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Eyes

American Kestrel (Courtesy NEBRASKAland Magazine/NGPC)

If anyone ever calls you "Eagle Eyes," you can feel very flattered. Raptors such as eagles and hawks have marvelous vision. (Some hawks can see every mouse in a field while flying over on the hunt.)

Most all birds have better vision than humans and other mammal species. A buzzard's vision is six to eight times better than yours.

You can just look at a bird and know that vision is its key sense. You see it in the size of a bird's eyes – they are huge in proportion to a bird's body.

In a human, the head is about a tenth of the total body weight. This ratio holds true for many bird species.

But when you compare the size of the eyes to the size of the head, you find that in humans, eyes make up less than one percent of the weight of the head. In birds, eyes make up about 15 percent of the weight of the head. The eyes of owls are even larger. In fact, some owls have larger eyes than humans!

The placement of a bird's eyes can tell you whether it's a predator or prey species. For example, the Red-tailed Hawk's eyes are located on the front of its head, just like a human's. For this reason, the hawk is able to judge distances and focus on prey at a distance. Having both eyes on the front of the head gives a bird (and a human) binocular vision (the ability to see the same object with both eyes at the same time).

On the other hand, a songbird such as the Indigo Bunting has eyes on the sides of its head. This arrangement gives these birds very little binocular vision, but gives them better vision all around them, which allows them to watch for predators. They get the whole picture all at once. This type of sight is called monocular vision. A chicken gets along well with this type of vision because it doesn't need to see distances but it does need to see the ground in front of it to find food to peck.

Birds have moveable upper and lower eyelids, just like humans. But birds also have a "third eyelid" – a nictitating membrane which is located between the cornea (the membrane covering the eye surface) and the upper and lower eyelids. This thin, moveable membrane is used for protecting, lubricating, and cleaning the eye. The upper eyelid moves when a bird blinks. The lower eyelid closes when the bird is sleeping. The nictitating membrane opens and closes from left to right, while the upper and lower eyelids move up and down to open and close.

A bird's eyes are flat and fixed, fitting tightly into its skull, so a bird is unable to roll its eyes. To look around, it has to move its head. When you see a bird bob its head or cock its head to the side, it's getting more visual information.

Because of the inner structure of the eye, some birds of prey have to turn their heads upside down to get a good look at the sky above them.

More about a "bird's eye view":

• Diurnal birds (birds that go out in the day) can perceive more variations in color than you can, and it helps them find food as well as mates.
• Birds can focus their vision faster than we can.
• Some birds see better under different conditions – for example, penguins see better under water than on land.
• Some water birds have filters in their eyes that cut out blue light, helping them see their prey.