The Genius of the Barn Owl’s Feathers
A hungry barn owl, unfed for several days, perches in a completely darkened room at Drumlin Farm, a sanctuary run by the Massachusetts Audubon Society. A deer mouse is released into the room, rustling among the two inches of dry leaves that have been placed on the floor. As the mouse pauses, silent, the owl strikes, capturing it. Over the next few days, the owl makes 16 more strikes at mice, missing only four times, each time by less than two inches.
To eliminate the possibility that the owl was using its sense of smell or detecting heat from the body of the mouse (for instance, by sensing infrared light emitted by a warm body), the experiment is repeated with a mouse-sized wad of paper dragged through the leaves; the owl again successfully hits its target. When the owl’s hearing is altered by putting cotton in one ear, the owl flies toward the mouse but misses it by about 18 inches. This first demonstration that barn owls can successfully hunt in total darkness was carried out by Roger Payne — later famous for discovering, along with Scott McVay, the songs of humpback whales — while he was an undergraduate at Harvard in the late 1950s.
Payne’s experiments established that barn owls can hunt using hearing alone. The question that naturally follows is how the owl’s auditory system could localize a sound source with such astonishing precision.
Barn owls detect the location of a sound in two ways: by the time it takes for a sound to reach each ear, and by the loudness or intensity of the sound in each ear. If a sound is directly ahead of the owl’s face, for instance, it reaches both ears at the same time and is equally loud in each. But if it comes from one side, it reaches the nearer ear slightly sooner and is louder there. By detecting these differences, owls can determine whether a sound is to the left or right of the center of the face.
Of course, locating a sound in space also requires determining whether it comes from above or below. Many species of owl have ear openings at different heights on the left and right sides of the head. You can see this asymmetry in the skull of the northern saw-whet owl. In the barn owl, the left ear is higher than the right.
But it is not just the position of the ears that enables their exceptional hearing. The facial feathers play an important role, too. One of the most identifiable features of a barn owl is its heart-shaped facial disc. The disc has two specialized types of feathers that help pinpoint the exact location of a sound’s source: white auricular feathers, which fill the interior of the disc, and rust-colored reflector feathers, which form the ruff around the edge of the disc. The auricular feathers have more widely spaced barbs that form the vane of the feather than a typical contour feather, transmitting sound more easily through them, while the reflector feathers have barbs packed more closely together than usual, reflecting sound (as their name suggests). The position of the reflector feathers focuses sound into the ear openings just within the edge of the ruff.
The feathers of the ruff, too, are asymmetric: The left side of the ruff is more downward-facing than the right side, so that it preferentially focuses sound from below to the left ear, while the right side of the ruff is more upward-facing than the left, preferentially focusing sound from above to the right ear. Sounds from below are louder in the left ear, while sounds from above are louder in the right ear.
Andrew Moiseff, at the University of Connecticut, demonstrated this by fitting barn owls with miniature earphones. When the sound was louder in the left ear, they turned their heads downward, but when it was louder in the right ear, the owls turned their heads upward. And in experiments in which the ruff reflector feathers were removed, conducted by Eric Knudsen and Masakazu Konishi at Caltech, the owls had difficulty in identifying the height at which a sound originated. The barn owl’s facial disc, and especially the reflector feathers around the edge of the disc, are essential for it to be able to precisely locate the source of a sound, such as that of a mouse rustling among dead leaves. Fish-owls, which prey on fish near the surface of rivers, lakes, and seacoasts, and hunt by sight, not sound, have poorly developed ruffs.
Feathers can also suppress sound. Owls that hunt by hearing fly nearly silently thanks to microscopic structures unique to their primary and secondary flight feathers. The serrated leading edge of the wing and the velvety surface of the feathers are thought to reduce turbulence as air flows overhead, while fluffy fringes on the edges of the feathers mesh together like the teeth of two combs, preventing fluttering. This allows the owl to both hear its prey moving as it flies towards it and prevents the prey from hearing the owl approaching.
The barn owl’s solution to quiet flight has proved especially intriguing to engineers.
Interestingly, in owls that eat fish and hunt by sight, rather than hearing, such as Blakiston’s fish-owl of northeast Asia and Pel’s fishing-owl of central and southern Africa, the flight feathers lack the features that give rise to nearly silent flight; since fish are underwater and can’t hear an owl approaching, there is no advantage for a fish-owl to fly silently. In flight, the wings of the brown fish-owl have even been reported making a singing noise.
The barn owl’s solution to quiet flight has proved especially intriguing to engineers. Siemens, for example, has introduced low-noise wind turbine blades with a trailing edge featuring “comb teeth” that reduce trailing-edge turbulence; the teeth mimic the fringes at the trailing edge of owl flight feathers. Others are experimenting with materials inspired by the velvety surface of owls’ flight feathers. In wind-tunnel experiments with airfoils shaped like a barn owl wing, air flows more uniformly over the surface, with less turbulence, if the airfoil is covered with a velvety material mimicking the surface of barn owl flight feathers than if it is smooth.
In other species of birds, feathers sometimes produce sound. During courtship dives, male Anna’s hummingbirds separate one tail feather from its neighbors, so that the air flowing past it causes it to flutter, producing a sudden, high-pitched sound. And club-winged manakins turn their wings into instruments by rubbing modified feathers together to make an almost electric buzzing sound. Microscopic variations in feather structure not only control sound, but also, as I describe in “Birds Up Close,” give rise to the brilliant iridescence of hummingbirds, the water repellency of duck feathers, and the exceptional thermal insulation of eider down.
Lorna J. Gibson is Matoula S. Salapatas Professor of Materials Science and Engineering and a MacVicar Faculty Fellow at MIT. A lifelong birder, Gibson is a member of the Board of Directors of the Massachusetts Audubon Society and the author of “Birds Up Close,” from which this article is adapted.
