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
Theories of Sound Location by Owls
In some species, such as the barn owl and the saw-whet owl, the left and right ear openings differ from each other in their size and/or position. In the barn owl the ear openings are about the same in size, but the left one, together with the skin flap in front of it, is located higher than the right ear hole and skin flap (Figure 11).
There does not seem to be any individual difference in this asymmetry—no left- or right-handed owls. Also, in the barn owl the asymmetry is restricted to the ear opening without affecting the middle and inner ears. The binaural methods of sound location make use of the lateral displacement between the ears to determine the azimuth of the source. By the same token, the vertical displacement has been thought to enable the owl to determine the elevation of acoustic targets (Norberg 1968; Payne 1962, 1971).
I tested this idea by a simple experiment that involved plugging one ear. The owls (I used two) with one ear plugged veered toward the side of the target opposite to the plugged ear. This would be expected if the owl uses binaural comparison of intensity; the signal should sound louder to the intact ear, and thus the owl estimates the target position too far toward that side.
One of the owls that I tested more extensively made systematic errors in the vertical direction. When its right ear was plugged, it struck short of the target, and blocking its left ear caused it to land slightly beyond the target on the average (Figure 12). These results suggest that the vertical displacement between the ears is not used in the same simple way described for the lateral displacement. To make the matter more complex, both owls struck closer to the target with their right ear unplugged than with their left ear unplugged. This may be due to differences in the degree of ear blockage, which could not be precisely controlled, although the same bird produced a similar set of results twice. It is also possible that the right ear plays a more important role than the left one in location. A person deaf in one ear is known to be able to locate sound. The pinna seems to play a crucial role in monoaural sound location in man (Batteau 1967).
Owls do not have a structure homologous to the mammalian pinna, but some of them have a fold of skin extending from the forehead above the eye and along the orbit behind the ear to the base of the lower mandible. In the saw-whet owl, this skin fold is quite large around the ear. In the barn owl, the skin fold itself is not so prominent, but it carries a tall curved wall of densely packed feathers which encircles each half of the face. The left and right halves meet along the midline of the face, where the feather walls from the two sides form a pointed ridge (see Figure 13). When the owl is not attentive, this ridge broadens.
On each side of the face, the curved wall looks like a trough with a paraboloid inner surface. At the level of the ear opening, the skin flap covers the trough, forming a tunnel in which the ear hole is located. The entire facial structure makes up the well-known heart-shaped outline of the owl’s face, called the facial disc. When one sees the whole design of the facial disc, one cannot help thinking of a sound-collecting device.
Does the facial disc facilitate directional hearing? Payne (1962, 1971), using a stuffed barn owl, and Norberg (1968), using a stuffed Tengmalm’s owl, measured the directionality of the ears by monitoring sound near the eardrum. According to Payne, for low frequencies the barn owl’s ear is only moderately directional. For high frequencies above 8.5 kilohertz, the ear becomes highly directional and also the pattern of directionality reflects the vertical displacement of the ears. Slight changes in the shape of the facial disc and the orientation of the skin flap affected the pattern of directionality for higher frequencies.
I removed the facial-disc feathers of a barn owl to find out whether and how its errors of location would be affected. The owl was tested with a continuous noise broadcast at the lowest sound level that assured accurate location, so as to be able to detect any slight change in the accuracy of location in the absence of the facial disc.
The operated owl made large errors by landing short of the target. However, when I increased the sound level by 10 decibels, the owl improved its accuracy of location considerably. No greater improvement resulted with an increase of 20 decibels. A 5-decibel increase did not reduce errors at all (Figure 14). These observations suggest that the facial disc may be a sound amplifier; it collects sound from a large area and focuses it onto a smaller area. Payne (1962) thought the facial disc was too small to be an effective amplifier in the frequency range audible to the owl.
The amount of amplification (gain) of a paraboloid antenna is a function of its diameter and the wavelength of sound expressed as G = η(πD)/λ)2, where G is gain, η is the aperture efficiency and is larger than 0 and smaller than 1, D is diameter, and λ wavelength. The widest part of the facial disc is about 7 centimeters in diameter, and the wavelength of 7 kilohertz is 4.9 centimeters. Using these values in the above equation and assuming η = 0.5, we obtain G = 10, which means a gain of 10 decibels. This is a small amount of amplification but should be useful when the owl must detect faint noises.
The facial disc may not function as a paraboloid antenna, but the above calculation should provide some idea as to the operating conditions and effectiveness of such a sound-collecting device. The facial disc seems also to contribute to directional hearing, since the owl, even with increased sound intensities, failed to recover the degree of accuracy attained before the operation. The directionality of a parabola is also a function of its diameter, shape, and wavelength. It is not yet known to what extent the owl controls the shape of the facial disc and the orientation of the skin flap during sound location. Solution of these problems seems essential for the understanding of the mechanism of sound location in this species.
Payne (1962, 1971) used his directionality data to conclude that, if the owl moves its head so that the amplitudes of all frequencies are maximized at both ears, it must be directly facing the target. Since the ear becomes sharply directional for higher frequencies, these would help obtain a fine azimuthal bearing. Since the asymmetry of the ears causes a vertical displacement in their directionality at higher frequencies, these would enable the owl to align its head precisely in the vertical direction.
Two lines of evidence make this theory untenable. First, the barn owl does not need such high frequencies as 8.5–13 kilohertz, which Payne’s theory requires. The owl can locate noises containing frequencies between 6 and 8.5 kilohertz accurately. Second, a simple experiment will show that the theory fails to explain the ability of the owl to recognize the direction of sound before it moves its head. Man can locate sound quite well without head movement, although it seems essential in the absence of the pinna (Freedman and Fisher 1968). I have not tried to restrain the owl’s head, but I used a trick to get the same effect.
If the owl turned its head toward a signal lasting shorter than the time required to initiate or complete the turning of the head, the owl should not have been able to align its head direction with the target by successive steps of readjustment, which Payne’s theory requires. I examined the direction of the owl’s face in infrared pictures taken during tests in which the owl was allowed to hear only one brief noise burst to redirect its flight course from one speaker to another. In every case the owl’s head continued to turn well after the signal had stopped. The owl oriented its head in the general direction of a signal lasting as short a time as 10 milliseconds, which is too brief an interval for the owl to initiate head movement.
Pumphrey (1948) developed a theory for owls with asymmetric ears to explain the location of sound without head movement. This theory also uses the frequency-dependent asymmetry of the ears’ directionality. It requires two ears and at least three bands of frequencies. As mentioned before, there are many points around the head at which a tone can produce a given inequality in intensity between the ears. These points are contained in a surface; each band of frequencies defines a surface. Because of the asymmetry, some surfaces intersect one another, and three of them can define a point in space unambiguously. The test of this theory must be done without head movement.
There are other general theories of sound location which will not be described here. The errors of sound location discussed so far consist of two components—errors in auditory location and deviations in the control of flight direction. We have recently designed a different type of experiment to measure the true accuracy of auditory location without flight. This work is still in progress. My studies demonstrate what the barn owl can do under the experimental conditions used. In nature, it must hunt under different and varying conditions which might render some of these potentials unusable or require capabilities not uncovered by my studies. Combinations of field and laboratory experiments will be necessary to learn more about the natural acoustic behavior of the barn owl. It should also be emphasized that other species of owls may have different acoustic capabilities.
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