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
Location of Pure Tones
Let us first consider how man locates pure tones in order to provide some theoretical framework for experiments with owls. Man can locate pure tones by binaural comparison of intensity, phase and time of arrival. We discuss here the first two methods. Figure 6 shows the angular errors of location of pure tones of different frequencies in man. Notice that man can locate low and high frequencies rather well. There is a curious hump around 2–4 kilohertz where man makes larger errors. The theory to explain these results is as follows.
Tones of long wavelengths (i.e., low frequencies) bend around the head without creating intensity differences in the sound field around the head, whereas shorter wavelengths (higher frequencies) can be bounced back by the head, causing differences in sound intensity around the head. Since the magnitude of intensity differences between two ears varies with the direction of sound propagation, man can determine the direction by binaural comparison of intensity. The shorter the wavelength relative to the diameter of the head, the more distinct is the sound shadow so created. Therefore, man can locate higher frequency tones relatively accurately.
Low-frequency tones are located by detecting phase differences between the ears, which are due to differences in the paths traveled by sound to reach the two ears. For each frequency, the magnitude and sign of phase differences vary according to the position of the sound source relative to the median plane of the head. This is the basis for location by binaural phase comparison.
For this method to be effective tones of wavelengths longer than at least twice the interaural distance are necessary, that is, d < λ/2, where d is the distance between the ears and l wavelength. When d > λ/2, a binaural phase difference of more than 180 degrees results, and it be comes impossible to discern which ear is in the leading phase, since a phase difference of 180° + φ is equivalent to an opposite phase difference of 180° - φ (from 180° + φ - [180° - φ] = 360° = 0, where φ is the excess angle over 180°). It is this ambiguity that makes the phase method ineffective with higher frequencies (Gulick 1971; Mills 1972; Steven and Newman 1934).
These conditions, higher frequencies for intensity comparison and lower frequencies for phase comparison, create for man a frequency range (2–4 kilohertz) in which neither the phase nor the intensity method is very effective. This explains the hump in Figure 6. Whether or not the above theory applies to the owl, it can suggest useful research strategies.
Since continuous pure tones can produce differences between the ears only in two acoustic parameters, namely intensity and phase, they are suitable for analyzing the acoustic method used by the owl. Tone signals were broadcast at a constant intensity of 4 decibels (re 0.0002 dynes per centimeter squared) at the perch. The signals lasted until the owl landed. The results from one owl are graphically summarized in Figure 7. Low- and high-frequency tones such as 3 kilohertz and 10 kilohertz were harder for the owl to locate than those between 6 and 9 kilohertz. These differences in the error of location are due neither to the variation in the owl’s auditory sensitivity nor to the directionality of the speakers, both of which depend on frequency. Adjustment in sound intensity according to the owl’s audibility curve did not significantly affect the error curves. The speakers did not become sharply directional at higher frequencies.
The results can be partly explained in terms of binaural intensity comparison, although this cannot account for the sudden increase in the error of location above 10 kilohertz. The barn owl does not seem to use the phase method, at least in the same way that man does, because it located low-frequency tones poorly and because it did not have any intermediate frequency range in which the error of location increased. Since the distance between the owl’s ears is shorter than that of man, the frequency range unsuitable for both the intensity and phase methods, if it exists, should be higher for the owl than for man.
The mouse rustles are not steady but discontinuous noises. Abrupt inflections in these noises would be useful for binaural comparison of time. Instead of binaural phase differences of a continuous tone, the time method uses differences in time of arrival which are caused by differences in the paths traveled by the first wave of sound. This method is independent of frequency. A series of tone beeps should provide the owl with sufficient time cues, because each beep has an onset and a cutoff. I compared the errors of location obtained by using tone beeps (50 or 100 milliseconds in duration separated by silent intervals of 80 or 150 milliseconds) and sustained tones and found no consistent differences between them. Moreover, just as the errors of location with sustained tones depended on frequency, so did those with tone beeps (Figure 7).