Widespread Cryptic Dichromatism and Ultraviolet Reflectance in the Largest Radiation of Neotropical Songbirds: Implications of Accounting for Avian Vision in the Study of Plumage Evolution

Avian coloration has played a central role in the study of sexual selection and other aspects of animal behavior. However, only recently have analyses of avian coloration been able to incorporate avian visual abilities. Although several studies have broadly sampled species for evidence of plumage coloration visible to birds but invisible to humans, few studies have quantified these data for all species in a single taxonomic group. We quantify ultraviolet (UV) plumage reflectance and document cryptic sexual dichromatism in the largest radiation of Neotropical songbirds, the cardinals and tanagers. Ultraviolet reflectance was common in the patches measured, with almost half of the species reflecting >20% of light in the UV range in at least one patch. High UV-reflecting patches, including 73 of the 91 patches that were found to be primarily UV colored, belonged to species in either Passerina or 2 of 13 major clades of tanagers. This indicates that high UV reflectance is not randomly distributed across the phytogeny. Sexual dichromatism was much more widespread in the group than previously thought. From a human visual perspective, about half the species in the group are sexually dichromatic; but from an avian visual perspective, 97% of species are dichromatic. We compared the implications of using human-perceived versus avian-perceived sexual dichromatism by mapping these traits onto tanager phytogenies. Quantifying dichromatism using an avian visual model provided a more accurate and detailed history of plumage coloration change across evolutionary history.