Most mammals and birds rely primarily on their sense of sight to get by in the world. They are guided mostly by their eyes as they search for the resources they need and steer clear of dangers that lie in their path. Certain conditions, however, make it difficult for animals to get around by sight alone. Nighttime is the most obvious of these. Without the aid of artificial light, we humans move through the dark with great caution, at risk for injury from unseen obstacles. Many animals are little better than humans at maneuvering in total darkness and so spend nighttime at rest. Other animals, however, are most active at night, and this requires a set of adaptations that heighten not only their sense of sight but their sense of hearing as well.

Owls, especially those species that hunt almost exclusively at night, have evolved a particularly heightened sense of hearing. Some, including the barn owl (a species whose hearing has been studied more than almost any other animal), can locate prey even in complete darkness. To do this, owls rely, in part, on some obvious physical features. Many owls have a large "facial ruff," the concave disc of stiff feathers at the front of their head that helps channel sound into their ears. Their ear holes are also quite large compared to those of other animals. These physical traits, however, tell only part of the story.

Far more important to the owl's heightened sense of hearing is the complex way the owl's brain processes sounds. To assess whether a sound comes from the right or left, the owl's brain measures the difference in time that the sound takes to reach each ear. The difference is usually less than 200 millionths of a second, depending on how far to the right or left the sound originated. Owls analyze sounds originating from above or below in a similar manner. With ear holes positioned at slightly different heights on the sides of their heads -- one slightly above eye level and the other an equal distance below -- owls can detect subtle differences in the vertical positions of sound sources. Sound coming from above, for example, will be slightly louder in the higher ear. A sound that is equally loud in both ears must come from a source at eye level.

Perhaps most amazing is the ability of the barn owl (and probably other species of owls) to create in its mind an image of the world based on sound alone. Scientists have discovered that the owl's brain interprets information from its ears in much the same way our brain interprets information from our eyes. The owl's ears are linked to an area in the midbrain, which contains about 10,000 cells called space-specific neurons. Only one small subset of neurons in this network will respond to sounds from any given location. Thus, based on the pattern of neurons being stimulated, the owl's brain can determine, with great accuracy, the location of a potential prey animal, its direction of movement, and, to some degree, its size.

Experiment in this NOVA classroom activity with what people hear when conversations are taking place simultaneously.