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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, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket.〔 In addition to the two eyelids usually 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 oculi, unique to birds. Birds, unlike humans but like fish, amphibians and reptiles, have four types of colorreceptors in the eye. One of these receptors gives some species of birds the ability to perceive not only the range visible by humans, but also the ultraviolet part of the spectrum, and other adaptations allow for the detection of polarized light or magnetic fields. 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 maximize 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 colordetectors, but a high density of rod cells which function well in poor light. Terns, gull and albatrosses are amongst the seabirds which have red or yellow oil droplets in the colorreceptors 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 holds 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. Birds with eyes on the sides of their heads have a wide visual field, useful for detecting predators, while those with eyes on the front of their heads, such as owls, have binocular vision and can estimate distances when hunting. The American woodcock probably has the largest visual field of any bird, 360° in the horizontal plane, and 180° in the vertical plane.〔 The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third concealed eyelid that sweeps horizontally across the eye like a windscreen wiper. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds when they are under water. When sleeping, the lower eyelid rizes to cover the eye in most birds, with the exception of the horned owls where the upper eyelid is mobile. The eye is also cleaned by tear secretions from the lachrymal gland and protected by an oily substance from the Harderian glands which coats the cornea and prevents dryness. The eye of a bird is larger compared to the size of the animal than for any other group of animals, although much of it is concealed in its skull. The ostrich has the largest eye of any land vertebrate, with an axial length of 50 mm (2 in), twice that of the human eye.〔 Bird eye size is broadly related to body mass. A study of five orders (parrots, pigeons, petrels, raptors and owls) showed that eye mass is proportional to body mass, but as expected from their habits and visual ecology, raptors and owls have relatively large eyes for their body mass. Behavioural studies show that many avian species focus on distant objects preferentially with their lateral and monocular field of vision, and birds will orientate themselves sideways to maximize visual resolution. For a pigeon, resolution is twice as good with sideways monocular vision than forward binocular vision, whereas for humans the converse is true.〔 The performance of the eye in low light levels depends on the distance between the lens and the retina, and small birds are effectively forced to be diurnal because their eyes are not large enough to give adequate night vision. Although many species migrate at night, they often collide with even brightly lit objects like lighthouses or oil platforms. Birds of prey are diurnal because, although their eyes are large, they are optimized to give maximum spatial resolution rather than light gathering, so they also do not function well in poor light.〔Martin, Graham. "Producing the image" in Ziegler & Bischof (1993) 5–24〕 Many birds have an asymmetry in the eye's structure which enables them to keep the horizon and a significant part of the ground in focus simultaneously. The cost of this adaptation is that they have myopia in the lower part of their visual field.〔 Birds with relatively large eyes compared to their body mass, such as common redstarts and European robins sing earlier at dawn than birds of the same size and smaller body mass. However, if birds have the same eye size but different body masses, the larger species sings later than the smaller. This may be because the smaller bird has to start the day earlier because of weight loss overnight. Overnight weight loss for small birds is typically 5-10% and may be over 15% on cold winter nights.〔 In one study, robins put on more mass in their dusk feeding when nights were cold. Nocturnal birds have eyes optimized for visual sensitivity, with large corneas relative to the eye’s length, whereas diurnal birds have longer eyes relative to the corneal diameter to give greater visual acuity. Information about the activities of extinct species can be deduced from measurements of the sclerotic ring and orbit depth. For the latter measurement to be made, the fossil must have retained its three-dimensional shape, so activity pattern cannot be determined with confidence from flattened specimens like ''Archaeopteryx'', which has a complete sclerotic ring but no orbit depth measurement. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bird vision」の詳細全文を読む スポンサード リンク
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