What Makes Bird Vision So Cool
They use their right and left eyes differently. Some may sleep on the wing. They may even be able to see the earth's magnetic field. Welcome to the amazing world of bird vision.
While conducting research on common murres on Skomer Island off the tip of South Wales, I constructed hides at various colonies to be able to watch the birds' behavior at close range. One of my favorite hides was on the north side of the island, where, after an uncomfortable hands-and-knees crawl, I could sit within a few meters of a group of murres. There were about 20 pairs breeding on this particular cliff edge, some of them facing out to sea as they incubated their single egg. Being so close to the birds, I had the sense of being almost part of the colony and had become familiar with all their displays and calls.
On one occasion a murre that was incubating suddenly stood up and started to give the greeting call--even though its partner was absent. I was puzzled by this behavior, which seemed to be occurring completely out of context. I looked out to sea and visible, as little more than a dark blob, was a murre flying toward the colony. As I watched, the bird on the cliff continued to call and then, to my utter amazement, with a whirr of stalling wings, the incoming bird alighted beside it. The pair proceeded to greet each other with evident enthusiasm. I could hardly believe that the incubating bird had apparently seen--and recognized--its partner several hundred meters away out at sea.
Of all the avian senses, vision--and color vision in particular--is the area where the most spectacular discoveries have been made, mainly because this is where researchers have focused the most effort.
Compared with mammals, birds have relatively large eyes. In simple terms, a bigger eye means better vision, and excellent vision is essential for avoiding collisions in flight or for capturing fast-moving or camouflaged prey. Birds' eyes, however, are deceptive--they are bigger than they look. As William Harvey (famous for discovering the circulation of blood) said in the mid-1600s, birds' eyes "outwardly appear small, because excepting the pupils they are wholly covered with skin and feathers."
The size of eyes is important precisely because the larger the eye, the larger the image on the retina. Imagine watching a 12-inch television screen compared with a 36-inch screen. Bigger eyes have more light receptors in the same way that larger TV screens have more pixels, and hence a better image.
Among diurnal birds, those that become active soon after dawn have larger eyes than those that become active later after sunrise. Shorebirds that forage at night have relatively large eyes, as do owls and other nocturnal species. The kiwi, however, is an exception among nocturnal birds, and, like those fish and amphibians that live in the perpetual darkness of caves, seems to have virtually given up vision in favor of its other senses.
The Australian wedge-tailed eagle has enormous eyes, both in absolute terms and compared with most other birds, and as a result has the greatest visual acuity of any known animal. Other birds might benefit from the eagle's acute vision, but eyes are heavy, fluid-filled structures, and the larger they are the less compatible they are with flight. Compared with our eyes, those of birds are relatively immobile in their sockets (space and weight are limited, and the reduction of muscles needed to move the eyes constitutes an important saving), so raptors and owls in particular have to move their head when they are scrutinizing something. Flight, and the need for large eyes, may also be responsible for the loss in birds of teeth, which have been replaced by a powerful muscular stomach, the gizzard (used to grind up food), located near the center of gravity in the abdomen.
Birds are among the most colorful of animals, which is, of course, one reason we find them so appealing. One of the most brilliantly colored of South American birds is the Andean cock-of-the-rock. The male has the most intensely red body, a jet-black tail and outermost wing feathers, and unexpectedly silvery-white innermost wing feathers. So-named because it nests among rocks on cliff ledges and because of its cocky Mohican-like crest, this pigeon-sized bird is a major draw to birdwatchers visiting Ecuador and other South American countries. The males display in groups, referred to as "leks," deep in the cloudforest.
I was once watching some male cocks-of-the-rock from a viewing platform, and the birds were surprisingly difficult to see. The vegetation was dense, and although the males were actively chasing one another from tree to tree, they came into view only occasionally. I kept willing them to perch in the sun so that I could see them properly. Eventually when one did, it was stunning and put me in mind of a fleck of glowing volcanic lava amid a mass of green foliage.
The most memorable thing about my brief encounter was that despite the birds' brilliant color, as soon as they moved out of the sun they became almost invisible. It was like watching an actor step out of a spotlight into the darkness and disappear. This effect is no accident. Evolution has designed these birds so that when they're illuminated by the sun they appear utterly brilliant, but in the shade, with the light filtered through green forest vegetation, their plumage has an almost drab quality, rendering the bird surprisingly well camouflaged.