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.
Several other lekking bird species choose their display sites with great care. The satin bowerbird of Australia selects sunny spots, but some birds-of-paradise in New Guinea and manakins in South America actually create their own sunny spot on the forest floor by pruning adjacent trees. It was once thought that this "gardening" was to minimize the risk of predation, but as our understanding of avian vision improved, it became clear that the birds were manipulating the background color to maximize the visual contrast of their plumage and the overall effectiveness of their sexual displays.
I was thrilled by the sight of male cocks-of-the-rock and their brilliant color in the sun, but I wondered whether a female would see them as I did. In fact, females see them even more brilliantly.
Humans have three types of photoreceptors, or cones, in the retina, defined by the color of the light they absorb: red, green, and blue. These are directly equivalent to the three color "channels" on a television or video camera, which in combination produce what we consider to be the full spectrum of color. Compared with many mammals, humans and primates have relatively good color vision, because most others--including dogs--have only two cone types, which must be like having only two color channels on a television. However good we (arrogantly) think our color vision is, compared with that of birds it is rather poor, because they have four single-cone types: red, green, blue, and ultraviolet (UV). Not only do birds have more types of cones, they have more of them. What's more, birds' cone cells contain a colored oil droplet, which may allow them to distinguish even more colors.
It is now known that many birds, probably most, have some degree of UV vision, which they use to find both food and partners. The berries that some feed on have a UV bloom, and European kestrels can track their vole prey from the UV reflecting off the voles' urine trails. The plumage (or parts of it) in hummingbirds, European starlings, American goldfinches, and blue grosbeaks reflects UV light, often more markedly in males than females. In certain species, like the blue grosbeak, the degree of UV reflectance may also reflect male quality, though females don't currently use this aspect of plumage to discriminate between potential partners.
the fact that birds use their right and left eyes for different tasks is one of the most extraordinary ornithological discoveries of recent times. As in humans, a bird's brain is divided into two hemispheres, right and left. Because of the way the nerves are arranged, the left half of the brain processes information from the right side of the body, and vice versa.
This bias in the role of each eye is difficult for us to imagine, but it may occur in all birds, albeit in different ways. Domestic fowl chicks, for example, use their left eye to approach their parent. Male black-winged stilts are more likely to direct courtship displays toward females seen with their left eye than with their right. When peregrine falcons are hunting they home in on their prey in a wide arc rather than in a straight line, and mainly use their right eye. New Caledonian crows, famous for their construction of tools--fashioning hooks from palm-like leaves--show a strong individual bias toward making tools from either the right or left side of leaves. Similarly, when actually using these tools to hook prey out of crevices, they show an individual preference for their left or right side, but no bias exists toward left or right in the population as a whole.
Given how widespread sidedness is, it is natural to assume that it has a function. And indeed it has. Intriguingly, the more biased the sidedness (at both the individual and species level), the more proficient those individuals are at particular tasks. It has long been known that parrots consistently prefer to use one foot to grasp food or other objects. The more biased parrots are toward using one particular foot (and it doesn't matter whether it is the left or the right), the better they are at solving tricky problems--like how to obtain a food reward dangling from the end of a string. The same thing is true of fowl chicks--those with strong sidedness are much better at foraging (discriminating between food grains and gravel) and keeping an eye open for predators in the sky.
Sleeping with one eye open is something we now know birds share with some marine mammals (which need to return to the surface to breathe), but certainly not with us. It is not even true of all birds, and so far it is known that songbirds, ducks, falcons, and gulls can sleep with one eye open. A complete survey has yet to be undertaken.
A bird sleeping with its right eye open is resting the right hemisphere of its brain, and there are two circumstances in which the ability to sleep with an eye open is incredibly useful. The first is when there is a predator about. Ducks, chickens, and gulls often sleep on the ground and are vulnerable to predators like foxes, so it pays to keep an eye open. A study of mallard ducks showed that individuals sleeping in the center of a group spent much less time with an eye open than those on the edge, and that ducks on the edge of the group were more likely to open the eye facing outward from the group in the direction from which a predator might approach.