1. Barn owls (Tyto alba) can locate prey in total darkness using only the sense of hearing, with an error of less than I° in both the vertical and horizontal planes.
2. Differences between the behaviour of barn owls flying at prey in complete darkness (analysed from films taken under infra-red illumination) and their behaviour in the light are correlated with the problems they must face in acoustic orientation.
3. Experiments with owls trained to strike a concealed loudspeaker show that they depend on frequencies of sound above 5 kHz.
4. Measurements of sound pressure in the region of the owl's eardrum, made with a probe-tube microphone while moving a loudspeaker around the owl's head, reveal that for frequencies above 8-5 kHz the ear is highly directional. At such frequencies, regions of high sensitivity (tightly isolated by peripheral regions of low sensitivity) are directed along different paths for the two ears.
5. These regions of good sensitivity are correlated with the asymmetry of the barn owl's external ears. Movements of a flap of skin in front of the ear opening changes the overall directional sensitivity patterns by redirecting the regions of maximum sensitivity.
6. A theory is presented to explain how a barn owl might locate the position of a sound source by moving its head until the intensity of all frequencies comprising a complex sound is brought to a maximum in both ears (aided perhaps by differences in inter-aural time delay that are enhanced by intensity disparities).