UV plumage color is an honest signal of quality in male budgerigars

Elaborate and colorful feathers are important traits in female mate choice in birds. Plumage coloration can result from pigments deposited in feathers such as carotenoids and melanins, or can be caused by nano-scale reflective tissues (structurally based coloration), usually producing ultraviolet (UV) coloration. Structural colorations remain the least studied of the three most important feather colorations. Previous studies have found a female preference for UV color in the budgerigar, Melopsittacus undulatus, but it is not clear what information this ornament conveys, nor what is the possible cost associated with its production. We investigated possible correlations between immune response and plumage color of wild-type (green) male budgerigars. In particular we measured the wing web swelling resulting from injection of phytohaemagglutinin (PHA). We did not detect any correlation between the sedimentation rate and morphological and color variables. We found that UV chroma is the best predictor for the cutaneous immune activity. Indeed, male budgerigars with high UV reflectance in the breast feathers showed stronger immune responses. These results are consistent with the idea that UV colors are special signals conveying information about a bird's condition.