Sunday, August 9, 2015

Owl Eyes

Whether it be the reddish-orange of the Eurasian Eagle Owl or the stunning yellow of the Great Horned Owl, when most people see an owl for the first time they notice their eyes first. 

What most people notice first are the eyes (photo: Wm. Oberbeck III)

An owl’s eye is a highly specialized light gathering organ that has been refined over hundreds of thousands of years of evolution. The structure of an owl's eye is not all that dissimilar to the human eye, but it has evolved to see better at night. 

First of all, the eyes are large to improve the light gathering capability, particularly in the low light conditions in which they prefer to hunt.  The owl eye shape is more long and tubular rather than the round shape of our eyes. 

Second, they are held in place by bony plates in the skull called sclerotic rings, which is why owls cannot move their eyes within their sockets. 

Third, the cilliary muscles that control the shape of the cornea, and the iris muscles that control the amount of light passing through the lens, are composed of striated muscle which contracts faster than smooth muscles.

 Fourth, the eye has a special structure called a pectin (a thin, folded structure that projects out from the back of the eye into the eye).  The pectin likely helps supply nutrients and oxygen while removing carbon dioxide from the retina. 

Lastly, an owl's retina has a higher rod (light sensing cells)-to-cornea (color sensing cells) ratio, which allows them to see better at low light levels at the cost of reduced capacity for color vision and generally lower visual sharpness.

In this photo you can see the nictitating membrane at the edge of the right eye (photo: Wm. Oberbeck III)

To shield their eyes from harm, owls come equipped with three eyelids.  They have the normal upper and lower eyelids, but they also have a third eyelid called a nictitating membrane.  This membrane is a thin layer of opaque tissue that closes diagonally across the eye, from the inside outwards.  This membrane moistens the eye surface while cleaning it of dirt and microscopic organisms.  They also use this special membrane right before impact with potential prey, presumably to protect the cornea from scratches or other possible damage.

The next time you visit the World Bird Sanctuary be sure to pay special attention to the eyes of our resident owls.  Not only are they a remarkable adaptation, but they make for great close-up photos.

Submitted by William Oberbeck III, World Bird Sanctuary Naturalist/Trainer

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