The Eyes Have It!

Photo Group
Red-tailed Hawk
Photo by: 
Richard Gooch

Did Hawkeye Pierce really have the eyesight of a hawk? We know that birds have great vision just by their ability not to run into things in flight, but does great vision come without cost?

While it doesn’t look like it from outward appearance, birds have large eyes. When you see a bird skeleton, you quickly notice how large the eyes really are. Couple that with the fact that eyes are filled with a heavy fluid, large eyes come at a high cost for an animal that flies.

Experts believe that the eyes are the primary reason birds had to sacrifice teeth. Teeth are very heavy and two heavy biological features in the same part of the body are not conducive with the light weight needed for flight. Teeth and the heavy muscles needed to operate them were replaced over time with a lightweight bill and a food-grinding organ known as a gizzard that is more centrally located.

Large eyes have a larger retina for a larger image. Larger eyes also have more rods and cones. Cones are necessary for seeing color. Human eyes have three color receptors while birds have four. We now know that the fourth receptor is for Ultra-violet. This led us to the realization a few years ago that birds like kestrels can actually see urine trails of voles and mice.

Another sacrifice bird’s make for these larger eyes is that they have little to no ability to move their eyes in their sockets. They have to move their heads to change their view. They also have little to no binocular vision, making focusing on objects more difficult.

How strong are their eyes? It really isn’t a question of strength. We now know that it is a question of clarity. Songbirds are known to have twice as many cones per square millimeter than humans, raptors have something like 5 times the number. While we would see a black speck in the sky, the hawk would see a clearly defined bird (a small bird, but clearly a bird).

So, the great baseball player Ted Williams was a pilot in WWII. I have heard that his vision was so good that he always flew the point in formation because he could see enemy planes coming long before anyone else could. Now he was a true “hawkeye”.

By Mark McKellar

SUMMER 2015