The ability of birds to perceive ultraviolet light (300-400 nm), to which humans are blind, has been neglected in avian behavioural ecology, and the occurrence and significance of ultraviolet (UV) colouration in plumage signals are important to explore. The support for a role of UV in avian communication comes mainly from demonstrations of UV-components in avian plumage reflectance, but most of these might be nonadaptive secondary effects of colouring mechanisms operating in longer wavelengths. Only colouration principally in the UV strongly suggests signalling in this spectral region. Here is presented the first example of nearly pure UV colouration in passerines, the Asian whistling-thrushes (Myiophonus spp.). As in a previously investigated parrot species, the barb-born UV-structure seems to be a modification of the taxonomically widespread `spongy structure' producing blue and violet feather colours. Lacking an obvious morphological constraint, the rarity of pure UV-signals is more likely to be explained by ambient light conditions and ecology. In the whistling-thrushes, a combination of high altitude, skylight-dominated habitat, mammalian predators, and semi-aquatic lifestyle might explain the evolution of UV-communication in this genus.