Because of their predominantly nocturnal tendencies, owls have evolved several physical adaptations which facilitate catching prey in the dark. All owls have large forward facing eyes giving good stereoscopic vision, vital for judging distances. Indeed, owls have the most forward facing eyes and hence the best stereoscopic vision of all birds. In smaller species the head often appears flattened so that the eyes can be as widely spaced as possible to increase the stereoscopic effect. This is often further enhanced by bobbing or weaving the head to give a differing perspective known as the parallax effect.
The eyes are very large, those of a Snowy Owl weighing as much as our own. They are modified in nocturnal species to improve sensitivity in low light intensities. They are tubular, rather than round, giving a relatively large cornea in proportion to the overall size of the eye and enabling more light to enter the eye. The light passes through the pupil (which can be closed by the iris to a small pinprick in bright light or opened so wide that virtually no iris is visible at night) to the lens. This is large and convex, causing the image to be focused nearer to the lens hence retaining maximum brightness. One drawback is that owls are long sighted and cannot focus on objects which are too close. Tactile bristles around the beak partially compensate for this. The tubular shape also gives a comparatively large retina size which is packed full of light sensitive rods, about 56,000/square mm. in the Tawny Owl. These rods are far more sensitive than cones at low light levels. The phenomenal light gathering properties of the owls eye is further enhanced in many species by a reflective layer behind the retina, called the tapetum lucidum, which reflects back onto the rods any light that may have passed through the retina without hitting one the first time. Tawny owls would appear to have the best developed eyes of all the owls, indeed of all vertebrates, being probably about 100 times more sensitive at low light levels than our own.
As well as rods, all owls possess colour sensitive cones in their eyes. Although having fewer light sensitive cones than humans, they can probably detect colours to some extent. They are certainly not blind in daylight and some, like the Eagle Owl, have better day time vision than us. Our night time vision, however, is better than some diurnal Pygmy Owls.
Owls are unable to move their eyes in the sockets because of the size and tubular shape. To compensate, they have a deceptively long flexible neck which enables them to turn their head 270° in either direction horizontally and at least 90° vertically.