Nestling birds put their best flange forward

The offspring of caring parents may evolve specialized traits uniquely adaptive during their dependence on parental care. For example, the mouths of passerine nestlings are often bordered by enlarged and colorful rictal flanges expressed only during the nestling period. Although these traits are commonly hypothesized to act as visual signals during begging, noncommunication functions for the specialized mouth have been proposed as well. To test the hypothesis that nestling flange colors have evolved largely or exclusively as visual signals, I compared the reflectance of flange tissue that would be visible to parents during begging to that of flange tissue not exposed during begging in nestling house sparrows Passer domesticus and cliff swallows Petrochelidon pyrrhonota. Specifically, I tested the prediction that both condition-dependent color parameters and those associated with visual conspicuousness would be expressed more intensely in tissue displayed during begging. Consistent with this prediction, flange tissue exposed during begging was brighter (reflected more total light), more UV-rich, and had more intense carotenoid-based coloration than hidden tissue. These differences do not exclude a non-signaling function for flanges, but are consistent with the hypothesis that flange colors have evolved as visual signals.