Two weeks ago I wrote a post entitled, “How do Birds get their Color?”. This got me thinking about eye color in birds. As humans we place exceptional value in the beauty of an individual’s eyes. Most of us will have heard the saying, “Eyes are the windows to the soul”. But besides the cosmetic value, the color of our eyes provides little other benefit to our lives. I have been wondering if it is the same with birds.
Eye color is even more varied in birds than it is in humans. For example, you don’t hear of people with red eyes. Unless they have conjunctivitis or have been smoking too much grass. Eye color in birds can vary from black to brown to red to orange to yellow to blue to green to white and many colors in between. The color of a bird’s eye, as in the color of a bird’s feather, can be caused by both pigments and refraction of light. In fact, many birds exhibit more pigment coloration in their eyes than humans (for the scientifically-minded these pigments are called pteridines, purines and carotenoids).
The actual benefits of eye color in birds, as in humans, appears to be mostly limited to the cosmetic, although its quite possible that certain eye colors are more sensitive to certain light conditions. In humans, many babies are born with blue eyes, only for their eye color to gradually change from blue to green or from blue to green and then to brown. Likewise in birds, eye color can be an indication of age. And this is often a great tool for identifying a young bird in the field. But a fundamental difference between eye color in birds and humans is that for quite a number of birds, eye color actually changes according to the breeding season. For example, the iris of the Brown Pelican becomes a stunning blue color during the breeding season. Its probably a good thing that this doesn’t happen in humans.
Changes in eye color that are dictated by age and by breeding season are likely to be under some form of hormonal control. Age-dictated color changes are found in a wide variety of birds including raptors, woodpeckers, grebes, thrushes, ducks, gulls, loons and vireos. In many birds that require long periods between juvenile and adult plumage there is a gradient of color change. For example, a first cycle Western Gull has dark brown, almost black, eyes. Some Western Gulls begin to show paler, lighter brown eyes in their second cycle. In their third cycle many individuals begin to display olive-green eyes. And by their fourth and adult cycles, most Western Gulls display the pale greenish-white eyes that they are known for (refer to Steve Howell and Jon Dunn’s book Gulls of the Americas for great info on gull ID).
Another example of a dramatic age-related color gradient change can be found in the bright yellow to red eye color changes in Sharp-shinned Hawks. Exactly what purposes these color changes serve is uncertain and it could be that, as in human babies, there is no benefit or purpose for these changes whatsoever. But, could it be that these eye color changes help certain bird species to establish the maturity of potential mates? Now there’s a thesis topic for a Cornell graduate student if it hasn’t been done already!
The most obvious difference between eye color in birds and humans is the fact that in certain species of birds, males and females have radically different eye colors. For example male Saddle-billed Storks have dark brown, almost black eyes, and females have bright yellow eyes.
A male Saddle-billed Stork has dark brown eyes
Similar examples exist amongst North American birds. Female Boat-tailed Grackles and Brewer’s Blackbirds have dark eyes whereas the males have bright pale yellowish/greenish-white eyes.
A brightly colored iris in a bird could signify various things. It could function as a “badge”, distinguishing between different age classes. It could advertise sex during the breeding season. It could even help to differentiate between the sexes year-round. The possibility exists that iris color is helpful to birds that live predominantly in certain light conditions. Or it could simply be that the coloration is caused by the chemical make-up of the eye, with little to know function at all. Whatever the reasons for eye color in birds, it makes for a fascinating macro-study of our favorite avian subjects.