Fluorescent and ultraviolet sexual dichromatism in the blue-winged parrotlet

The presence of sexual differences in plumage coloration (sexual dichromatism) is frequent in birds. However, in many cases, humans cannot detect colour differences that are discernible to birds and it is therefore necessary to employ objective methods that contemplate the characteristics of the avian visual system for the study of plumage coloration. An understudied property of feather coloration is the occurrence of fluorescence, which has been described almost exclusively in parrots from the Eastern Hemisphere using non-objective methods and has been attributed to yellow pigments that are only present in psittacids. In this study, we explore fluorescence and sexual dichromatism through objective and quantitative methods in the plumage of a Neotropical species, the blue-winged parrotlet Forpus xanthopterygius. We measured plumage reflectance and fluorescence emission on museum skins using spectrophotometry and spectrofluorometry, respectively. The reflectance analysis revealed the presence of ultraviolet sexual dichromatism that adds to the differences in the visible range of wavelengths that are detectable by humans. The spectrofluorometric analysis showed that fluorescence is indeed present in this species, both in green plumage patches, where fluorescent pigments are presumably located, and in the blue rump of males, where colour is considered to be purely structurally based. The sexes differed in the intensity and wavelength of their fluorescence emission, representing the first finding of fluorescence sexual dichromatism in birds.