Nestling Plumage Colour Variation in a Sexually Dichromatic Hole‐Nesting Passerine Bird—Potential Functions and Mechanisms

Animal colouration is subject to various selection pressures, which often result in the phenomena of sexual dichromatism and gradual colour development. Despite extensive knowledge about adult colouration, the significance of nestling or fledgling plumage colouration in birds remains understudied. Focusing on the collared flycatcher (<i>Ficedula albicollis</i>), this explorative study investigated colour variation in the pre-fledgling stage of nestlings. We collected reflectance spectra from the brown primary coverts and the yellow tip of these coverts of the nestlings from 71 nests and applied DNA-based sex determination. We revealed significant sex differences in offspring colour: females had brown coverts with higher brightness and lower UV chroma, and their yellow stripe had lower brightness, UV chroma and saturation. We detected significant but low repeatability of colouration between nestlings in the same broods. Nestlings did not show phenotypic integration between the colour variables of coverts and those of the terminal stripe, suggesting that these could be independent traits. The results also suggested that the yellow colouration of the stripe was probably caused by a white structural background and porphyrin pigmentation. Based on our results, we offer testable hypotheses on the potential adaptive functions of early-life sex-specific colouration patterns in birds for different contexts, including parent-offspring communication or hiding from predators.