Friday, May 14, 2010

Eyes like a hawk - Friday's facts, fascinations and folklore

Several times this week we had a red-tailed hawk come perching on the vines near our house.  There is a nesting pair in the woods nearby but normally we don't see the hawks perching near the house.

He looked quite majestic and I loved his yellow feet ( I wonder why they are yellow?).  And he got me thinking about the saying "eyes like a hawk".

A red-tailed hawk's eyes are nearly as large as a human's, although the bird may weigh only three pounds.  Such size results in a relatively large image cast upon the retina.  The retinas have a dense concentration of cones and a great many optic nerve fibers. The area of critical focus, known as the fovea, in the back of a hawk's eye has 1,000,000 visual cells or cones per square millimeter, compared to 200,000 in a human eye. Even more impressive, the hawk's eye has two foveas in the retina where the highest concentration of visual cones occur, while we have but one. Thus raptors can see eight times better than man and have an exceptional ability to judge distance.

If you hold a finger 2 feet in front of your face., you can see it clearly because you are focusing the image on your one fovea.  Now hold the finger 2 feet to the left of your head and move your eyes but not your head to see it.  It is not in focus because the light from it is not hitting the fovea.

For a red-tailed hawk with mutliple fovea, most of what they see is in focus.  So human's will never have eyesight as good as hawks!

As an aside, other birds - like seabirds and shorebirds have a ribbon-like fovea across the back of the eye in order to orient themselves better to the horizon.

Guess we are lucky we don't have to catch little creatures and birds to sustain ourselves.....

Here's a red-tailed hawk's call - Warning: it's pretty strident so don't play this is you are in the office and should be working!

1 comment:

Audrey said...

What great pictures you were able to capture of him! Interesting article too.