Owl Ears and Hearing
Because Owls are generally active at night, they have a
highly developed auditory (hearing) system. The ears are located
at the sides of the head, behind the eyes, and are covered by the
feathers of the facial disc. The "Ear Tufts" visible on some
species are not ears at all, but simply display feathers.
The shape of the ear opening (known as the aperture) depends on the species of Owl
- in some species, the opening has a valve, called an operculum
covering it . The opening varies from a small, round aperture to an
oblong slit with a large operculum. All owls of the family Tytonidae
have rounded openings with large opercula, while in Strigidae,
the shape of the outer ear is more varied.
An Owl's range of audible sounds is not unlike that of humans, but an Owl's hearing is much more acute at
certain frequencies enabling it to hear even the slightest movement of
their prey in leaves or undergrowth.
Some Owl species have asymmetrically set ear openings (i.e. one ear is higher than the
other) - in particular the strictly nocturnal species, such as the Barn
Owl or the Tengmalm's (Boreal) Owl. These species have a very pronounced
facial disc, which acts like a "radar dish", guiding sounds
into the ear openings. The shape of the disc can be altered at will,
using special facial muscles. Also, an Owl's bill is pointed downward,
increasing the surface area over which the soundwaves are collected by
the facial disc. In 4 species (Ural, Great Gray, Boreal/Tengmalm's & Saw-whet), the ear asymmetry is actually in the temporal parts of the skull, giving it a "lop-sided" appearance.
An Owl uses these unique, sensitive ears to locate prey by listening for prey movements through ground cover such
as leaves, foliage, or even snow. When a noise is heard, the Owl is able
to tell its direction because of the minute time difference in which the
sound is perceived in the left and right ear - for example, if the sound
was to the left of the Owl, the left ear would hear it before the right
ear. The Owl then turns it's head so the sound arrives at both ears simultaneously
- then it knows the prey is right in front of it. Owls can detect a
left/right time difference of about 0.00003 seconds (30 millionths of a
second!)
An Owl can also tell if the sound is higher or lower by using the asymmetrical or uneven Ear openings. In a Barn Owl,
the left ear left opening is higher than the right - so a sound coming
from below the Owl's line of sight will be louder in the right ear.
The translation of left, right, up and down signals are combined instantly
in the Owl's brain, and create a mental image of the space where the
sound source is located. Studies of Owl brains have revealed that the medulla
(the area in the brain associated with hearing) is much more complex
than in other birds. A Barn Owl's medulla is estimated to have at least
95,000 neurons - three times as many as a Crow.
Once the Owl has determined the direction of its next victim, it will fly toward
it, keeping its head in line with the direction of the last sound the
prey made. If the prey moves, the Owl is able to make corrections mid
flight. When about 60 cm (24") from the prey, the Owl will bring
its feet forward and spread its talons in an oval pattern, and, just
before striking, will thrust it's legs out in front of it's face and
often close it's eyes before the kill. Click
here to see a Great Gray Owl using it's hearing to catch a small
rodent concealed under snow.
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References: Campbell, Wayne. 1994. "Know Your Owls (CD-ROM)". Axia Wildlife
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