensitive than man in the frequency range from about 500 hertz to 10 kilohertz. No other birds that I studied are so sensitive as the barn owl, although some songbirds may be able to hear frequencies as high as the barn owl can (Konishi 1970).
Besides these quantitative data, I compared people and the owls under the same conditions. The owls could hear sounds which were so faint that none of my young undergraduate students and assistants could register them at the distance of the owl’s perch; however, above 12 kilohertz man is more sensitive than the barn owl. The barn owl thus can hear a large portion of the prey’s rustles, but it does not necessarily follow that the entire audible part of the rustles is equally important for sound location by the owl.