The superior performance of the eagle’s eyesight
is due to several anatomical features.  They have two foveae or centers of focus, that
allow the birds to see both forward and to the side at the same time. Humans
have only one fovea – one for binocular or forward vision. Bald eagles are
capable of seeing fish in the water from several hundred feet above, while
soaring, gliding or in flapping flight. This is quite an extraordinary feat,
since most fish are counter-shaded, meaning they are darker on top and thus
harder to see from above. Fishermen can confirm how difficult it is to see a
fish just beneath the surface of the water from only a short distance away.

   Young bald eagles have been known to make mistakes, such as
attacking objects like plastic bottles floating on or just below the surface of
the water. Bald eagles will locate and catch dead fish much more rapidly and
efficiently than live fish, because dead fish float with their light underside
up, making them easier to see.

   Eagles have eyelids that close during sleep. For blinking,
they also have an inner eyelid
called a nictitating membrane. Every three or four seconds, the nictitating membrane slides
across the eye from front to back, wiping dirt and dust from the cornea.
Because the membrane is translucent, the eagle can see even while it is over
the eye.

   Eagles, like all birds, have color vision. An eagle’s eye is
almost as large as a human’s, but the eagle’s eyes are so large that they occupy most of the space
of its head. The large eye allows for a large retinal area where the visual image
appears. The eagle can
identify its prey moving almost a mile away. That means that an eagle flying at
an altitude of 1000 feet over open country could spot prey over an area of
almost 3 square miles from a fixed position.