The Condition-Dependent Development of Carotenoid-Based and Structural Plumage in Nestling Blue Tits: Males and Females Differ.

Despite the importance of proximate mechanisms for understanding costs and functions of signals, relatively little is known about physiological processes underlying colorful plumage production. We investigated yellow carotenoid-based breast plumage and ultraviolet (UV)/blue structural color of tail feathers in nestling blue tits. At peak plumage production and at fledging, we examined plasma concentration of protein, the substance of feathers, and levels of carotenoids, which are deposited in yellow plumage and also required for support of growth and immune development. Males showed more UV-shifted hue and higher chroma and UV chroma of the tail feathers. In males only, tail chroma and UV chroma were strongly but negatively related to circulating plasma protein at the period of peak feather production. Breast plumage was more chromatic in males, and yellow coloration was strongly condition dependent in males only. Production of more chromatic plumage appeared to deplete carotenoids from plasma, since male fledglings had less circulating carotenoids than females. This had no obvious consequences for these males, since coloration was unrelated to cell-mediated immune responses, humoral immune status, stress, and parasitism. Nonetheless, the pronounced sex specificity of condition dependence of structural and carotenoid-based coloration is suggestive of a signaling function of the fledgling plumage.