| How Does My Bird See? A Bird's Eye View |
| To survive, to eat, to mate, to live, a bird relies heavily on its sense of vision. Because sight is so important to them, birds have highly developed eyes, even more so than humans in many aspects. Birds see in color, which is important in recognizing food and danger and is possibly used in mating rituals. Birds also see with enormous accuracy and at long distances. Some have very advanced depth perception and motion detection capabilities, while others see well at night. A bird's eyes also are much larger in comparison to the size of its body than our human eyes are when compared to our body size. The position of a bird's eyes on its head varies by type of bird. Birds of prey have wider heads with the eyes set apart and facing forward, similar to humans. This gives them binocular vision, or depth perception, and enables them to judge the speed and distances of prey and other objects. Birds such as parrots, which are prey for other animals, usually have eyes set on the side of the head. This gives the bird better peripheral vision and enables it to detect danger from a wider field of vision. Birds can move their eyes a bit, to extend their range of vision, but do not have the extensive ability of some reptiles to rotate the eyes. The color of a bird�s eyes depends on the species. All of the same species will normally have the same color of eye. The only exception is when an albino bird occurs. This bird will not have the normal color eye of its species but will have a white pink colored eye. Some species of baby birds have a distinctive color change of the iris (the colored portion of the eyeball surrounding the pupil) that occurs as they mature, including macaws, African greys, and most poicephalus. Until recently, scientists thought that what most birds see is similar to what we see because the physical structure of a bird's eye is fairly similar to humans. But there are some very important differences. Most birds' have eyes that are flatter than human eyes. This flatness allows birds to have a larger area in focus at one time while we focus one smaller area at a time. There are also �foveae� in all eyes, which are zones of maximum visual sharpness. People have one fovea. Some types of birds, such as hawks and eagles, have two foveae, allowing them to have a type of double vision. Other differences exist in the color vision of humans and birds. For instance, cells called �cones� are present in all eyes. Cones allow the brain to perceive colors. Scientists have learned that while a human may have only 10,000 cones per square millimeter, some birds may have up to 120,000 per square millimeter! Also, aside from the number of cones, in the past few years scientists have discovered two even more significant ways that a bird�s color vision differs from that of humans. First, all bird species that have been studied have at least four types of cone, while humans have only three types of cone. This means birds have four-dimensional, not three-dimensional, color vision! To explain a bit further: Our human color vision comes from three different cone types in our retina, each producing a different sensation of color which we have named "red", "green" and "blue" respectively. All hues that our eyes can see are produced by mixing red, green and blue light. Therefore, we say humans have three-dimensional color vision because our eyes have three cone types. Animals with only two cone types, such as most mammals other than old-world primates, have two-dimensional color vision, which is hard for us to imagine. But it is even harder for us to imagine what color vision is like for creatures like birds with more than three cones in their retina. Think about this, for example: If you were red/green "color-blind" and you came up with conclusions about color in the natural world, would anyone believe you? Probably not! But that is what we humans are doing every time we think we are seeing the color world in the same way as non-human animals. Secondly, birds can see very well in the ultraviolet range light. The notion that plumage may be even more colorful to birds, or even colorful in different ways, has not been widely considered until recently, even though we readily accept that a dog's sense of smell is far richer than our own. Because their ultraviolet vision increases the range of wavelengths over which they can see, a bird�s color perception probably cannot be translated into human experience. These colors that birds can see are not simply variations of the hues that humans see - these are colors totally unknown to the human eye. Science has realized that color is not an inherent property of the object being viewed; it is a property of the nervous system of the animal perceiving the light. Many fruits, flowers, and seeds contrast with their background much more strongly in ultraviolet light than they do in the human-visible light range. Even more interesting, so do many species of birds' plumage. This fact is of particular interest to researchers studying birds� sexual selection and mate choice. Let�s look at sexual dimorphism, which in birds means color differences between the sexes. Male and female blue tits look similar to us, but there is a significant difference in the coloring of the sexes when seen in the ultraviolet range of light, particularly in the 'blue' crest. Also, male and female starlings look similar to us, yet several regions of their plumage are dimorphic when seen in the ultraviolet. Experiments have shown that the ultraviolet aspect of plumage colors is very important in mate selection for species such as starlings and zebra finches. So � a male and female sun conure, for example, may look like identical twins to our human eyes, but because of their ultraviolet vision range, those conures can probably see sexually dimorphic color differences in each other. So what does all this mean to pet bird lovers when we consider these new facts we have learned about bird color vision? Maybe not much� But the next time your parrot flies shrieking off its perch for no apparent reason, instead of thinking he is flighty or silly, you might just wonder what he saw that you can�t see! And whenever you gaze lovingly into your parrot�s big, beautiful eyes, you may remember this article and marvel at the amazing capabilities of �a birds� eye view�. |