Parrots will fluoresce for sex. (Biology).

A budgerigar's head literally glows for its mate, and both males and females of this parrot species prefer to court radiant partners.

Only birds in the parrot family have feathers that fluoresce, explains Kathryn E. Arnold of the University of Glasgow in Scotland. For example, the crest of a cockatoo absorbs ultraviolet (UV) light and reemits it at longer wavelengths that birds and people can see. Arnold ran across old references to the phenomenon and confirmed it by examining under a black light some 700 skins of Australian parrots from museum collections. "All the areas that fluoresced were display feathers that were waggled or fluffed up or showed off in courtship," she says.

To see if fluorescence really did matter in romance, Arnold and her colleagues studied budgerigars with the coloring they have in the wild. The researchers offered birds a choice of two companions, one smeared with petroleum jelly carrying a UV-blocking agent and the other with plain petroleum jelly.

Both males and females preferred unsmeared birds of the opposite sex, Arnold and her colleagues report in the Jan. 4 SCIENCE. Yet when Arnold offered birds same-sex companions, they no longer based their choice on fluorescence. --S.M.