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
Sound Tracking in Flight
Small rodents make noises by moving. How does the barn owl catch a moving prey in the dark? What happens if, after the owl takes off, the prey moves or stops making sound? When I simulated these conditions electronically, I found that all three owls made larger errors when the signal stopped upon take-off than when it continued until landing (Figure 8). For example, when the signal continued until landing, one owl hit 46 times out of 58 trials within the area covered by the four 10 centimeter x 10 centimeter plates surrounding the target, while the same bird missed that area 69 times out of 86 trials in the absence of a post-take-off signal. Making the signal louder did not help the owl locate the target better without post-take-off signals.
The above results suggest that the owl can make mid-flight course corrections, like the moon shots, in order to strike the target accurately. Small rodents make noises intermittently, and the owl must be able to adjust to this condition. In another series of tests, I let the signal stop upon take-off and reappear after the owl had flown for varying periods. When the signal reappeared after the owl had flown for 0.5 second out of the total flight time of 1.2 seconds, the owl still struck the target as accurately as when the signal continued until landing.
The accuracy of location was not affected until about 80 percent of the total flight time was devoid of signal. When the owl had to fly for a period of 1 second without signal, it located the target as poorly as when the signal stopped completely upon take-off. Another factor that affects mid-flight corrections is the timing of post-take-off signals. When the owl could hear a faint and brief (50 milliseconds) noise burst three times (0.3, 0.6, and 0.9 second after take-off), it could locate the target as accurately as with a continuous noise.
The most crucial test of the owl’s ability to make mid-flight corrections involves the use of two loudspeakers: the signal shifts from one speaker to the other during flight. Figure 9 shows an owl changing its flight direction as the signal shifted from one speaker to another. Notice the direction of the owl’s face. It turns its face toward the new target position before orienting its body.
In Figure 9 infrared flashes were delivered at constant intervals of 250 milliseconds. Notice that the second and third exposures are closer together than the others; this is because the owl reduces its flight speed as soon as it hears a shift in the target position. When the owl has to make a large course correction, it comes to a sudden halt in midair and hovers before advancing toward the new target position. Because of this deceleration and the longer flight path required, the owls took a significantly longer time to reach a speaker when it was used as a second target than as a single source.
To hear faint and brief noises in flight and correct the flight course must be a difficult feat. One would wonder how the owl manages to do this when its own wing noises might mask the signal. Owls are known to fly much more quietly than other birds. Their body feathers are soft, and the leading edge of their wings has a fine comb, which is supposed to suppress the wing noises (Graham 1934). A recent study, however, reports that the removal of the comb had no effect on the wing noises of the tawny owl (Neuhaus, Bratting, and Schweizer 1972).
When I recorded and analyzed the wing noises of one of the barn owls during location tests (Figure 10), I found that the flight noises are not only faint but also lack high-frequency components. Most of their energy is concentrated below 1 kilohertz; above 3 kilohertz there is too little energy to record even with a very sensitive set of equipment. Similar results were reported for other species of owls (Gruschka, Borchers, and Coble 1971; Neuhaus, Bratting, and Schweizer 1972).
These findings imply that the owl’s wing noises would not interfere with the detection of acoustic clues for mid-flight correction, since useful cues are noises between 6 and 9 kilohertz. The lack of high frequencies is also advantageous for the owl, because small rodents capable of hearing high frequencies cannot hear and locate the approaching owl. The house mouse and some deer mice are rather insensitive to frequencies below 3 kilohertz (Ralls 1967).
Some rodents, such as the kangaroo rat, however, are quite sensitive to low frequencies, which might enable them to hear and discover the owl. The resonance frequency of the kangaroo rat’s middle-ear cavity, which is low due to its enlarged mastoid bulla, increases the sensitivity of the rat ear to low frequencies (Webster 1972). If the bulla cavities of a kangaroo rat are obliterated, its chance of being caught by an owl greatly increases, which is perhaps due to the inability of the rat to hear the flight noises of the owl (Webster 1962).