Owls—Designed for Nightlife
OWLS are everywhere—almost. They are found on every continent except Antarctica. They range in size from that of a sparrow to that of an eagle. The littlest are the elf and pygmy owls; much larger are the eagle owls of Eurasia, the great gray owls, the great horned owls, and the strikingly beautiful snowy owls of Arctic regions. Some 140 species are widely distributed in such diversified habitats as grasslands, prairies, deserts, marshes, deep woods, rain forests, and arctic tundra. Their diets are as varied as their habitats: worms, insects, frogs, rodents, small birds, and fish.
With big head, large round orange or yellow eyes, both facing forward and staring out of saucer-shaped disks of radiating feathers, Mr. Owl looks so wise. No wonder he is called the wise old owl. Part of the impression of wisdom comes from the big eyes that stare with such a steady and unwavering gaze. That unwavering gaze, however, is not due to any deep meditative powers—his eyes are set in sockets that prevent their rolling or swiveling about. Even so, from ancient times the owl was credited with wisdom—it was the sacred bird of Pallas Athene, the Greek goddess of wisdom.
Not all owls exude such an aura of wisdom. The little elf owl does not have the impressive air of the great horned owl, nor does the burrowing owl. Elf owls live in desert areas and occupy woodpecker holes left in the giant saguaros. They have loud voices for such small birds, and when male and female sing duets—if you can call it singing—it sounds like the yips and chucklings of puppy dogs.
Burrowing owls live in the holes of prairie dogs or ground squirrels and are often seen bobbing up and down on mounds or perched on fence posts. Nestlings threatened in their burrows emit a frantic buzzing that mimics the rattlesnake’s warning signal. It discourages entry by unwanted visitors.
Many think that owls cannot see very well in daylight. They also think that owls can see fine in the dark. But they are wrong on both counts. Owls have very good eyesight. During the day their vision is excellent. At night they also do very well. The nocturnal owls—and most are—have retinas superabundantly packed with rods to enable them to see in the dimmest of light. In such surroundings their eyes gather in the faint light a hundred times better than ours do. But in total darkness, they might as well be blind. One researcher scattered dead mice on the floor in a totally dark room and put owls in it. Those owls did not find a single mouse.
When Ears Become Eyes
However, when a barn owl (page 15) was put in a totally dark room with leaves on the floor and live mice rustling among them, it caught them all. The same feat could have been accomplished by other nocturnal owls, but certainly the barn owl is a specialist. In total darkness its ears become its eyes. Barn owls have a sense of directional hearing that is more accurate than that of any other land animal studied.
When we want to hear a very faint sound, we turn our ear toward its source and may cup a hand behind our ear to collect the sound waves and channel them into our ear openings. The barn owl’s face is designed to do this automatically, and extremely faint sounds not perceptible to us are easily heard. World Book’s Science Year 1983 explains: “The barn owl’s great sensitivity to sound is largely due to the sound-collecting property of the facial ruff—the wall of stiff, densely packed feathers that makes a heart-shaped outline around the face. . . . Like a hand cupped behind an ear, the large surface of the ruff collects sound and channels it into the ear openings.”
The design for hearing does not stop with the barn owl’s ruff. Another ‘cupped hand’ is available for channeling the sound to the ear opening. Science Year 1983 describes it: “The pink flap that lies over the barn owl’s ear opening has a structural resemblance to the human outer ear. Feathers on the outside of the earflap and in the ruff behind the ear act like cupped hands to funnel sound into the opening.”
This earflap, however, is not just another ‘cupped hand’ to reinforce the sound-gathering power of the facial ruff. It, along with the ruff, is especially designed to add an entirely new dimension to the barn owl’s directional hearing capabilities. The ear openings in the barn owl’s skull are symmetrical, that is, the right and left ear openings are placed exactly opposite in the skull. The external ear structures, however, are not symmetrical. Both the right earflap and the external ear opening are lower and directed upward, whereas the left earflap and the external ear opening are higher and directed downward. Hence, the right ear, with its earflap and opening cupped upward, is more sensitive to sounds coming from above, whereas the left ear, with its earflap and opening cupped downward, is more sensitive to sounds from below. If the sound is more intense in the right ear, the owl knows its source is above; if more intense in the left ear, the source is below.
Similarly, if the sound’s source is more horizontal than vertical and is heard by the right ear before the left one, it is immediately perceived as coming from the right; if heard first by the left ear, it is perceived as coming from the left. The owl’s head is small, so the difference in time of arrival of the sound at one ear as compared to the other is minimal, measured in microseconds (a microsecond is one millionth of a second). The owl’s directional response to sound is immediate—within a hundredth of a second, the owl’s face turns toward the source. Its ability to process instantaneously these split-second cues is critical to pinpointing the sound’s source.
As mentioned before, the owl’s eyes are virtually immovable. This is not a mistake in design, however. The owl’s neck is so flexible that some owls can turn the head as much as 270 degrees, enabling it to see even directly behind itself. Moreover, that the eyes are immovable is an asset. It means that whenever the owl hears a sound and turns its head toward the source, its eyes are automatically aimed in that direction. It sees the source of the sound a hundredth of a second after it hears it.
Wings Designed With Silencers
The feathers of most birds make noise as they whir through the air in flight. Not so with owl feathers; they are specially designed for silence. They are soft and downy, with a velvety feel, so wind makes no noise as it passes over them. The flight feathers do not have straight, hard edges like those of most birds, which produce a whirring noise as they fan the air in flight. The barbs on the owl’s feathers are uneven in length, leaving soft fringed edges that make no sound as they sweep through the air.
This devotion to silence, however, is abandoned when owls engage in owl talk—hoots, warblings, whistlings, clacking of beaks, and claps of the wings in flight. Some researchers refer to these noises as owl songs, and to owl ears some of the noises may pass for singing, since they do play a role in courtship communication between mated pairs.
Though they may not originally have been created for this purpose, owls are valuable today in controlling insects and rodents. The barn owl especially is considered the farmer’s friend, ridding his fields of mice and rats and other pests that eat his crops. In some places owls are encouraged by special “owl doors” to provide easy access to farm buildings. In Malaysia, oil-palm growers put up nesting boxes for barn owls—and that’s not charity. The pair that lives there pays rent, annually ridding the farmer of up to 3,000 rats that otherwise would have eaten his crop. And the barn owls add a touch of beauty. They are among the most beautiful of birds, distributed worldwide, and they have one of the most intriguing heart-shaped faces in nature.
When you think of the big yellow eyes that gather in the faintest light, the ears that catch a whisper of sound from any direction, and the flight feathers that slip silently through the air, you must marvel at those nocturnal owls that are created so well designed for nightlife.
[Pictures on page 16, 17]
Left and above: Great horned owl and baby
[Credit Line]
page 16 left, Robert Campbell; page 16 right, John N. Dean
Right: Burrowing owl
[Credit Line]
Paul A. Berquist
Far right: Elf owl
[Credit Line]
Paul A. Berquist
[Picture Credit Line on page 15]
Photos: page 15, Paul A. Berquist