Owl Eyes and Vision
Of all an Owl's features, perhaps the most striking is its eyes. Large and forward facing,
they may account for one to five percent of the Owl's body weight, depending
on species. The forward facing aspect of the eyes that give an Owl its
"wise" appearance, also give it a wide range of "binocular" vision
(seeing an object with both eyes at the same time). This means the owl can
see objects in 3 dimensions (height, width, and depth), and can judge
distances in a similar way to humans. The field of view for an owl is about
110 degrees, with about 70 degrees being binocular vision.
humans have a field of view that covers 180 degrees, with 140 degrees
being binocular. A woodcock has an amazing 360 degree field of view,
because its eyes are on the side of its head. However, less than 10
degrees of this is binocular.
An Owl's eyes are
large in order to improve their efficiency, especially under low light
conditions. In fact, the eyes are so well developed, that they are not eye balls
as such, but elongated tubes. They are held in place by bony structures in
the skull called Sclerotic rings.
For this reason, an Owl cannot "roll" or move its eyes - that is, it can
only look straight ahead!
The Owl more than makes up for this by being able to turn its head up to 270 degrees left or right from the forward facing position, and almost upside down. There are several adaptations that allow this, outlined in the Owl Skeletal system article.
Cross-section of an Owl's Eye
As most owls are
active at night, their eyes must be very efficient at collecting and
processing light. This starts with a large cornea (the transparent
outer coating of the eye) and pupil (the opening at the centre of the
eye). The pupil's size is controlled by the iris (the coloured
membrane suspended between the cornea and lens). When the pupil is larger,
more light passes through the lens and onto the large retina
(light sensitive tissue on which the image is formed).
The retina of an owl's eye has an abundance of light-sensitive, rod-shaped
cells appropriately called "rod" cells. Although these cells are very
sensitive to light and movement, they do not react well to colour. Cells
that do react to colour are called "cone" cells (shaped like a cone), and an
Owl's eye possesses few of these, so most Owls see in limited colour or in
Since Owls have extraordinary night vision, it is often thought that they
are blind in strong light. This is not true, because their pupils have a
wide range of adjustment, allowing the right amount of light to strike the
retina. Some species of Owls can actually see better than humans in bright
To protect their
eyes, Owls are equipped with 3 eyelids. They have a normal upper and lower
eyelid, the upper closing when the owl blinks, and the lower closing up when
the Owl is asleep. The third eyelid is called a nictitating membrane, and is a thin layer of tissue that closes
diagonally across the eye, from the inside to the outside. This cleans and
protects the surface of the eye.
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Campbell, Wayne. 1994. "Know Your Owls (CD-ROM)". Axia Wildlife