UNUSUAL CAROTENOID USE BY THE WESTERN TANAGER (PIRANGA-LUDOVICIANA) AND ITS EVOLUTIONARY IMPLICATIONS

I used spectrophotometric, chromatographic, and chemical means to establish that rhodoxanthin, a 3-keto-retrodehydro carotenoid, was the only red pigment in the head feathers of the Western Tanager (Piranga ludoviciana). In contrast, the red head and body feathers of a close relative, the Scarlet Tanager (P. olivacea), exhibited several 4-keto-carotenoids. Other tanagers and emberizids also displayed 4-keto-carotenoids. The deposition of presumed canary xanthophylls and phaeomelanins differed quantitatively between the Western Tanager and other tanagers belonging to the genus Piranga. Uniquely among the Piranga spp. examined, the head feathers of the Western Tanager had flattened bards without barbules. Partly because the head colors of the Western and Scarlet tanagers were indistinguishable either in the hand or when examined by reflectance spectrophotometry, I excluded selection for a variant color as the basis for the observed chemical and morphological differences. Biochemical costs, including putative costs associated with the endogenous production of 4-keto-carotenoids, could have led to the conversion in the Western Tanager to an available dietary pigment. This tanager, unlike the other species studied, has access to an abundant source of rhodoxanthin in the coniferous forests of western North America. The pigment changes in the Western Tanager could have taken place with minimal effect on head color.