The avian world is a kaleidoscope of colors. Every possible hue of color imaginable is found upon birds, and the study of this color and its evolutionary functionality is at the heart of one of the larger branches of ornithology. What makes birds colorful, why display such colors especially when they make you an obvious target for predators? These are the questions that researches such as Geoffry E. Hill of Auburn University have devoted their life’s work toward trying to understand.
One of the most puzzling aspects of bird coloration however is eye color. Like the plumage of the birds, eye color seems to come in nearly every possible color as well. Canary Yellow, Mandarin orange, scarlet red, coal black, you name it and you will find it as prominantly displayed in birds as the showy feathers they use to attract mates.
One thing we do know is how the eyes are colored, we are just not sure exactly why they are. Coloration in birds eyes is usually a result of both pigmentation and crystalline deposits of pigments. Dark browns and blacks are due to melanins – that same pigment that is now being studied for evolutionary advatages it gives carriers in thier fight against infections. Diving ducks and great blue herons exhibit yellow eyes which are produced by carotenoids. However, most yellow, red, and orange eyes are colored by pterin – a class of pigments that are completely unrelated to carotenoids. Then of course there is hemoglobin, that same stuff that makes our blood red, that is the source of the red eyes in the bronzed cowbirds.
Even though we can break down in labratories just what exactly causes the rainbow of eye coloration in birds, we are pretty much clueless as to why it occurs. One thing that we do know though, is that eye color tends to change with many species from juvenile to maturity and with the seasons. In these birds therefore, eye color is beleived to be attached to hormones and therefore serve the purpose of mate selection. Changes of eye color with age are found in a wide variety of avian families including the loons, grebes, ducks, hawks, pheasants, gulls, alcids, woodpeckers, mimic thrushes, vireos, and blackbirds. Species requiring more than a year to pass from juvenile to adult plumage (such as the Bald Eagle and Herring Gull) generally show a concurrent change in eye color. Some specific examples of age-related changes are Lesser Scaup and Northern Harrier (from brown to yellow), Sharp-shinned Hawk (bright yellow to red), Red-tailed Hawk (yellow to red-brown), American Crow (blue or blue-gray to brown), Dark-eyed junco (gray or gray-brown to red-brown), and Common Grackle (brown, turning paler with age).
Another fascinating aspect of color is that it would appear some species have even gone so far as to enhance the pupils of the eyes for signaling as well. Both the Black Oyster Catcher and the American Oyster Catcher exhibit a double pupil. This double pupil has been heavily studied in the Black Oyster Catcher – that species that associates with rocky shorelines as opposed to sandy coasts like the pie bald American. These studies have found that the presence of a double pupil can be used with an 80% success rate of determining the sex of the bird. Double pupils mean female. Experiments on American Oyster Catchers like this have either not been conducted, of the findings have yet to be published.
Eye color, like plumage color, is therefore most likely all about signally for a mate.
