The evolution of carotenoid coloration in estrildid finches: a biochemical analysis

The estrildid finches (Aves: Passeriformes: Estrildidae) of Africa, Asia, and Australia have been the focus of several recent tests of sexual selection theory. Many estrildids display bright red, orange, or yellow colors in the beak or plumage, which typically are generated by the presence of carotenoid pigments. In this study, we used high-performance liquid chromatography to investigate the carotenoid content of feathers and other colorful tissues in seven species of estrildids. Star finches (Neochmia ruficauda) and diamond firetails (Stagonopleura guttata) circulated two main dietary carotenoids (lutein and zeaxanthin) through the blood and liver and used both to acquire a yellow plumage color. However, five other estrildids (common waxbill, Estrilda astrild; black-rumped waxbill, Estrilda troglodytes; zebra waxbill, Amandava subflava; red avadavat, Amandava amandava; and zebra finch, Taeniopygia guttata) circulated these same dietary carotenoids along with two metabolites (dehydrolutein and anhydrolutein) through the blood and/or liver and used all four as yellow plumage colorants. We subsequently tracked the distribution of these pigments using a published phylogeny of estrildid finches to determine the evolutionary pattern of carotenoid metabolism in these birds. We found that finches from the most ancient tribe of estrildids (Estrildini) possessed the ability to metabolize dietary carotenoids. Although carotenoids from the most ancestral extant estrildid species have yet to be analyzed, we hypothesize (based on their relationships with other songbirds known to have such metabolic capabilities) that these finches inherited from their ancestors the capability to metabolize carotenoids. Interestingly, later in estrildid evolution, certain taxa lost the ability to metabolize dietary carotenoids (e.g., in the Poephilini), suggesting that the occurrence of carotenoid metabolism can be labile and is likely shaped by the relative costs and benefits of color signaling across different species.