Structurally based plumage coloration is an honest signal of quality in male blue grosbeaks

We investigated the signaling function of blue plumage in male blue grosbeaks (Guiraca caerulea) to determine if structurally based coloration may act as a reliable signal of quality to conspecifics. Blue plumage results from the microstructure of feather barbules rather than from pigment granules, and thus it is possible that structurally based plumage ornaments may function differently from sexually selected ornamental coloration that is pigment based. The plumage of male blue grosbeaks reflects maximally in the blue-ultraviolet range, so most variation in plumage coloration among males is invisible to human observers. In previous research, we showed that increased area of blue plumage on the body is associated with a shift in the wavelength of maximum feather reflectance toward the ultraviolet and with high intensity of light reflected at that maximum, and that extreme expression of the male ornament is condition dependent. These observations suggest that blue plumage may be an honest advertisement of male quality. We tested this hypothesis in a wild population of blue grosbeaks. We quantified male quality in three broad categories. (1) Physical condition was assessed from subcutaneous fat deposits, ectoparasite load, and body size. (2) Territory quality was assessed from territory area, prey abundance, and predation risk. (3) Paternal investment was assessed from male feeding rate. We found that the bluest males have the largest body size, maintain the largest territories with the greatest prey abundance, and feed nestlings in the first nest of the season at the highest rates. We conclude that structurally based plumage coloration functions as an honest, intraspecific signal of quality.