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Nature Notebook – Owl eyes

Our Eastern screech owl, Otis, recently spent some time in our office. He studied the new surroundings; we studied his eyes.

His pupils quickly changed from small to almost fully dilated and back. When he turned his head, one pupil changed but the other didn’t. In humans, this is a sign of brain damage. In owls, it demonstrates their ability to control the amount of light transmitted to their retina. They can accommodate even small differences in light levels.

The visible part of the eyeball is the front half of an elongated tube. The back part, where the light image is received and transmitted to the brain, is wider than the front part. The tube shape places the focusing lens relatively far from the retina…better for distance vision. The wide back provides a larger surface area for a large number of light-sensitive “rod” cells…better for night vision. Owls have very few “cone” cells so their world is always gray.