by Stan Tekiela
January 7, 2005
Photo by Stan Tekiela©
I am often asked how birds find food. Or another great question is, if I put up a new birdfeeder, how will the birds know it’s there or find it. The answer is simple—eyesight.
Eyesight is very important to birds. In fact, that might be understating the roll of vision in birds. It is darn right critical for survival. It is thought that the birds have the best vision among all animals. And not just some birds, nearly all birds have excellent eyesight.
This excellent vision comes from large eyes. A bird’s eyes are so large that in some species the weight of their eyes is equal or more than the weight of its brain. In many species the eye accounts for about 15 percent of the mass of the birds entire head. Human eyes by comparison account for less than 2 percent of the head and weight a fraction of the brain. The largest eyes of any land animal are those of the Ostrich which are nearly two inches in diameter. All of this space allows for more photoreceptors called "rods and cones" in the back of the eye. The human eye contains about 10,000 cones per square millimeter while many of our songbirds have up to 12 times this amount or 120,000 cones per square millimeter (650 million per square inch) which gives them the sharpest vision in the animal kingdom. The Golden Eagle for example exceeds the visual acuity of humans by two or three times allowing them to see movement of small prey from more then a mile away.
All of these extra receptors in a bird’s eye allow many species the ability to see in specific light frequencies, including ultraviolet that humans cannot see. Humans have three types of cones, each sensitive to different wavelengths of light or colors—red, green or blue. This is called trichromatic color vision. Birds have an extra cone for quadchromatic color vision (some have five cone types) that allows them to see the ultraviolet light frequencies. In addition bird eyes contain specialized oil droplets that act as filters, altering color sensitivity in the same manor as sunglasses. Human eyes don’t have these oil droplets.
Seeing in ultraviolet light helps birds in all sorts of ways. Many birds have feathers that reflect ultraviolet light. It is thought this is used to communicate a bird’s species, gender or perhaps its social standing. Seeing in ultraviolet light allows some birds of prey to locate their food by looking for visual clues left behind such as mouse urine, which reflects a bright yellow when seen in UV light. The bird just follows the trail of urine to the mouse.
Most birds can see well in low light conditions. We humans rely on photoreceptors called "rods" to help us see in low lighting. We have about 200,000 rods per square millimeter. Some birds such as owls have up to 1 million rods per square millimeter allowing them much greater vision in the dark. Rods don’t help in seeing colors but allow for amazing black and white vision after dark.
Nearly all of the ten thousand species of birds in the world have in addition to the familiar upper and lower eyelids, a third eyelid called a nictitating membrane. It moves from side to side across the eye at right angles to the regular eyelids. This third eyelid cleans the eye’s surface and keeps it moist. In many aquatic birds such as the Common Loon, the nictitating membrane has a special window-like area in the center that presumably lets the membrane act like swim goggles to improve their underwater vision.
The act of focusing on an object is termed accommodation. It is accomplished by muscles changing the curvature of the lens in your eyes. At the same time, the amount of light entering the eye is regulated by changing the size of the pupil. In birds both of these processes occur very quickly. Much faster than the human eye. This allows birds to quickly focus on objects near and far and to change from light to dark situations in the blink of an eye. A Peregrine Falcon diving (stooping) for example can keep focus on its prey even while traveling up to 200 mph and a songbird can see well enough ahead to avoid hitting tree branches while flying through a forest with pockets of light and dark.
As you can see (pun intended) most bird species see in the full range of color like humans and much more. Birds see the world very differently than humans but not because of their excellent eyesight but because of the placement of their eyes in the head, and that is what I will talk about next time. Until then…
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