Cosmetic enhancement of signal coloration: experimental evidence in the house finch

Plumage coloration plays an important role in visual communication by signaling aspects of individual quality. It has been proposed that the need for preserving plumage condition and hence signaling content may have promoted a novel signaling mechanism: cosmetic coloration. Here, we investigated whether preen waxes may act as cosmetics by affecting feathers' ornamental coloration in male house finches (Carpodacus mexicanus). We (1) blocked the access to uropygial gland of a group of males and compared their coloration with a control group and (2) applied preen wax extracted from live individuals to museum skins and measured color before and after wax application. We found that red feathers of control males were more colorful than those of experimental males and that applying waxes to museum skins' red feathers increased their color saturation. Our results provide evidence that preen waxes act as cosmetics by enhancing plumage ornamentation, thus revealing the existence of novel mechanisms operating in signaling coloration. Although preen waxes per se would not constitute a signal, selection for signal efficacy might favor individuals that are able to increase signal intensity beyond the optimum necessary for plumage maintenance. Our results support the hypothesis that preen waxes reinforce the honesty of signals, which may be particularly useful in a social context where interactions between individuals are mediated by the intensity of the signals.