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.
Humans have three types of photoreceptors, or cones, in the retina, defined by the color of the light they absorb: red, green, and blue. 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 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.
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