Understanding the relative costs of different forms of ornamental displays is fundamental to research on the evolution of such traits. For traits such as elongated tails, assessing the degree of exaggeration is straight-forward: bigger is more exaggerated and more costly. In contrast, assignment of such a hierarchical ranking of costliness and exaggeration to color displays has not been possible. In this paper, I consider whether red carotenoid displays represent a more energetically costly form of ornamentation than yellow or orange carotenoid displays. Several lines of evidence support the idea that red carotenoid pigmentation is particularly costly. Red carotenoid pigments are less abundant than yellow pigments in the diet of virtually all vertebrates. Although many vertebrates can convert some yellow carotenes and xanthophylls to red pigments, all species appear to be restricted in the types of metabolic conversions of which they are capable as well as by the costs of such conversions. In virtually all avian taxa with carotenoid-based coloration, when females and juvenile males show plumage hue that is different than that of adult males, their plumage is less red (more orange or yellow). Finally, in a comparative study of cardueline finches, there was a significant positive relationship between the degree of sexual dichromatism and the redness of male plumage, suggesting that red is a particularly costly color display. The finding that red is a more costly color display than yellow or orange has important implications for comparative studies of the evolution of ornamental plumage.
