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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 experimentally reduced fluorescence on its crown (F treatment)(2). We reduced fluorescent emission by applying sunblock to the crown. This …