Structural Plumage Coloration, Male Body Size, and Condition in the Blue-Black Grassquit

In birds, the colors ultraviolet, blue, and green originate from feather microstructure rather than pigmentation.Structural plumage coloration may be maintained by sexual selection, yet there exists only limited evidence that structural colors can honestly signal individual quality.In this study, I found considerable individual variation in the blueblack to ultraviolet coloration of the wing coverts and rump of male Blue-black Grassquits (Volatinia jacarina), thereby revealing the potential for sexual selection on structural plumage coloration in this species.I used a principal components analysis (PCA) of four color characteristics to combine reflectance data into a single color score.Birds with high color scores have brighter, more intensely colored, and more saturated blue-black feathers.There was a significant positive relationship between color scores for both body regions and male condition.There was also a positive trend between the color score of the rump region and male body size.These results support recent findings that structural plumage coloration is condition-dependent, and this type of coloration may be an honest signal of male quality in the Blue-black Grassquit.