īird eye size is broadly related to body mass. The ostrich has the largest eye of any land vertebrate, with an axial length of 50 mm (2.0 in), twice that of the human eye. 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 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. When sleeping, the lower eyelid rises to cover the eye in most birds, with the exception of the horned owls where the upper eyelid is mobile. The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds when they are under water. Instead the eye is lubricated by the nictitating membrane, a third concealed eyelid that sweeps horizontally across the eye like a windscreen wiper. The eyelids of a bird are not used in blinking. The nictitating membrane of a masked lapwing 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. Unlike the mammalian eye, it is not spherical, and the flatter shape enables more of its visual field to be in focus. The eye of a bird most closely resembles that of other reptiles. Terns, gulls, and albatrosses are among the seabirds that have red or yellow oil droplets in the colour receptors to improve distance vision especially in hazy conditions. Nocturnal species have tubular eyes, low numbers of colour detectors, but a high density of rod cells which function well in poor light. The placement of their eyes gives them good binocular vision enabling accurate judgement of distances. Birds of prey have a very high density of receptors and other adaptations that maximise visual acuity. Some bird groups have specific modifications to their visual system linked to their way of life. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds. In addition to the two eyelids usually found in vertebrates, bird's eyes are protected by a third transparent movable membrane. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. Birds are theropod dinosaurs, and the avian eye resembles that of other reptiles, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have 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". Vision is the most important sense for birds, since good eyesight is essential for safe flight. With forward-facing eyes, the bald eagle has a wide field of binocular vision.
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