Eagle eyes

During the thirty-five years we have lived in Portland, Oregon, sightings of bald eagles have become much more common.  The species is thriving.  It is still a thrill to see one soaring over our house or sitting in a tree.  They are beautiful birds. A couple of years ago I had the good fortune to be sitting in our backyard with my camera and a telephoto lens at hand when a juvenile bald eagle landed on the wall about twenty feet from me. (The white head and tail feathers don’t come in until age four.)

As he turned to face me, I noticed that his pupils were unequal in diameter. The pupil on the sunny side of his face was smaller than the pupil in the shade. 

In humans and most other mammals, the pupils constrict or dilate together with changes in light.  This is called the consensual pupillary response, and it is a test used by neurologists checking for certain problems in the brain. My curiosity was piqued, so I did some research on the consensual pupillary response in birds.  It turns out that in general it is only partial or sometimes absent altogether in birds. This magnificent young eagle taught me a neurological lesson I won’t forget.