Fluorescent pigments appear to glow because they absorb and reemit ultraviolet light at longer wavelengths. Such pigments decorate the crown and cheek feathers of budgerigar birds, commonly known as budgies. (In the image at the right, short-wavelength illumination reveals the budgies' fluorescent markings.) But whether the fluorescence serves a specific purpose or is merely a by-product of the birds' brilliant coloring has remained somewhat of a mystery. To answer that question, Kathryn E. Arnold of the University of Glasgow and her colleagues devised a clever experiment. They gave budgies of both sexes their choice of two birds of the opposite sex, one of which retained its fluorescent plumage and the other of which had its glow snuffed with sunblock. Both males and females, the researchers found, showed a strong sexual preference for the fluorescent birds.
The team also considered the bird's visual apparatus and determined that the fluorescent yellow feathers are ideally placed for chromatic detection by another lovelorn budgie. "These findings show that the fluorescent plumage of parrots is an adapted sexual signal, rather than a by-product of plumage pigmentation," the investigators conclude. "Given the elaborate biochemical pathway by which fluorescent pigments are produced, they may be costly and thereby honest indicators of individual quality."