Laura Erickson's For the Birds

Vision

(Recording of a Peregrine Falcon)

That Peregrine Falcon looks at the world through a pair of the most highly evolved organs in all of creation–its eyes. Birds depend almost entirely on vision to find food, detect others of their kind, and to discover and elude predators. Some hawks and owls weighing less than five pounds have eyes which are actually larger than an adult man’s eyes. In many birds the two eyes together weigh nearly as much as the entire brain, and are often as big as the head can possibly fit. Of course, a beady little chickadee eye doesn’t look all that big to us, because the cornea–the only part not hidden by feathers and skull, is relatively small. But the eyeballs themselves are enormous masses inside that little chickadee head.

Actually it isn’t quite correct to refer to a bird’s “eyeballs,” because the shape isn’t spherical. In owls, the eyes could more correctly be called eye tubes–they’re cylindrical. When you see a picture of a cute little owl with eyes demurely cast down and to the side, you know that the artist doesn’t know a thing about owls–their eyes are absolutely fixed facing forward in their sockets. To compensate, an owl’s head can rotate over 180 degrees in either direction.

Most other birds can move their eyes at least a little, but the movements are very slight compared to mammals. The most remarkable thing about these eye movements is that each eye moves and sees independently of the other. Studies show that many birds turn their heads and look at objects with one eye alone to make difficult visual discriminations. We humans normally have depth perception because of the simultaneous appreciation of slightly different images on each eye’s retina. Integration of these images occurs in our brain–optic nerves from each eye connect mainly on the opposite side of the brain, but there’s at least some overlap. With birds, the two optic nerves are completely crossed–each side of the brain receives its information from one eye only. That doesn’t mean that birds don’t have some depth perception–there’s good evidence that a unique area of the bird brain, called the Wulst, which isn’t found in other animals, permits stereoscopic vision.

How do people know just what a bird’s eye view really is? After all, we can hardly present a Bald Eagle with an optometrist’s eye chart. The first way is to carefully examine a bird’s eye and brain to compare it with the human eye. Birds have a sharply defined pit in the middle of the retina called a fovea–this structure is absent on all mammals except man and other primates. The avian fovea is much more highly developed than our own, with extremely fine, tightly packed cones, each connected by its own ganglion to the optic nerve.

To discover whether this complicated eye actually sees better than a human eye, scientists train captive birds to peck at particular colors or shapes. Once the bird learns what it’s supposed to be pecking at, the image can be made smaller and smaller, or lighter and lighter, in order to discover the limits to avian vision. These studies only work on birds that adapt well to captivity and training, but they prove that birds like crows and falcons have far greater visual acuity than man’s. Who says only humans can be visionaries?

(recording of a Peregrine Falcon)

This is Laura Erickson, and this program has been “For the Birds.”