Bird vision

Vision is the most important sense for birds, since good eyesight is essential for safe flight, and this group has a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". Güntürkün, Onur, "Structure and functions of the eye" in Sturkie (1998) 1–18 ] The avian eye resembles that of a reptile, but has a better-positioned lens, a feature shared with mammals. Birds have the largest eyes relative to their size within the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids always found in vertebrates, it is protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten, unique to birds.

Birds, like fish, amphibians and reptiles, have four types of colour receptors in the eye. Most mammals have two types of receptors, although primates have three. This gives birds the ability to perceive not just the visible range but also the ultraviolet part of the spectrum, and other adaptations allow for the detection of polarised light. Birds have proportionally more light receptors in the retina than mammals, and more nerve connections between the photoreceptors and the brain.

Some bird groups have specific modifications to their visual system linked to their way of life. Birds of prey have a very high density of receptors and other adaptations that maximise visual acuity. The placement of their eyes gives them good binocular vision enabling accurate judgement of distances. Nocturnal species have tubular eyes, low numbers of colour detectors, but a high density of cone cells which function well in poor light. Terns, gull and albatrosses are amongst the seabirds which have red or yellow oil drops in the colour receptors to improve distance vision especially in hazy conditions.

Extraocular anatomy

The eye of a bird most closely resembles that of the reptiles. Unlike the mammalian eye, it is not spherical, and the flatter shape enables more of its visual field to be in focus. A circle of bony plates, the sclerotic ring, surrounds the eye and hold it rigid, but an improvement over the reptilian eye, also found in mammals, is that the lens is pushed further forward, increasing the size of the image on the retina.Sinclair (1985) 88–100] Most birds cannot move their eyes, although there are exceptions, such as the Great Cormorant. [
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