A Virginia Rail at Taking Waters Garden in Albany (Courtesy/Albert Ryckman)
Birds differ from us and other mammals in many ways. There’s flying, and being toothless, plus all those feathers. Among both the feathered and the furry are nocturnal species—kiwis for them, bats for us—who aren’t always visually directed. But for most birds, and most mammals, eyesight is very important.
We now know that birds’ eyes function better than peoples’ in specific ways, under many conditions. They can see colors that are invisible to us. They can perceive other aspects of the natural world that are beyond our abilities.
Physically, eyes of mammals and birds share many characteristics. But there are some crucial differences.
Our eyes are attached to specialized muscles that move them in their sockets. Birds’ eyes are fixed in place. Flight requires minimum baggage, and muscles weigh too much. Some birds have eyes that bulge out and give them almost a 360-degree field of vision. Our field of vision is 120 degrees, much of that fuzzy.
Some ground-feeding birds can see straight up, the better to spot predators on the attack. Owls’ vision is enhanced by extra neck vertebrae that let them turn their heads all the way around, to look backwards over their shoulder.
Birds have eyelids, like us, plus a special transparent membrane to protect the eyes. It’s called the nictitating membrane. It folds and unfolds from side of the eye and lubricates the eyeball when it moves across it. Because of this membrane, a tern or pelican can dive into the ocean with open eyes, and a falcon can dive through air at a hundred miles per hour.
Birds see frequencies of light we don’t—the ultraviolet end of the spectrum—so they see colors we can’t perceive. Eagles discern sixteen times more differing colors than we do. Most birds have many more light receptors in their eyes than we do.
Nocturnal birds have relatively big eyes, with some bigger owls’ eyes as large as ours, though the owl weighs less than five pounds. Know why you and I can’t hunt a small rabbit from five hundred feet in the air? Bad eyesight. Our eyes have a single fovea in each retina. The fovea is where the most light reception is concentrated and where we get our finest focus. Because our two fovea concentrate on one single area, that’s where we can focus and see clearly. An eagle has four fovea and each focuses on a different central spot so that bird can survey a huge expanse of water or grassland with fine focus.
Finally, we are finding out that birds’ eyes contain special proteins that react to the lines of magnetic force across the Earth. This ability to “see” or sense the magnetic field allows birds to navigate on dark nights and know where they are without use of a map or GPS. In a bird’s world, the eyes have it.
For information about upcoming Salem Audubon programs and activities, see www.salemsudubon.org, or Salem Audubon’s Facebook page.
Harry Fuller is an Oregon birder and natural history author of “Freeway Birding.” He is a member of the Salem Audubon Society. Contact him at [email protected] or http://www.towhee.net/. His “Some Fascinating Things About Birds” column will be appearing regularly in Salem Reporter.
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