Eyes

The daunting gaze of a female golden eagle in Shell Ridge
Don Jedlovec © 2002

Have you ever noticed how eyes are set onto animals in different ways? The chicken sees out from only one side of its head with each eye and therefore has to turn its head once in a while so it can see other things, including what's in front of it. Predators, however, have both eyes in the front of their head so they can see their prey more easily. This is called binocular vision and it has two advantages – the animal can track its prey in front of it, and it can tell how far away the prey is. The visual acuity of hawks is regarded as being some twenty times better than human vision. To test what bird vision is like, stand on top of Shell Ridge and watch the White-Throated Swifts zoom by at over thirty miles per hour. They are not randomly flying around gobbling up tiny insects that happen to be in their way – they can actually see them approaching on the wind and these birds adjust their flight pattern to intercept them.

Owls have larger eyes for their heads than most other predators since they require more light-gathering power for their crepuscular (dawn and dusk) and nocturnal (night time) activities. Hawks are diurnal (day time) and soar along looking for movement. They can distinguish such motions where everything around the object stays still. Many birds have pigmentation in their eyes that limits the range of colors they can see. This permits then to focus more sharply because the eye doesn’t have to accommodate for chromatic aberration, as does the eye of man. Other birds, such as the black-shouldered kite, hover above an area and then speed down to their prey. Peregrine falcons have been clocked at over 120 miles an hour as they dive – all as a result of their being able to see something that we cannot. During this dive, they never lose sight of their prey. Try this sometime!

Non-binocular vision

Many animals that are preyed upon do not have binocular vision, but all predators do.

Humans, too, have binocular vision.

Jerry Fritzke
June 21, 2003