Abstract
Unlike humans, birds perceive ultraviolet (UV) light (320 to 400 nm), a waveband which is known to play a role in avian mate choice. However, less attention has been paid to the role of UV light in avian foraging. Some blue, violet and black berries reflect UV light. The colour of berries might be an effective advertisement for avian seed dispersers and indicate the stage of fruit ripeness. We conducted an experiment to test how the UV reflection of berries affects birds' foraging. Redwings were allowed to choose between UV-reflecting bilberries and rubbed bilberries (UV reduced) in the presence and absence of UV light. We used wild-caught adult and hand-raised juvenile birds to assess possible differences between experienced and naive birds. We found that adult redwings preferred UV-reflecting berries when UV illumination was present, but when UV illumination was absent, they did not distinguish between the two berry types. Our study therefore shows, for the first time, that UV wavelengths are used when birds feed on fruit. However, naive birds showed no preferences, suggesting that age and/or learning may affect frugivore preference for UV reflectance.