SVR Murthy
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Eagles, like other birds of prey have unbelievable eyesight. To keep their vision sharp, eagles clean their eyes with the help of an inner eyelid called nictitating membrane. Besides protection, the membrane keeps the eye moist and clean of dirt and dust. It slides horizontally every three to four seconds. And, by being transparent, the eagle never loses sight of the prey even when the membrane is drawn across the eye. It is also called Juliet’s eyes.

An eagle’s eyeball is almost the same size as a human eye. Given that the eyeball is so large relative to the size of the head, an eagle’s eyes fill most of the skull. Each eyeball is “fixed” in the skull, held in place by a sclerotic ring. Eagles are unable to move their eyeballs within the socket

Eagles use both monocular and binocular vision, meaning they can use they eyes independently or together depending on what they are looking at.

An eagle eye has two focal points (called “fovea” [singular] or “foveae” [plural]) one of which looks forward and the other to the side at about a 45 degree angle. These two foveae allow eagles to see straight ahead and to the side simultaneously. The fovea at 45 degrees is used to view things at long distances. An eagle can see something the size of a rabbit at more than three miles away.

 The visual field of an eye must extend contralaterally in order to contain a centre of expansion of the optic flow field that encompasses the object or surface towards which the animal's head is moving. For movement towards a relatively distant target, the disparity of viewpoint of two eyes with overlapping contralateral fields will be negligible. This will result in the two eyes receiving identical optical flow field information, and this may of itself enhance accuracy through redundancy (as, for example, in humans where two eyes are superior to one eye, even when binocular viewing is devoid of stereoscopic cues Oones and Lee, 1981].


Schematic horizontal section through the retinal fields of a Short-toed Eagle. A, B, e. D are points within the cyclopean field, and their projections onto the retina of each eye are indicated by similar leiters. The large arrow at the bottom of the diagram indicates the direction of forward movement of the bird's head. The arrows around the edge of the cyclopean field indicate schematically the directions in which elements in the optic flow field appear to travel as the head moves forward. The directions of movement across the retina of the images of these elements are indicated by arrows within each eye. Indicated by hatching is the section in each retina that corresponds to the binocular field. In these sections the images of elements in the optic flow field travel in both directions away from the image of the point towards which the head is moving.


References:

Berkhoudt H. (1985) Structure and function of avian taste receptors. ill Fonn and Function in Birds (cd. by A.S. King and J. Mclelland). Academic Press, London. pp. 463~96. British Ornithologists' Union (1992) Checklist of Birds of Britain and Ireland (ed. 6). British Ornithologists' Union. Tring. Herts .• U.K. Brown. L.H .. E.K. Urban. and K. Newman (1982) The Birds of Africa. Vol. I. Academic Press, London. New York