Data from: 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 results in the phenomenon of sexual dichromatism and gradual colour development. Despite extensive knowledge about adult colouration, the significance of nestling or fledgling plumage colouration of birds remains understudied. Focusing on the collared flycatcher (Ficedula albicollis), this explorative study investigated colour variation in pre-fledging 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 low but significant repeatability of colouration between nestlings within the same nests. Nestlings did not show phenotypic integration between the colour of coverts and their terminal stripe. The results also suggested that the colouration of the yellow plumage part was probably determined by white structural background and porphyrin pigmentation. Based on our results we offer future testable hypotheses on the potential adaptive functions of early-life sex-specific colouration patterns in birds in different contexts, including parent-offspring communication or hiding from predators.