Almost all birds have extremely well developed eyes (very few exceptions exist in the case of some nocturnal birds, like the kiwis Apteryx spp.) which reflects the paramount importance of the sense of vision in birds. They are dependent on it for finding food, recognising their mates, enemies, etc. Birds have relatively large eyes; some owls (Strigiformes) and hawks (Accipitridae) have eyes as large as man. The eyeballs are sometimes as big as the head can accomodate and almost touch each other at one point. Because of their large size, they usually fit very tightly in their orbit and birds can therefore move the eyes only a little bit, the movement being mainly in the horizontal direction. This general lack of eye movement is compensated by an extremely flexible neck, especially in species with frontal eyes (like owls) of which the neck can turn well over 180°. Birds, like many other vertebrates, have a nictitating (winking) membrane or third lid which lies under the other two lids on the nasal side and can be drawn horizontally across the eye. In diurnal (active in daylight) birds it is normally transparent and can be used to clean or moisten the eye without shutting out the light. It is generally believed that the nictitating membrane covers the eye during flight to prevent drying out of the cornea by air currents. In some diving birds it forms an auxiliary lens for underwater vision. The basic structure of the avian eye is like the general vertebrate pattern. Because birds have no blood vessels within the retinal tissue, they developed a peculiar feature, the pecten; a pigmented, highly vascular structure in the posterior chamber of the eye and most probably a device for providing nutrition and oxygen for the retina; which is unusually thick and well developed in birds. The visual acuity (the ability to distinguish fine details) of birds is generally very high and comparable to that of man, although some birds-of-prey are thought to have about five times better visual acuity. It is mainly achieved by a small, highly developed pit in the central retina, the fovea (also present in primates) of which sometimes a second one exists (especially in birds-of-prey) to obtain better binocular vision. As most birds have their eyes placed laterally in the head (and one central fovea) this means that the most accurate vision is usually to the sides and that stereoscopic vision (which needs a binocular vision field) and the judgement of distances is often poor in birds. On the other hand the visual field of these birds is very large and together with some head movements they can see all round. Some birds have their eyes set so far back in the head that they can look behind just as well or even better (e.g. the Woodcock). Diurnal birds possess colour vision of which the range and distinguishing capabilities are generally comparable to that of man, but nocturnal ones do not have colour vision.

Alternative forms for eye : acuity, binocular, eyes, nictitating, pecten.