|Eyes may be even more important to birds
than to humans. One indication of this is that bird eyes are much larger, relative to
total face area, than human eyes. The common European Starling's eyes account for fifteen
percent of the mass of the bird's entire head, while human eyes amount to less than two
percent. I'll bet the Great Horned Owl's eyes in the picture at the right (photo by Dr.
Dan Sudia) account for even more than fifteen percent.
Of course, no one really knows what birds see, but we can make educated guesses after studying bird eye anatomy. For instance, the retina is a thin membrane at the back of the eyeball. It receives the image from the eye's lens and is connected to the brain by the optic nerve. Among the retina's receptor cells are cone cells, used for daylight vision. The denser that cone cells are, the sharper is the perceived image. The human eye has at most 200,000 cones per square millimeter, while House Sparrows have approximately twice that number. Hawks, who must spot small prey from the sky, possess about five times as many as humans! Songbirds and predators such as hawks are believed to have the sharpest vision among birds. They can see details at distances two to three times farther away than humans.
A bird's retina actually has three types of photoreceptors that "translate" light into nervous impulses:
The interesting thing here is that humans only have two types of photoreceptors, rods and cones. Thus birds may see more colors than humans. In fact they may be able to perceive ultraviolet or near-ultraviolet light, which humans cannot. Moreover, bird retinas, in contrast to human retinas, contain no blood vessels. This prevents shadows and light scattering, which cut down on human vision.
Some nocturnal birds such as owls and whippoorwills have a layer at the back of the eye called the tapetum lucidum that, like a mirror, reflects light back through the retina, making it more likely that light will strike sensory cells in the retina. As a result, birds with a tapetum lucidum see much better at night. The tapetum lucidum produces the "eyeshine" you see when you shine a flashlight into the eyes of a nocturnal bird or mammal, or take its picture with a flash.
It's been suggested that sharpness of vision may not even be the birds' main sight-advantage over humans. Rather, their advantage may lie in their brain's ability to capture at a glance a picture that a human eye would have to scan back and forth to see and understand. This is more a matter of brain circuitry than eye structure.
Bird vision isn't superior to human vision in every respect, however. The next time you see a bird taking a close look at something, notice how it turns its head sideways. This is because eyes in most bird species lie at the sides of their heads, and bird eyeballs can't be rolled like human ones. Therefore, when most birds look closely at something, they use only one eye at a time, and they must turn their heads. This means that most birds have little or no binocular vision, which makes judging distances difficult.
This often accounts for why many birds bob their heads. They look at something from below, then from above. If the object's perceived position changes a lot, it's close up, but if there's little or no change, it's far away. Owls, as the picture at the top of this page shows, are one kind of bird with eyes directed forward, and thus owls have good binocular vision, needed when pouncing on rodents in the night.