IMPRINT, the Bell Museum's quarterly magazine for members, offers stories of scientific adventure and discovery, insight into today's rapid environmental changes, updates on museum programs and exhibits, and fun activities for kids. IMPRINT is published quarterly and is available as a benefit of Bell Museum membership.

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by Jennifer Amie

From the hummingbird's ruby throat to the oriole's black head, we rely on appearances to identify bird species and distinguish males from females. In fact, our observations of plumage patterns and color have for more than a century formed the basis of many theories of bird evolution and behavior. As it turns out, we've been looking through a flawed lens.

When human beings look at a pair of Parus caeruleus (European blue tits—see cover photo), we see a male and female that look alike, each with an identical blue patch on top of its head. To another bird, however, the male in this pair is quite distinct from his mate. His "blue" patch is another color entirely—an ultraviolet-enhanced blue that is not visible to the human eye. His feathers reflect light in the ultraviolet range, at a frequency just outside the spectrum of colors the human eye can detect.

In the mid-1980s, scientists first discovered that birds can see what humans can't. To humans, the rainbow of visible colors spans the range from wavelengths of 400 nanometers (violet) to 700 nanometers (red). In between are the familiar purples, blues, greens, yellows, and oranges.

Birds, on the other hand, also perceive colors below the 400 nm wavelength, in the ultraviolet range between 340 nm and 400 nm. This slight extension of the spectrum of visible color results in a markedly different perception of the world.

"For every color that a human sees, a bird sees many, many more," says Bell Museum director and ornithologist Scott Lanyon.

The recent discovery that birds have access to a broader range of sensory information points out the limitations of our own powers of perception.

Imagine that you are a birdwatcher walking through the woods and trying to count birds—but you're wearing a Walkman turned up to high volume. How many birds will you find? Now imagine that your headphones have been removed. How many birds will you find? Additional sensory information, such as the ability to hear bird calls, would dramatically change your perception of the environment.

Likewise, says Lanyon, birds' perception of the world is dramatically affected by their ability to see in the ultraviolet range.

"In the two centuries since Linnaeus got us started classifying birds, we've been colorblind," says Lanyon. "We've looked at these species through one pair of glasses, and our prescription was all wrong."

Birds' remarkable range of vision may affect all behaviors and adaptations related to sight, causing scientists to reevaluate longstanding theories on how birds find food, avoid predators, migrate, choose mates, and find nesting places.

Of particular interest is the long-held theory of sexual selection, first proposed by Charles Darwin to explain why, in some species, males are more brightly colored than females. Such species are called dichromatic.

In most dichromatic species, females are choosy about their partners. Scientists believe that they prefer males with bright colors or extravagant feathers.

Scientists have generated many hypotheses to explain this process of sexual selection:

• The effect of parasites is more visible in brightly colored feathers, enabling females to evaluate at a glance the health of a potential mate.
• Males' bright plumage signals other males to keep away, helping to defend territorial boundaries.
• Bright colors make male birds more visible, and therefore more vulnerable. Males that survive despite this "handicap" have demonstrated their strength and worth as mates.

Underlying all these theories has been the assumption that birds see the world the way we do.

"Suddenly, we realize that birds look really different to each other," says Lanyon. This discovery calls into question all of our existing ideas about dichromatic species. Scientists now think it is possible that UV coloration is an important component in plumage displays of dichromatic birds. Another, even more intriguing, possibility is that many more birds are dichromatic than previously thought. In many species of birds, males and females are monochromatic—that is, they look alike to the human eye. What if such birds, in fact, have UV color differences that humans cannot see?

What does this mean for existing theories about birds? Eaton is addressing this question by correlating his data about UV reflection in various bird species with information about their habitats, flock size, nesting habits, and locations.

"I want to see if UV coloration has evolved differently in different habitats," says Eaton. If a bird lives on the forest floor, for example, the sunlight reaching the bird is filtered through a canopy of trees, creating a distinct visual environment. Quite different light conditions exist in open country. Eaton hopes to discover whether such habitat differences affected the evolution of UV coloration in birds' plumage.

Social structures may also be a factor in the development of UV coloration. For example, UV plumage may be important to flocking birds, who use visual signaling as a critical means of communication.

Last but not least, Eaton is attempting to discover whether birds that appear to humans to be monochromatic are dichromatic to each other. He has selected 240 species of birds in which males and females appear to humans to have the same coloration. Using spectrometer readings, he will discover whether some of these birds have color differences invisible to the human eye.

New discoveries about birds' visual capabilities are, in one sense, a cautionary tale, reminding scientists that the human perspective is but one among many. Though we take our own point of view for granted, it may not always reveal the many complexities of the natural world.