How An Owl Hears

How an Owl Hears: Five Key Facts

Strix_aluco_1_(Martin_Mecnarowski)

Owl at night (Photo: Wiki Commons, Martin Mecnarowski).

Owl Hearing Explained

Nature never does anything without a reason and there’s a reason for everything that birds do, such as why hawks and eagles hunt during the day and most owls hunt at night.  In ecology lingo it’s called “habitat partitioning,” which means using different parts of the habitat at different times or in different ways and not overlapping. This allows predatory birds like eagles and hawks to avoid conflict, and mice to be terrified 24 hours a day (it’s rough being lowest on the food chain).

Hunting at night isn’t nearly as simple as hunting during the day. Night hunters can’t use their eyesight very well (except during times of the full moon), prey animals can hear them coming (because it’s more still), and they have to land on a scampering wee beastie that is moving lightning fast (in, under, and around things) on the forest floor (or flying) with accuracy. Not much to ask eh? Owls are spectacular hunters, and I’ve come to appreciate their adaptations for catching prey, in particular their accuracy and hearing. That's why I thought I'd do a post on how an owl hears.

Barn_Owl_South_Acre_2

Barn owl flying (Photo: Wiki Commons).

FACT 1: Owls Fly Silently

First rule of night flight, if you can’t see well then you have to rely on another sense, namely hearing. If you rely on hearing you can’t be all noisy and flappy and generally scaring off prey or making so much sound you can’t hear your food scurrying around. For this reason owls have feathers that are muffled. Most bird wings are quite loud when they flap, this is because of how air flows over them (for physics nerds it’s called Bernoulli’s Principle). Owls have tiny barbs, hooks and bows on the feathers along the leading edge of their wings. These break up the air flowing over the wing, from one big airflow of turbulence to many small turbulences. These tiny air turbulences flow over and down the surface of the wing to the trailing edge of the wing. Owls also have flexible, raggedy fringes on the ends of their feathers. These further break up airflow and air noise, allowing them to fly silently.  Since prey animals don't really think about looking overhead when they are running for safety, the silent flight of owls makes them especially deadly.

barred owl flicker A. Davey

Barred owl (Photo: Flicker Sharing, A. Davey).

FACT 2: Owls Have Satellite Faces

If you look at the face of an owl you can’t help but notice how it’s shaped.  There are some pretty specialized terms for the feathers on an owls face. These are:

  1. Facial disk-the satellite face (sometimes heart shaped).
  2. Facial ruff –this is the outer ring of feathers on the owls face (sort of like the lip of a cereal bowl). They form a curved wall of feathers that give the facial disc its cupped appearance.  The symmetry or asymmetry of an owl’s ruff is species specific. Diurnal owls (like burrowing owls) have symmetrical faces, while nocturnal owls (barred owls) usually have asymmetrical faces, we’ll talk more about why this is important later.
  3. Auricular (are-ick-you-lar) feathers-the feathers covering the facial disc. These feathers cover the facial ruff too, so much so that often it can’t be seen. You have to be pretty chummy with an owl to see its ruff. The auricular feathers are what we usually see. All of the feathers of the face (and the ruff) can be controlled by the owl, though facial muscles, which means they can shift and change the shape of their facial satellite disc, to channel sound.

The feathers of the ruff acts like reflectors for sound, they increase the area over which sound can be collected by making the owl’s cupped satellite face. The feathers also increase the surface area over which sound is collected, thereby increasing sound sensitivity by about twenty decibels.  In terms of sound ratios, that's  a lot! The interesting thing is that diurnal owls (like burrowing owls) have symmetrical ruffs, while nocturnal owls have asymmetrical ruffs. It’s thought that this asymmetry helps the owls in orienting sound. This relates directly to Fact 3 below.

Bubo_virginianus_06

Great horned owl (Photo: Wiki Commons).

Fact 3: Owl Ears Are In Different Places

We’ve all seen pictures of great horned owls or owls with funky tufts of feathers on their heads. These tufts are not really ears, they’re just feathery bits that can tell you what an owls mood is, sort of like how a cat swivels its ears to tell you how it feels. The tufts can’t here bupkis. An owl's ears are really located behind the feather ruff and on the edges of the facial disc.

jjwilliams_1362174530_boreal1280

Owl ears in different places (Photo: JJ Williams, Star Tribune).

Now here’s the funky part, in most nocturnal owls, their ear holes are not directly across from each other like ours are (see owl skull picture above). Picture taking one of your ears and moving it about four inches up the side of your head. By having ears in two different locations owls can triangulate sounds. It's estimated that sound coming from the left of an owls reaches its left ear about 200 microseconds faster than the right ear. The asymmetry of the ruff and ears allows the owls to be maximally sensitive to sound. If an owl hears a mouse scurry below it, the sounds that the ears receive differs in intensity (the technical term for this is “interaural distance differentiation”) based on where the mouse is moving around. Their owley nervous system can separate out where the sounds come from based on which ear hears the sound more intensely.

flicker jeff whitlock

Screech owl, red phase (Photo: Flicker Sharing, Jeff Whitlock).

Fact 4: Owls Have a Big Map-Brain

Owls also have a very developed medulla, a part of the brain that allows for spatial mapping. It's thought that an owl has 3x more neurons (computing power) than other birds. An owl that is sitting on a perch and hunting has to be able to mentally map (in the dark) an object in not only two but three dimensions. Think of their medulla as being the triangulation calculator. To visualize their triangulation of prey triangulate, if the prey is below and in front of and owl, visualize a mouse on the ground at the point of a triangle with two uneven lines coming off the mouse and stretching up to the owl on on the branch. The owl would be in the middle of the base of the triangle, and it would move its head back and forth until the two sides of the triangle were equal. An owl does this triangulation continually, without thinking, and even in rapid sequence while flying. There are lots of fun experiments owl researchers have done to study owl hunting/hearing. In one study they had a mouse walk across foam (in the dark) with a rustling piece of paper tied to their tail to see what the owl would strike on (spoiler alert: the owl struck at the paper not the mouse).

Barred_Owl_(Strix_varia)

Barred owl up close (Photo: Wiki Commons).

Fact 5: An Owl’s Beak Faces Down

Most birds of prey have beaks that extend off their faces, which allows them more surface area for breaking the necks of prey, ripping and tearing food, and for creating a more aerodynamic head. Owls don’t have this. They are built to maximize sound reception. By having a beak that is snug against their face, the sounds that come to them bounce off the beak and are redirected to the facial disc and the ears.

Owls are amazing creatures, and how an owl hears is even more amazing. They have many unique adaptations that make their habitat partitioning work. If you see an owl out flying at night, stop to admire how it moves and the computing brain power it takes just to fly without running into things in the dark (much less landing on prey!).

 
Posted in Birds, Birds of Prey and tagged on by .

About Infinite Spider

I am currently the Education Program Coordinator at the Smithsonian Environmental Research Center. I have also been a curriculum developer for the Smithsonian Science Education Center and a contract curriculum writer for the Discovery Channel. In my spare time I am a blogger here at "The Infinite Spider" (Infinitespider.com), a science and nature blog for naturalists and outdoor educators. I love rowing crew, birding, hiking, kayaking, and being outdoors. My undergraduate degrees are in Environmental Science and Philosophy, and my graduate degree is in Biology. Currently I live in Annapolis, MD.