Mouth coloration in nestling birds: increasing detection or signalling quality?

The bright colours of nestlings' mouths are conspicuous when they beg. Three main hypotheses suggest that nestling mouth colour (1) increases detectability in dim ambient light, (2) signals health or quality, or (3) provides a means of offspring recognition. To distinguish these hypotheses, we manipulated both the flange colour (visual and UV spectrum) and the ambient light in the nest of three cavity-nesting species: a woodpecker and two passerines. Parents avoided nestlings with black-painted flanges to a greater extent in dim than in bright ambient light conditions, and the effect was most pronounced in the woodpecker, which had the deepest and darkest nest cavity of the three species studied. There was no difference in mass gain of nestlings with flanges painted white or yellow or with UV-block cream relative to controls, suggesting that neither UV reflectance nor carotenoid colour was a cue of nestling quality. These results support the detectability hypothesis and not the health-signalling or recognition function for bright flanges. Black-painted flycatcher nestlings were more disadvantaged in short trials (<3 h) than in longer trials (>3 h), and we developed a model to explain how nonvisual components of begging, such as vocalizations or movements, may become more important than visual cues as nestlings become hungrier and beg more intensively over time. Our results suggest that bright flanges in cavity-nesting birds evolved primarily so that parents can locate offspring in dim light, but competition among nestlings may lead to exaggerated brightness. Furthermore, other components of begging such as movements or vocalizations may temporarily override a disadvantage of dull mouth colour.