Does biofluorescence enhance visual signals in birds-of-paradise?

Visual signals are important for mediating numerous behaviours in organisms. Frequently, brightly coloured feathers are used in signalling during reproductive behaviours in birds. Furthermore, an increasing number of studies have documented the additional use of biofluorescence as a visual cue. We investigate the presence of fluorescence in all 45 species of birds-of-paradise (Paradisaeidae), a group where males exhibit elaborate feather morphology, coloration, and mating displays. We show that all core birds-of-paradise are biofluorescent (37 species representing 14 of 17 genera); all genera except Lycocorax, Manucodia and Phonygammus, which comprise the sister group to the core birds-of-paradise. In males, biofluorescence occurs on plumage and skin used in reproductive displays. Biofluorescent regions vary among species but include the inner mouth and bill, as well as feathers on the head, neck, belly and plumes. In females, biofluorescence is usually restricted to plumage on the chest and belly. Emitted biofluorescent wavelengths are green and green-yellow, with emission peaks around 520 and 560 nm. Using an established framework of criteria for determining the functional role of biofluorescence in communication, our results provide evidence that within core birds-of-paradise, males likely utilize biofluorescence to enhance visual cues used during male hierarchy and mating displays.