All eagles are renowned for their excellent
eyesight, and the bald eagle is no exception. The expression “eagle
eye” describes quite accurately one of the unique physical adaptations of
the bald eagle. The bald eagle’s eyesight is three to four times stronger than
that of humans with perfect vision. They also have dual-focus ability – meaning
they can focus their eyes on an object ahead of them and another on the side at
the same time.
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
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.
contain a high number of features which allow for visual accuracy and color
perception. These features are called cones. Human eyes also
contain cones, but proportionately not as many. Because of the high number of
cones, as opposed to rods, the eagle’s vision is not particularly good at
The Bony ridge (the brow) protects the eye from
too much sun. The bristles between the eyes protect the eye from dust when
flying. Finally, because they can only move their eyes slightly within the
sockets, they are able to rotate their heads almost 270 degrees.