Information content of persistent plumage color traits in <i>Ficedula albicollis</i> nestlings

Abstract In animals, colors play a key role in communication. Coloration studies typically focus on the expression of color traits of sexually mature individuals. However, the same color-producing mechanisms are at work in immature individuals, so it cannot be excluded that inter-individual variation in coloration reflects individual quality in immatures too. Here, we investigated the plumage coloration in pre-fledging nestlings of Ficedula albicollis (Collared Flycatcher) in relation to the rearing environment and parental color ornamentation. We collected reflectance spectra from two nestling plumage parts with different proximal backgrounds (i.e., eu- and pheomelanin-based brown, and porphyrin-based yellow), and we also applied molecular-based sex-determination. The focal plumage parts are still present at yearling age, so their color could have importance in sexual competition, with potential fitness consequences. We found that the coloration was less developed in nestlings reared under supposedly unfavorable environmental conditions (higher mismatch with food peak, larger brood size, younger father), and paternal covert color predicted the sired offspring’s covert color. Overall, the brown plumage part of nestlings under presumably poor conditions was less brownish and brighter, and the yellow part was less yellowish and less bright. The correlations suggest a certain degree of sex-dependent sensitivity in color development. However, the fixed factors in our models explained only a small proportion of the total variance in nestling coloration, indicating that most of the variation remains unexplained. Based on the findings, the expression of plumage coloration in nestlings of F. albicollis could have the potential to be an honest indicator of certain aspects of individual quality.