Eagle's eyes

April 29, 2005

Eagle Vision

Before we talk about eagle vision, I wanted to answer a frequent question from today regarding the corncob in the eagles' nest.

Not far from the two cam nests is a field where the Refuge staff has grown corn for migrating waterfowl. Eagles do not eat corn (neither do ospreys), but the eagle parents bring cornstalks from the field to the nest for nesting material, and sometimes an ear of corn comes along for the ride. It looks like the eaglets are pecking the corn out of curiosity, but we don't think they're eating it. Eventually the cob will probably get buried or knocked out of the nest. :-)

And now on to the topic of eagle vision...

As the eaglets grow and develop, we continue to see more adult-like features on their bodies. One example is the bony ridge that can now be seen over the eaglets' eyes. Most raptors have this ridge over each eye as it helps shade their eyes from the sun and makes it easier to see and track prey. It serves the same purpose as a visor that a human would put on top of their head.

In addition to the helpful bony ridge, eagles also possess what is regarded as the sharpest, most powerful set of eyes in the animal kingdom. It's believed that their eyesight is about four times sharper than a perfect human eye. In fact, the eagle's vision is so legendary that it's responsible for the idiom "eagle eye," which means a person with unusually keen sight.

An eagle eyeball is almost the size of a human eyeball even though the eagle's head is smaller than an adult head. Along with their large eyeball, eagles also have two foveae, or centers of focus, on each eye, which allows them to see both forward and to the side at the same time. And as a further bonus, eagle eyes have around 1 million light-sensitive cells per square millimeter of retina, while humans only have about 200,000 cells.

Ornithologists state that an eagle can spot a moving rabbit from about a mile away. If the eagle is flying 1,000 feet high in the sky, it's believed it can spot prey over about three square miles.

Despite their tremendous eyesight, one of the hardest tasks for the eaglets will be learning to fish from the air. Fish often have dark coloring on top of their bodies, so they are difficult to see from above. In addition, water refracts objects and makes it difficult to judge their exact location underwater. Many juvenile eagles "catch" dead fish more frequently than live fish until they learn the art of sky fishing, which is an acquired skill that will take time to develop and will impact their chances for survival.

Until next time,
Lisa - webmaster

Posted by Webmaster at April 29, 2005 06:55 PM