Owls (Strigiformes)

This post was originally published on January 31, 2013. We’ve seen or heard these three owls at our Decorah or Decorah North nests. GHO have also been seen at Great Spirit Bluff.

Owls are a large and diverse group of mainly nocturnal birds. Over 200 species are distributed among 27 genera worldwide, ranging in size from the tiny Elf Owl (weighing as little as 31 grams or one ounce) to the massive Blakiston’s Fish Owl (weighing as much as 4.5 kilograms or ten pounds). Despite differences in size, wingspan, color, plumage patterns, and habitat, owls share some common characteristics, including:

  • Facial disks
  • Large forward facing eyes
  • ‘Soft’ feathers
  • Round heads, distinctive facial markings and/or ear tufts
  • Talons and a curved beak

We’ve seen and/or heard Great Horned Owls, Barred Owls, and Eastern Screech owls at our Decorah and Decorah North nests. Owls have a number of adaptations that help them survive and thrive after dark, including highly specialized hearing, strong feet and talons, night-adapted vision, and silent, cryptic plumage. Click the down arrows to learn more about them!

Specialized hearing
Owl sound localization map

An owl’s medullary 3D sound map

An owl’s ears are located on the sides of its head, under the feathers of its facial disk. Its facial disk splits into left and right sides down the middle of its face: a living parabolic microphone that magnifies sound by channeling it into the ear on its side of the owl’s head. The disk also aids directional hearing. An owl can move its facial disk feathers to change the disk’s shape, channeling more sound into one ear than the other. Some owls can establish prey directionality based on differences in sound wave arrival times just 30 millionths of a second apart.

Directional hearing doesn’t end with the owl’s facial disk. Owls have asymmetrically set ear holes – that is, one ear opening is higher than the other. Sound waves arrive in either the upper or lower ear first, establishing the prey’s vertical location. These X and Y-axis locations create a three-dimensional image of the space where the sound source is located, enabling the owl to accurately strike prey it can’t see.

An owl perched in a bare oak tree hears a mouse rustling under snow in a wintery nighttime forest. The sound reaches her right ear slightly faster than her left ear, so she knows which direction the rustling is coming from. She turns her head until the sound reaches both of her ears at the same time, signaling that the mouse is right in front of her. Now she knows the general location of her prey, but she needs to pinpoint its location more accurately in the darkness.

Sound reaches the owl’s lower ear hole first, which tells her that the mouse is below her line of sight. She dips her head until she hears the sound equally in both ears. Her prey is deep in the snow. She silently launches into the air.

Strong feet and talons
Barred Owl

Barred Owl with Prey

Owls have four sharp, curved talons specialized for sensing, catching, and killing prey. In addition to filoplume feathers that help some species of owls sense when they’ve made contact – a useful adaptation for hunting in the dark – the underside of an owl’s foot is covered in a rough, frictional surface that helps owls grasp and hold prey. Many owls live in cold weather, and feathered or ‘booted’ feet protect them against the cold and bites or scratches from their prey.

An owl’s feet appear massive in comparison with the rest of its body. When perching, two talons are directed forward and two behind. A flexible joint allows three talons to swivel forward during flight. When striking, owls spread their talons wide in a rough oval shape, increasing their chance of success.

Once they’ve made contact with prey, owls knead and crush it with their large, powerful feet, using their sharp, hooked beaks to grip and tear tissue.

About two feet from her target, she brings her feet forward, spreads her talons, and strikes. Success! The mouse can’t escape her rough feet and sharp, curving talons. She quickly dispatches it, crushing it and wolfing it down head first.

Night-adapted vision
An owl's field of vision

An owl’s field of vision

Owls have very large eyes, accounting for one to five percent of total body weight depending on the species. Like many predatory animals, their eyes face forward, increasing the range of binocular vision. Binocular vision, or seeing an object with both eyes at once, makes depth perception possible and aids owls in everything from capturing prey to accurately landing on a branch in the middle of the night. According to the Owl Pages, the field of view for an owl is about 110 degrees, with about 70 degrees being binocular vision.

Like many nocturnal animals, owls have tubular eyes and an abundance of rod cells, which are excellent at collecting light but don’t see color well. They can see well at night, but have monochromatic vision and cannot roll or move their eyes within their sockets. How do they see their surroundings? Owls can swivel their heads up to 270°, which means that they can see behind themselves without turning around.

However, owls don’t see well in the dark just because their large eyes are good at gathering light. Their fixed tubular eyes give them a long axial eye length, which means that there is an unusual amount of distance between the cornea and the retina. Think of a projector! A projected image grows larger as you move a projector back from the screen, and the same is true of an owl’s eye. Its larger retina also has room for a lot of photoreceptors, giving it heightened visual acuity since each photoreceptor processes a smaller portion of the image. In short, an owl’s long, tubular eye projects a large image over a large, dense field of photoreceptors, enabling it to see detail and depth even in very low light conditions.

Finished with her mouse, the owl flies back to her favorite perch. A human might not see the branch and would be unable to judge depth and distance in the darkness, but her highly specialized eyes can see clearly even at night. As she approaches it, she raises her body and flares her wings to slow down. With feet outstretched, she has enough momentum to keep moving forward until her rough feet grasp the perch. She hears something in the distance and swivels her head to look at it.

Silent, cryptic plumage

Many birds have stiff primary or flight feathers, but owl feathers have a special soft leading edge that helps them fly almost silently through the darkness.

When birds fly, air rushes over their wings, creating lift and turbulence. The stiff edge of a normal bird’s wing produces a larger area of turbulence, which makes a whooshing noise. However, the comb-like soft edge of an owl’s wing breaks down the turbulence and muffles the sound of air rushing over the wing surface. The lack of owl-related flight noise helps owls hear prey and keeps prey from hearing their approach.

The plumage of owls is generally cryptic – that is, the colors and patterns of their feathers camouflage them. For example, Snowy Owls live in the tundra and are primarily white, while Great Horned Owls live in the woods and are brown, grey, and white. Changes in cryptic patterns can even be seen within species inhabiting different ranges: Great Horned Owls that live in birch forests tend to be a little lighter than those that live in darker oak or maple forests.

Although cryptic plumage aids camouflage, many owl species have facial and head markings: face masks, ear tufts, and brightly colored irises. Wikipedia states that these markings are more common in species inhabiting open habitats and are thought to be used to signal other owls under low light conditions. Other people have suggested that, like cryptic patterns, tufts and facial markings serve to camouflage owls.

The owl lifts off her branch. Subnivean voles and mice flare briefly in her awareness as they scrabble beneath the snow. Her directional hearing, acute vision, muffled flight, powerful talons, sharp beak, and cryptic plumage make her a formidable predator as she flies silently through the dark woods to her nest.


Things that helped me learn about this topic:

Image credits
Medulla map: Solis, Michele M, and Perkel, David J(Jan 2006) Neuroethology. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0003380] Owl feather: By Kersti (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons
Barred Owl: By NaturesPhotoAdventures (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons
Owl Vision: The Owl Pages: http://www.owlpages.com/articles.php?section=owl+physiology&title=vision