Birds of Wisconsin, Wisconsin Nature

Nocturnal Adaptations

Owls are widely regarded as elusive and stealthy nighttime predators. Of the approximately 450 known raptor species worldwide, it is the roughly 200 species of owls that are nocturnal. This hunting strategy requires special owl adaptations that differ from other raptor species. Eleven species of owls are commonly found in Wisconsin, and five of those species currently reside as permanent educational animals in our Raptor Program.

Owl Adaptations for Seeing at Night

Low-light situations mean that owls need an enhanced sense of sight. This is aided by the size of their eyeballs. Human eyes take up 5% of the space inside of our skulls, whereas owl eyes take up 75% of theirs! This is especially apparent when looking at an Eastern Screech Owl, one of the smallest species in Wisconsin. If an owl was the height of an average human, their eyes would be as large as softballs. Owl eyes also have a higher density of light-sensing receptors called rods.

As it often goes in nature, there is a trade-off for having incredible eyesight. Owls also have a low density of color-sensing receptors, so they are basically colorblind. Though owls have large eyes and precise nighttime vision, they lack ocular muscles to move those eyeballs. This means that an owl is constantly looking forward, and must move its entire head to see what is happening on the periphery.

Owls are famous for the solution they have regarding this problem. They have flexible necks, allowing them to turn their head up to 270 degrees left or right. Owls accomplish this due to the unique structure of their neck. All mammals, sized from mice to giraffes, have seven cervical vertebrae. This means our necks are not especially flexible, often keeping us from turning our heads past our shoulders. Owls, however, have 14 cervical vertebrae that are smaller and able to rotate more easily. Their jugular veins are also highly elasticized to not cut off the blood supply to their brain while turning their heads. The Great Horned Owl offers a regional example of this behavior, as their tufts visually exaggerate this capability.

Owl Adaptations to Hear at Night

Not only do eyes and necks serve owls well when hunting, but so do their ears. In fact, most owls rely more heavily on sound than sight when hunting. Many of Wisconsin’s owl species must use sound exclusively to hunt during winter, when their prey is underneath snow. They can pinpoint the location of prey using their ears, and capture animals through the snow and ice with their razor-sharp talons.

Many species of owl, including the Barn Owl, have asymmetrical ear openings. When sound is coming from one direction, the waves reach the ears at slightly different times, allowing the owl to pinpoint the source with incredible accuracy from any direction. Since human ears are on the same horizontal plane, we generally have trouble finding a sound source if it is directly in front of or behind us.

Sound Gathering Faces and Feathers for Silent Flight

All species of owl, regardless of ear symmetry, have a “facial disk.” This is a stiff ring of feathers around the face of the owl, and it acts like one large outer ear. While looking at the face of the Barred Owl, this is especially apparent. The feathers concentrate sound waves into the ear openings, and the bird can control how the feathers are shaped to filter sounds coming from different directions, much like a maneuverable satellite dish.

In order to hear and capture their prey, owls must minimize the noise they make while hunting. Not only will they refrain from vocalizing, but their wings have built in sound-mufflers. Tiny fringe feathers, which look similar to eyelashes, are located on the leading edge of owl wings to disrupt the air moving over and under the wing, thereby minimizing the noise. One experiment conducted by BBC sound engineers used an array of extremely sensitive microphones to measure the sound produced by a flying Barn Owl. Not only were the wingbeats unregistered by human ears, but the sounds recorded by the microphones were infinitesimal.

Owls are spectacular creatures that are brilliantly adapted to the dark. Check our calendar for our programs where you can learn about owl adaptations – then go explore our trails in search of wild owls!