Preferences for ultraviolet partners in the blue tit

The preference of female blue tits, Parus caeruleus, is correlated with the brightness of the ultraviolet (UV) crest; there is also assortative mating with respect to the crest's UV/violet chroma. However, manipulation of plu mage reflectance is necessary to infer a direct causal link between UV plum age and mate choice. We gave both male and female blue tits a choice betwee n a W-reflecting ('UV+') partner and a partner whose UV plumage reflectance had been removed ('UV-'). Male blue tits significantly preferred UV+ femal es. Similarly, female blue tits tended to prefer UV-reflecting males, but t heir UV+ preferences were nonsignificant. Neither sex showed a preference w hen conspecifics were replaced by a heterospecific. This study suggests mut ual mate choice but male choice may be more strongly influenced by the visu al appearance of potential mates. This is one of a few studies to show male mate preferences and the first demonstration of a direct relationship betw een UV reflectance and;male mate choice. (C) 1999 The Association for the S tudy of Animal Behaviour.