Nestling mouth colors mediate parental favoritism but do not influence detectability.

This article investigates how variation in the coloration of nestling house sparrow (Passer domesticus) mouthparts, specifically the carotenoid-richness, brightness, and ultraviolet (UV) reflectance of rictal flanges, influences parental feeding behavior. Through experimental manipulations simulating differences in these color components, the study found that parents preferentially allocated more food to nestlings with carotenoid-rich (yellow) flanges but showed no feeding bias related to brightness or UV reflectance. Additionally, none of the color manipulations affected the efficiency of prey transfer from parents to nestlings, suggesting that while carotenoid-based coloration may signal offspring quality and elicit parental favoritism, brightness and UV reflectance do not independently affect detectability or feeding efficiency within natural variation. These findings highlight the importance of considering distinct color components separately in studies of offspring-parent communication and the evolution of visual signals in birds.