The production of some types of signals is often assumed to be cost-free, yet we know relatively little about the factors that affect signal development. In birds, many plumage signals are produced by the pigment melanin being deposited into growing feathers. Birds synthesize melanin in a complex pathway involving many potentially limiting resources, one of which may be dietary calcium. We tested the effect of dietary calcium on the development of the black throat patch (bib) of juvenile male House Sparrows (Passer domesticus). Subjects were maintained throughout their molt on artificial diets containing either a low, medium, or high level of calcium, or a bicarbonate control diet with a low level of calcium but an acid—base balance equivalent to that of the high-calcium diet. Dietary calcium affected the size of the bib, but not in the predicted direction: birds on the low-calcium diet produced the largest bibs. Birds on the bicarbonate control diet produced bibs intermediate in size that were significantly smaller than those on the low-calcium diet, which suggests that acid—base balance may also influence melanin-based signals. Dietary calcium also had a broader physiological effect; birds on the low-calcium diet were in better condition than those on other diets, but only the calcium treatment explained significant variation in bib size. Although our treatments affected bib size, our results indicate that access to dietary calcium is not a general mechanism for condition dependence of melanin-based ornaments.
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Vol. 127 • No. 3