Q: Why do so many birds have red eyes? I’ve noticed common loon, western grebe, Cooper’s hawk, and spotted towhees all have red eyes. I know of no mammal or reptile with red eyes. What is the evolutionary advantage of red eyes in birds?
A: Color qualifies as one component in the realm of biological information known as "nonverbal communication": the exchange of information between two or more individuals but without the use of sound. Size, scent, and motion are just three examples of many modes of communicating without sound.
An opossum can curl its lips to show big teeth that say, "Leave me alone!" A plant can release fragrance that says, "I want you to pollinate me!" A dog can wag its tail to say, "I am so glad to see you!" Likewise, colors make announcements and advertise intentions.
A good read about this color business is in Niko Tinbergen’s 1958 book, Curious Naturalists. Two points—time frame and experimentation—make the book worth reading. Much of what Tinbergen describes about learning specific attributes of insect and bird behavior occurred during the 1930s into the 1950s. This means we are but two generations out from understanding how color influences behavior, making our knowledge incredibly recent. Tinbergen also explains that making an assumption about behavior and then expressing it as fact is not acceptable as science. Only through carefully planned and executed experimentation can alternative explanations be eliminated to identify the one factual explanation.
A researcher can document the behavior of male red-winged blackbirds, then obscure the red shoulder patches and document the same behaviors on the same individual birds. The comparison provides direct cause-and-effect evidence for how a bright color influences success of specific behaviors. But how does one experimentally alter the color of a bird’s eyes without blinding or otherwise disabling it? This problem greatly complicates direct experimentation. Still, research has identified at least four ways eye colorserves birds as nonverbal communication:
• Species recognition, which allows individuals to find within a group or at distance other birds of their own kind for flocking and social interaction;
• Sex recognition, which allows individuals to assess other birds as either potential mates or potential competitors for mates;
• Age recognition, which allows individuals to distinguish other birds as either adult competitors or nonadults that are not competitors because they lack experience;
• Health recognition, which allows individuals to gauge fitness of a potential mate.
These things we know because eye color changes with age and may change when certain chemical pollutants accumulate through diet and thereby indicate health. Birds’ eyes and corresponding brain areas are exquisitely adapted to recognize color. Mammals, by contrast, are very limited in their ability to discern colors. Ergo, birds are colored various greens, yellows, blues, reds, and oranges whereas mammals are limited to black, white, gray, and various hues of brown.
Primates—humans included—see colors very well; and not coincidentally, a few monkeys have brightly colored eyes. Reptiles’ eyes vary: Some crocodilians have golden yellow eyes; some geckos and a few snakes have bright red, orange, or even green eyes.
From all this, understand that eye color confers information that birds subsequently use to govern their behaviors, and that we are not yet done learning about this topic.
Taken from Bird Watcher’s Digest