Sexual traits often signal quality ( Grafen 1990 Kotiaho 2001). Zebra finches ( Taeniopygia guttata) exhibit brightly red colored bills. Thus, we conclude that females do prefer males with redder bills when there was sufficient opportunity for sexual imprinting, but to what extent this is causally related to the bill color remains to be established. We conclude, therefore, that the role of bill coloration in mate choice cannot be solely due to an association with song rate. We show, however, in a separate meta-analysis on a different set of studies that the correlation between bill coloration and song rate ( r = 0.14) was significantly lower than the r = 0.61 between bill color and attractiveness. The latter may either be due to aspects of the experimental design or due to bill color being correlated with another trait such as song rate as was previously suggested. Here, we report a meta-analysis of mate-choice studies that reveals that female zebra finches do prefer males with redder bills ( r = 0.61), except when there was reduced opportunity for imprinting or when bill color was experimentally manipulated, which both reduced preference for red bills to approximately zero. However, studies of female mate preferences for redder bills show mixed results. The zebra finch is extensively used in the study of carotenoid-dependent signaling. A red bill may thus function as an indicator signal.
This color is dependent on carotenoids, which enhance immune function and are antioxidants. Male zebra finches display multiple secondary sexual traits such as song and red bill coloration.
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