An owl's eyes are fixed in their sockets - their large size rendering them
almost immovable in the skull. They face forward, and provide a wide angle
of vision, the overlapping field of view providing binocular, stereoscopic vision.
Because their eyes are fixed, owls have evolved extraordinary flexible neck
muscles, that enable the birds to rotate their heads through a maximum of
270 degrees.
An owl's staring eyes are its most striking feature.
Owls can see in near darkness, due to their large pupils and retina - the
layer of light-receptive cells at the back of the lens. The large pupils
allow them to discern objects at one-tenth to one-hundredth the amount of
light a human needs: they see better in semidarkness than we see in broad
daylight. The retina is endowed with a greater proportion of cells than
any other diurnal birds. Because of these extremely sensitive cells,
called rods, owls see only in monochrome, ie., in one colour black-and-
white, with variations only in light and shade.
An owl's eyes are protected by a nictating membrane which can be closed at
will. The motion of the membrane also serves to remove dust and debris
caught on the surface of the eye.
The membrane in motion.
Excellent senses of sight and hearing are essential prerequisites for their
nocturnal lifestyle, and the combination of an owl's vision and hearing work
in harmony.
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