What Do They See?
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.
Look closely sometimes at the eyes of hawks or owls and notice how large they are in proportion to their head, much larger in proportion than human eyes.

The eyes of most birds are on the side of their heads. This placement allows them to be able to see the things on each side at the same time as well as in front of them. This type of vision is called monocular vision. Birds with eyes placed like this get a wide area of vision to be able to see danger as quickly as possible. For instance, Rock Doves (pigeons) can see 300 degrees without turning their head. Some birds, like Woodcocks, have their eyes placed far back on the sides of their heads, allowing them even to see danger behind them. However, with monocular vision, birds have a harder time judging distances and have worse depth perception.

The vision that occurs when the field of vision from each eye overlaps is called binocular vision. Birds, such as raptors, have their eyes placed far to the front giving them binocular vision as people have. These birds may have a 180 degree field of overall vision with much of that binocular. They have much sharper vision to the front than their monocular cousins. For instance, the Rock Doves may only have 30 degrees of binocular vision.

Owls, with wonderful binocular vision, make up for the lower field of vision by an amazing ability to turn their heads to face backwards. Also most birds' eyes do not move in their sockets as our eyes do. Reflex movements of their necks usually replace the eye movements.

The structure of a bird's eye is similar to humans. However, 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.

At the back of eye is an area called the retina. It is made up of several different types of cells. Rod cells are sensitive to small amounts of light. Nocturnal birds that spend the night hunting, such as owls, have many more rods than humans or than birds that find food during the day. There are almost a million rods per square millimeter in some owls while we have only around 200,000 per square millimeter. That certainly explains why owls can see so much better at night than we can.

Other reason nocturnal birds see better at night than humans is that these birds' pupils are larger. These huge pupils allow more light to come into the eyes than our smaller pupils do.

Cells called cones are also present in eyes. Cones allow the brain to perceive colors. A human may have 10,000 cones per square millimeter while some birds may have up to 120,000 per square millimeter! Nocturnal birds who have the extremely high concentration of rods will have very few cones - it is much more important to be able to see in low light and not very important to distinguish colors. On the other hand, birds who forage and feed in the daylight see colors very clearly from far off. It's been said that a hummingbird can spot red flowers from over a half mile away.

There are also foveae, which are zones of maximum visual sharpness. People have one fovea. Some types of birds, such as hawks and eagles, have two, allowing them to have a type of double vision.

The color of a birds' eye depend 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 have a white pink colored eye.

You should be aware that there is an eye disease spreading among many birds called House Finch Disease. Infected birds have swollen red eyes that may have a crusty area or be runny. In some cases the eyes are so infected that the bird is practically blind. Badly infected birds will usually die of starvation, not being able to find food.

Infected birds have been found in the United States east of the Mississippi River and into Canada as far north as Quebec. It is believed that there is no danger to people becoming infected, only other birds. If you see an infected bird, contact your local wildlife authorities.

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