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SEXUAL selection is one of the most actively studied areas of evolutionary biology^1–3, and ever since Darwin¹ birds have been probably the most popular taxon for testing the predictions about colour variation. Humans have been used to assess 'colour', an approach which may be flawed^4,5 as many birds see in the ultraviolet (to which humans are blind), and have at least four spectral classes of retinal cone cells (humans have only three). Here we report experiments on zebra finches which test the hypothesis that the ultraviolet waveband (300–400 nm) is used in avian mate-choice decisions. We found that the ultraviolet is used, and that it probably contributes to hue perception. This finding may have wide implications for future studies of avian sexual selection and colour, and supports one hypothesized function of avian ultraviolet vision, the role of which is largely unknown.^4,6,7