More to Bird Coloration than Meets the Eye

Undergraduate Nicole Spooner reports on a new study showing why songbirds appear

For many small songbirds, beauty truly is in the eye of the beholder: although these birds may appear colorful and flashy to their intended mates, potential predators often view them in a completely different light. Olle Hastad from Uppsala University in Sweden and colleagues discovered that songbirds and the birds that prey on them have different types of visual receptors. Some songbirds have capitalized on these differences by sporting plumage that appears brilliant to their mates but not to passing predators (Proceedings of the National Academy of Sciences, May 2005).

Many male songbirds, including thrushes, sparrows, and warblers, have bright plumage that reflects ultraviolet light, making them more attractive to potential mates. However, beauty has its price in the animal kingdom--males that stand out among their competitors risk catching the eye of predators as well. How can they show off their vibrant attire without greatly increasing their chances of becoming someone's lunch? Songbirds have apparently exploited colors visible to one another but not to their predators, taking advantage of what researchers call a "private communication channel." Previous studies have shown, for example, that some mammalian predators are blind to ultraviolet light.

Now the evidence from Hastad and colleagues shows that the same phenomenon occurs among different types of birds. Small songbirds have vision that is most sensitive to ultraviolet light, whereas hawks and nest-raiding crows and ravens see better in the violet region. Male songbirds take advantage of this difference in vision by sporting plumage that is distinctive to females but that helps them blend into the background when viewed by a predator.

This study shows that the importance of perception systems such as vision should not be underestimated when looking at animal behavior. In this case, when we observe the brilliant red breast of a robin or the golden coloration of an oriole, there is much more at work than simply what meets the eye.

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu