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 Noises
In theory, a single pure tone should be hard for the owl to locate, because even two ears are insufficient to define a point in three dimensions. For example, there is not one but a family of points around the head where a tone can produce a given inequality in intensity between the ears. The relatively good location of some pure tones by the owls is perhaps due to head movements. The rustling noises of prey contain many frequencies, as mentioned earlier, and the reason why the owl can use them so effectively will be sought next. The results of the pure-tone tests should provide bases for useful predictions. It would also be possible to find out which components of the noises are used by the owl by systematically removing different parts. However, I decided to use a more systematic and controllable approach.
I constructed signals with a constant center frequency and different bandwidths (maximum frequency minus minimum frequency) to study the effects of bandwidths on the error of location. The signals were broadcast at a constant intensity of -12 decibels (re 0.0002 dynes per centimeter squared) at the perch and lasted until landing. For comparison with the noise signals, the errors of location were measured for a 7 kilohertz tone delivered at an intensity of 4 decibels (re 0.0002 dynes per centimeter squared). The noise signals were delivered at a lower intensity in order to demonstrate their superiority as location cues over the most effective puretone broadcast at a higher sound level.
Since the differences in the errors of location were small, they were compared by scoring and statistical procedures that were immune to the errors in estimating the owl’s position. The distribution of strikes in Table 1 shows a general tendency for larger proportions of strikes to fall within the four-plate zone and its immediate vicinity with increasing bandwidths. Noises containing frequencies between 5.5 and 9.5 kilohertz were more accurately located than those involving other frequency ranges. A 4 kilohertz band noise centered around 7.5 kilohertz is sufficient for accurate location. Additional frequencies do not contribute to more accurate location.
The owl needs, therefore, only a small portion of the frequency spectrum in the prey’s rustles. No wonder the owl can precisely locate small rodents that make wideband noises rich in frequencies in the range most suitable for sound location. It should be noted that 5.5–9.5 kilohertz is the range in which the barn owl is most sensitive, although a higher sensitivity itself does not contribute to more accurate location, as mentioned before.