Last week, I saw a mature Bald Eagle apparently fishing. It was a dank day and she was circling high in a gray sky over gray swells in Eggemoggin Reach. (Sex assumed from her large size.) She was flying at a height of at least four football fields (1200 feet). My big lens did not reveal any ducks or other surface swimmers below her. What could she see under the moving water from such heights? A lot.
I recalled a fascinating lecture on Golden and Bald Eagle eyesight that I had attended years ago at the Blackwater National Wildlife Refuge on the Chesapeake Bay. I got out my notes and found the images I had taken there of two of the Refuge’s patients: a Bald Eagle and a Golden Eagle, both of which were recuperating from wounds. (Two of those images are shown here.)
Bald Eagle (Leighton Archive Image)e)
Notice the large eyes of these birds relative to the rest of their bodies. Those eyes are about the same size as the eyes of adult humans and are reportedly larger than the birds’ brains. They can see up to eight times better than humans, researchers say.
An Eagle’s eye sockets are angled to give the birds an amazing 340-degree field of vision, which is both binocular (when its two eyes are focusing on a rabbit or fish two miles away, for example) and peripheral (when it’s focusing each eye differently and scanning the surrounding sky to avoid other eagles or crows, for example).
Golden Eagle (Leighton Archive Image)
However, Eagles’ eyes are so tightly fit into their sockets that they usually have to move their heads slightly to focus both eyes on single targets. Thus, their large heads are seemingly always moving when they’re focusing both eyes on their surroundings. When perched, this sometimes looks like they’re shaking their heads to music, but it’s only their way of “looking around.” (Brooklin, Maine).