Ultraviolet reflection of berries attracts foraging birds. A laboratory study with redwings (Turdus iliacus) and bilberries (Vaccinium myrtillus)

Unlike humans, birds perceive ultraviolet (UV) light (320-400 nm), a waveba nd which is known to play a role in avian mate choice. However, less attent ion has been paid to the role of UV light in avian foraging. Some blue, vio let and black berries reflect UV light. The colour of berries might be an e ffective 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 U V-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 illumi nation was present, but when UV illumination was absent, they did not disti nguish between the two berry types. Our study therefore shows, for the firs t time, that UV wavelengths are used when birds feed on fruit. However, nai ve birds showed no preferences, suggesting that age and/or learning may aff ect frugivore preference for UV reflectance.