1. The Experiment: Barn Owls Catching Mice by Sound Alone in a Dark Room
Scientists placed barn owls in a dark, soundproofed room. In experiments by neuroscientists Konishi and Knudsen, it was shown that these owls could find and catch prey using only sound [1]. Another experiment by Payne in 1971 used speakers to create sounds like a mouse rubbing against leaves, rather than using actual mice. The owl’s accuracy was measured by looking at footprints in clay around the speaker to see how far off the owl was from the center [2]. The owl relies mainly on sounds higher than about 5 kilohertz [2].
In Payne’s experiment, the error was less than one degree in both vertical and horizontal directions. Another study summary mentions an error of about 1.5 degrees [5]. The figures differ between sources, and the reason for the difference is unclear. To put this in perspective, one degree of error at a distance of 10 meters is about 17.5 centimeters, and 1.5 degrees is about 26 centimeters. These distances are calculated from the angles and were not directly measured in the experiment [2]. Since the numbers vary by source, it is safer to note the difference rather than deciding which is correct. So, where does this accuracy come from? Let’s look at the ears first.
2. Why a Barn Owl's Ears Are Placed at Different Heights on Its Head
The ear openings of a barn owl are placed asymmetrically on the head. They differ in position and direction between the left and right sides. This asymmetry is also found in other owl genera [3]. Because the ears are offset vertically, the sounds received by the left and right ears are different. This difference helps create a three-dimensional map of the surrounding sounds [3]. Japanese explanations also state that because the ears are offset up and down, the owl can grasp the position of the sound source in three dimensions [8].
Imagine two microphones: one pointing up and one pointing down. A sound coming from above would be louder in the upward-pointing microphone, and a sound from below would be louder in the downward-pointing one. The owl’s ears are much more complex than this simple analogy, but the idea that "different directions make the same sound sound different" is similar. Because the ears face different directions, there is also a difference in volume. Additionally, there is a special feature on the face around the ears.
3. How the Barn Owl's Facial Feathers Collect Sound for Its Ears
The barn owl’s face has a round, dish-like surface of feathers. This is called the facial ruff. It is made of stiff feathers forming a concave surface that reflects sound toward the ears, according to Stanford Birds [1].
A study by Hausmann and others in 2009 investigated what would happen if the facial ruff were removed. In a virtual experiment, they calculated the results assuming the ruff was gone. They reported that the range of time differences used for left-right judgment became smaller, and the way volume differences appeared for up-down judgment changed. Both factors made it harder to locate the position [4]. This calculation shows that removing the ruff changes the clues for distinguishing left-right and up-down positions. However, this is the conclusion of just one study. So, how does the owl calculate position from the sound that reaches its ears?
4. How Barn Owls Use Timing for Left-Right and Volume for Up-Down
Having two ears allows us to know the direction of a sound. For left-right differences, the main clue is the time difference in when the sound reaches each ear. For up-down differences, the main clue is the difference in volume (loudness) between the ears [4]. The volume difference corresponds one-to-one with up-down position within a range of about plus or minus 40 degrees vertically and plus or minus 60 degrees horizontally from the front [4].
Konishi and colleagues conducted an experiment where they played sounds with time differences of 10 to 30 microseconds between the left and right ears using small earphones [5]. The owls reacted to these extremely small time differences. One microsecond is one-millionth of a second, so 20 microseconds is one 50,000th of a second. These numbers refer to the time differences added to the sound in the experiment, not the error in the prey’s position. Also, the rule "time difference for left-right, volume difference for up-down" is a simplified way to organize the main clues. How does the brain receive these tiny differences?
5. How a Barn Owl's Brain Builds a Map of Sound Positions
The small differences received from the ears become a map in the brain that represents sound positions. Knudsen and Konishi showed in 1978 that the midbrain of barn owls contains a map of sound positions. This map has two coordinates: horizontal and vertical [6].
Later, Knudsen found that the map for sound and the map for vision are located in the same place in the midbrain, called the optic tectum [6]. This means the map representing sound position and the map representing visual position overlap. However, the materials used for this explanation are mainly summaries from search results, not the original papers. For detailed content, it is best to check the original papers. Thus, the brain calculates position from sound. But if the owl’s own wing sounds were loud as it approached, it would interfere with hearing the prey’s sound.
6. How Barn Owls' Wings Let Them Fly Silently to Hear Better
Barn owl wings have three features for silent flight: serrations (zigzags) on the front edge, fringed edges on the back, and a velvety structure on the feather surface [7]. The velvety feather surface reduces friction sounds and steadies the airflow around the wing, while the serrated front edge and fringed back edge break up the airflow and reduce vortex noise [7]. Stanford Birds explains that the serrations on the front edge disturb the airflow to suppress sound caused by vortices [1].
The reason for flying silently is explained as preventing the owl’s own wing sounds from interfering with detecting the prey’s sound [1]. For an owl that hunts by sound, silent flight is like part of its ears. This wing shape has also been applied to human tools. Chiba University and Mitsui Chemicals demonstrated the application of the owl’s serrated wing structure to drone propellers to reduce noise [8].
7. An Experiment to Try: Pointing to a Sound with Your Eyes Closed
Try an experiment with a partner. One person sits in a chair with eyes closed, and the other makes a small sound from a short distance. The sound should be small and quiet. The person with eyes closed points in the direction the sound came from. Change the position to front, back, right, and left to compare which is easiest to guess. Rather than comparing your accuracy with an owl's, see whether some directions are easier to guess than others. Please ensure you do not make loud noises and perform this in an open space with no objects nearby, as you will be moving around.
Try cupping your hands behind your ears to see if the way you hear changes. Think about this alongside the explanation that the facial ruff collects sound. If you want to learn more, the Stanford Birds explanation [1] and the Sharp Japanese page [8] are good starting points.