Reflections in Nature
Hunting turkeys, especially in the spring, can be quite challenging. Through the years, I have called gobblers in to my location but not close enough for me to see the birds. Although I was always dressed in camouflage from head to toe, with my back against a large tree, the turkeys somehow detected my presence. The birds would stop gobbling and then melt back into the woods.
The visual ability of most species of birds allows them to forage, avoid predators and adjust for the rapid change of aerial movement all at once. A bird’s eyesight is its most critical sense and the one relied on most for survival.
Birds are able to see two to three times better than we humans. Of course, there are some birds, such as hawks and owls, that have better eye sight than other birds.
Birds that are active during the day, such as robins, cardinals, goldfinches and wrens, have densely packed retinas, with cones that are distributed in a uniform fashion across the retina. The back of a bird’s eye is flatter than that of most mammals, and this combined with the cones allow the birds to have a wider in-focus area.
Add to this the pigmented oil droplets that enhance cone color in diurnal birds. Scientists do not fully understand the function of these oil droplets; however, they appear to sharpen color perception and reduce glare.
Hawks have front facing eyes that make it possible for binocular vision, which allows stereoscopic vision that enables the accurate determination of distance that is very important to the hawks. Birds and animals that have eyes on the front of their heads (binocular vision) are predators.
Birds and animals (rodents, rabbits, etc.) that have eyes on the side of their heads are usually prey species. Although they do not have binocular vision, these birds and animals have periscopy vision (wide field of view), which almost allows them to see what is occurring directly behind them.
Compare this to nocturnal birds, such as owls, that have retinas with only a few cones; however, these retinas contain a large number of rods, which are useful in dim light for night hunting. Nocturnal birds have 100 times the light gathering power of humans’ eyes. Even the smallest owls have eyes as large as a man’s.
During the day, owls draw an unique third transparent eyelid over their eyes, acting as a window shade to keep out the sun. However, owls can see just as good during the day as they can at night. Since they do not have good periscopy vision, these nocturnal birds must turn their heads to see what is occurring to their sides. They have the ability to turn their heads almost 360 degrees.
Hummingbirds, which rely on seeing long distances, have a temporal fovea (a small hollow on the eyes) believed to improve binocular vision. Hummingbirds locate their food by eyesight. It is a well known fact that hummingbirds are attracted to red, orange and purple flowers. Flowers of these colors, especially the red, are good providers of nectar. Since most of good nectar sources are red, hummingbirds can tell, while in mid-flight, which one suits their feeding habits. Knowing which flowers provide a good food source saves the hummingbird precious energy that otherwise would be spent searching for a food source.
Nectar is vital to the process for flowers to become fertilized. The birds and insects fertilize the flowers with pollen picked up when sipping nectar from the flowers. The hummingbird ranks among the most important of the bird pollinators throughout North America. It is no accident that many of the plants that hummingbirds depend on for food are interdependent on hummingbirds as pollinators.
For instance, the fuchsias are a type of plant that has adapted specifically for hummingbirds instead of insect pollination. However, hummingbirds do eat a small amount of tiny insects and spiders as well as nectar.
Hummingbirds not only use colors to locate food, the structure and color of their feathers are used in finding a mate. We’ve all seen the hummingbird’s vivid iridescent feathers, especially on the gorget (throat patch) of the males. Light refracting off the feathers changes as the bird is viewed from different angles, often comparing to the iridescent oil on water.
How important is vision to birds? If a bird was blind-folded it could neither take off nor fly properly because it would be unable to orient itself.
Bower retired after 34 years as a wildlife conservation officer for the state Game Commission. He has published several books about his experiences. Questions and comments may be sent to him at 153 Redington Ave., Troy PA 16947.