Why can owls see well at night?

Owls - the night vision of owls is 100 times as keen as that of human beings, because their eyes are especially adapted for seeing in the dark. But most are almost colour blind and the pictures they receive are slightly blurred. This is because their eyes contain more rod shaped receptor cells than cone shaped ones. Operating in bright light, cone cells sharpen details and react to colour. Rod cells gather light and owls have 10 times as many of these as do human beings. Each cell contains "visual purple", a substance capable of transforming the slightest glimmer of light into a sight impression.

Owls have exceptionally large eyes and can control the amount of light entering by expanding or contracting the pupil. Each pupil can act independently of the other so that owls can see objects in the shadows and in bright light at the same time. Owls eyes are so large that they are supported by thin, bony, tubular structures called sclerotic rings. Because of this the eyes are almost immovable and nature has compensated for this by giving owls extremely flexible necks, which enable them to turn their heads through an arc of 270 degrees. These birds have excellent binocular vision as their eyes are in the front of their heads. This gives them a tremendous advantage in swooping on small lively prey, because distance judgment depends on binocular vision. To add to their advantages at night, the owls have outstanding hearing, keener than that of any other carnivorous bird. But owls can also see well in the daytime. Although most species hunt by night, others are active at dusk or in full daylight.