Prey localisation by hearing in owls

Adult owls hunt within a well-defined territory, which they know well and patrol regularly at night. During its patrol, an owl visits a number of observation perches, from which it can survey the ground round about. If it hears potential prey, the owl swiftly turns its head so that it directly faces the object of interest. Then, after adequate scrutiny, it flies down to capture the prey in its outspread talons (Fig. 6.1). An owl listening from a perch or in low-level flight must be able to pinpoint the sound source both in the horizontal plane (azimuth) and in the vertical plane (elevation). In fact, unless an owl is looking down from directly above its prey, its orientation is more critical in the vertical than in the horizontal plane (Fig. 6.1b).

The use of hearing in prey capture has been studied mostly in the ubiquitous barn owl, Tyto alba. Early behavioural studies were carried out using tame individuals, which proved able to locate prey even in total darkness, and a number of simple experiments demonstrated that hearing is used to accomplish this (Payne, 1971). For example, it was found that the birds could strike accurately at a concealed loudspeaker quietly broadcasting a recording of leaf-rustling noises. This result not only showed the barn owl's ability to locate prey by hearing alone, but also opened the way for testing which features of the sound are important in localisation.

Using this technique, the accuracy of localisation was found to vary with sound frequency. The greatest accuracy was achieved with a sound containing frequencies from 6 to 9 kHz. Typically, birds are not sensitive to frequencies above 5 kHz, but the barn owl can hear up to 10 kHz, and 6-9 kHz is the range to which its ear is most sensitive. In addition, more than half the auditory neurons in a barn owl's ear are devoted to this extended frequency range of 5-10 kHz. The usual pattern is for approximately equal numbers of auditory neurons to be devoted to each doubling of frequency (octave), but the barn owl devotes a disproportionate number to the higher frequencies (Koppl, Gleich & Manley, 1993). Hence, these important frequencies can be analysed in greater detail and this arrangement may be thought of as an acoustic fovea (cf. section 6.9 on bats).

The accuracy of sound localisation has been studied in more detail by exploiting the natural response in which an owl turns to face a novel sound. The owl is trained to remain on its perch and the angle through which its head turns in response to a sound is measured using an electromagnetic angle detector (Fig. 6.2 a). In each test, the head is first aligned by attracting the owl's attention with a sound from the zeroing speaker, and then the owl is stimulated with a sound from the target speaker.

When the target speaker is placed in front of its face, the barn owl's localisation is exceptionally accurate, with an error of less than 2° in both azimuth and elevation. But the owl's accuracy deteriorates in both planes as the angle between the source and the axis of the head increases (Fig. 6.2 b). The rapid flick of the head, with which the owl responds, is initiated about

Elevation Azimuth Barn Owl

70L 50L 30L 10L 10R30R 50R 70R -70 -50 -30 -10 +10 +30 +50 +70

70L 50L 30L 10L 10R30R 50R 70R -70 -50 -30 -10 +10 +30 +50 +70

Target azimuth (degrees) Target elevation (degrees)

Figure 6.2 Orientation of the head to sounds by the barn owl (Tyto alba). (a) The method used to measure the accuracy with which the owl locates sounds coming from different positions in space. Sound stimuli originate from either a fixed source (the zeroing speaker) or a movable source (the target speaker). The search coil on top of the owl's head lies at the intersection of horizontal and vertical magnetic fields generated by the induction coils. Movement of the head in response to sound from a speaker induces a current in the search coil, which is analysed by computer to give horizontal and vertical angles of movement. (b) Localisation accuracy as a function of the position of the target speaker, showing the mean degree of error in judging target position in the horizontal plane (left) and in the vertical plane (right) for an individual owl. (Modified after Knudsen, Blasdel & Konishi, 1979.)

100 ms after the onset of the sound. However, maximum accuracy can be achieved even with brief sounds (75 ms duration) that end before movement of the head begins. This shows that the owl does not locate the sound by successive approximation but can determine the sound's precise location in space without feedback. That is to say, the owl is operating under open-loop conditions (see section 1.6).

Figure 6.3 The barn owl's ability to locate sounds in elevation. (a) A plot of auditory space in front of the owl in degrees of azimuth (L and R) and of elevation (+ and —). The symbols show individual errors in open-loop localisation of a sound source in the centre of the plot (Target) produced by partly blocking one ear. A tighter ear plug (closed circles and triangles) produces a greater localisation error than a looser ear plug (open circles and triangles). (b) The facial structures of the barn owl that contribute to localisation of sound in elevation: the facial ruff is formed from tightly packed feathers projecting from the relatively narrow skull, and the ear openings are located behind the preaural flaps. These structures are revealed by removing the sound-transparent feathers of the facial disc, which give the owl's face a flat appearance. (Modified after Knudsen & Konishi, 1979.)

Facial ruff

Figure 6.3 The barn owl's ability to locate sounds in elevation. (a) A plot of auditory space in front of the owl in degrees of azimuth (L and R) and of elevation (+ and —). The symbols show individual errors in open-loop localisation of a sound source in the centre of the plot (Target) produced by partly blocking one ear. A tighter ear plug (closed circles and triangles) produces a greater localisation error than a looser ear plug (open circles and triangles). (b) The facial structures of the barn owl that contribute to localisation of sound in elevation: the facial ruff is formed from tightly packed feathers projecting from the relatively narrow skull, and the ear openings are located behind the preaural flaps. These structures are revealed by removing the sound-transparent feathers of the facial disc, which give the owl's face a flat appearance. (Modified after Knudsen & Konishi, 1979.)

An indication of just what cues are involved in locating a sound is provided by partially blocking one ear, which effectively reduces the sound pressure level at that ear without significantly altering the sound's time of arrival. A plug in one ear leads to significant errors in elevation but only slight errors in azimuth (Fig. 6.3a). A plug in the left ear causes the owl to direct its head above and a little to the right of the target, and a plug in the right ear results in the owl facing below and a little to the left. The tighter the ear plug, the greater is the degree of error. This result indicates that the intensity difference between the ears is the principal cue for locating a sound in elevation.

The owl is able to use a comparison of intensity between the left and right ears to locate a sound accurately in elevation due to the arrangement of feathers on its face. The ear openings and the protective, preaural flaps are vertically displaced, the left flap being above the midpoint of the eye and the right one below it (Fig. 6.3b). There is also a slight asymmetry in the facial ruff, which is composed of dense, tightly packed feathers and forms a vertical trough behind each ear opening: the left trough is orientated downwards and the right one upwards. Because of its dense feathers, the facial ruff acts as an effective sound collector at frequencies above 4 kHz. Consequently, the asymmetries in the ruff and in the ear openings give rise to a vertical asymmetry in the directionality of the two ears: the left ear is more sensitive to high-frequency sounds from below and the right ear from above the horizontal plane. If the ruff feathers are removed, the owl is quite unable to locate sounds in elevation and always faces horizontally regardless of the true elevation of the source, but it can still locate in azimuth.

When a source of sound is not directly in front of or behind the head, sound will reach the two ears at slightly different times. In order to test the importance of these time differences, sound stimuli are delivered using miniature earphones installed in the owl's ear canals, rather than sound delivered from a distant speaker, which would inevitably produce differences in both time and intensity. With the earphones, stimuli can be made equal in intensity but different in time of arrival at the two ears. An owl responds to such a stimulus by turning its head horizontally in a direction that the time difference would represent if it were an external source of sound. The owl turns its head to the side that receives the stimulus earlier, and the angle of turning is positively correlated with the magnitude of the time difference between the ears (Moiseff & Konishi, 1981). These experiments show that the owl depends mainly on differences in time of arrival at the two ears to locate sounds in azimuth.

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