Nestling mouth colors mediate parental favoritism but do not influence detectability

Young animals often solicit care from their parents using behaviors and morphologies collectively termed begging. Many nestling altricial birds add to their familiar postures and vocalizations by displaying colorful mouthparts, hypothesized to enhance their visual conspicuousness and/or be favored by choosy parents. We evaluated these two hypotheses with experimental manipulations of the flange tissue of house sparrow (Passer domesticus) nestlings. We allowed parents to interact with nestlings that appeared to have either carotenoid-rich or carotenoid-poor flanges, and then with nestlings that appeared similar in carotenoid-richness but differed in either brightness (total light reflected) or ultraviolet (UV) richness. These three features of flange color comprise much of the variation observed both within and among species. None of our three manipulations shaped parents’ ability to transfer prey efficiently to nestlings, a proxy for how visually detectable nestlings were. Parents preferentially allocated prey to nestlings that appeared carotenoid-rich, but did not bias allocation when we manipulated brightness or UV reflectance. These patterns suggest that nestlings displaying carotenoid-rich flanges benefit from parental favoritism, but that variation in brightness and UV reflectance do not independently shape parental care, at least within the range a house sparrow parent might encounter in nature. Efforts to understand mouth color in the context of offspring-parent communication and visual signal evolution may benefit from considering the component parts of color, as they may function, be constrained, and evolve independently.