Male Red-winged Blackbirds (Agelaius phoeniceus) respond with greater intensity to epaulets with UV reflectance

Male Red-winged Blackbirds (Agelaius phoeniceus) have red epaulets that are used as badges for intrasexual competition for territory acquisition and defense. Red epaulets reflect very little across ultraviolet (UV) wavelengths (300–400 nm) yet most birds, including Red-winged Blackbirds, are visually sensitive to UV light. Here, we conducted a field experiment to test if adding UV reflectance (UV+) to epaulets changed the effectiveness of signals to receivers relative to control epaulets. We constructed blackbird models with an epaulet made from red felt that reflected both long-wavelength (i.e., red) and UV wavelengths (UV+). To half of the models we applied sunscreen over the epaulets to block UV reflectance (UV–) and therefore mimic reflectance spectra of natural epaulets. UV+ and UV– models, along with small playback recorders playing oak-a-lee calls, were presented in established territories of male Red-winged Blackbirds. UV+ models received significantly more song flight displays (2017) and song spread displays (2019) from territorial males. Two territorial males physically attacked UV+ models but none attacked UV– models. To our knowledge, this is the first field experiment to add UV color to a plumage signal, resulting in a behavioral response from wild birds, raising questions for future research to explore a potential pre-existing visual bias for UV coloration in Red-winged Blackbirds and potential phylogenetic constraints on expression of UV reflectance of epaulets.