<P> 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 rod cells which function well in poor light . Terns, gulls and albatrosses are amongst the seabirds which have red or yellow oil droplets in the colour receptors to improve distance vision especially in hazy conditions . </P> <P> 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 . </P> <P> 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 . </P> <P> 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 rises to cover the eye in most birds, with the exception of the horned owls where the upper eyelid is mobile . </P>

Why do birds have eyes on either side of their head