Most owls are nocturnal or crepuscular birds of prey. Because they hunt at night, they must rely on non-visual senses. Experiments by Roger PaynePayne, Roger S., 1962. How the Barn Owl Locates Prey by Hearing. The Living Bird, First Annual of the Cornell Laboratory of Ornithology, 151-159 have shown that owls are sensitive to the sounds made by their prey, not the heat or the smell. In fact, the sound cues are both necessary and sufficient for localization of mice from a distant location where they are perched. For this to work, the owls must be able to accurately localize both the azimuth and the elevation of the sound source.
