Fluorescent signaling in parrots.

Fluorescent pigments appear to glow because ultraviolet (UV) light is absorbed and reemitted at longer wavelengths. Humans use fluorescence as a highlighter, but it remains a mystery whether naturally occurring fluorescence functions as a signal or is a by-product of pigment structure. Here, we test for evidence of signaling using the fluorescent plumage of parrots (1). We performed mate choice experiments on wild-type budgerigars (Melopsittacus undulatus), in which both sexes have fluorescent yellow plumage on their crown and cheeks (Fig. 1, A and B) that is used in courtship displays. “Focal” individuals of each sex were given a choice between two “stimulus” birds of the opposite sex, one retaining fluorescent plumage on its crown (F treatment) and the other with