Proximate Correlates of Carotenoid-Based Mouth Coloration in Nestling House Sparrows

Abstract

Abstract. The mouth coloration of passerine nestlings is hypothesized to attract parental care by increasing the visual conspicuousness of begging chicks and/or by signaling the reproductive value of nestlings. Specifically, carotenoids are often hypothesized to mediate the latter relationship. In House Sparrow (Passer domesticus) nestlings, we confirmed both the presence of carotenoids in rictal flanges and a positive relationship between carotenoid concentration and the intensity of yellow coloration. This carotenoid-based coloration was positively associated with nestling mass and with plasma carotenoid concentration. Red gape coloration also revealed titers of circulating carotenoids. Carotenoids reduced the overall brightness and the UV reflectance of flanges, an effect that may limit the detectability of carotenoid-rich mouth colors and the ability of brightness and UV coloration to function in communication. For example, flange brightness, likely the primary determinant of conspicuousness, was posi...