Data from: Information content of persistent plumage colour traits in collared flycatcher nestlings

In animals, colours play a key role in communication. Colouration studies typically focus on the expression of colour traits of sexually mature individuals. However, the same colour-producing mechanisms are at work in immature individuals, so it cannot be excluded that inter-individual variation in colouration reflects individual quality in immatures too. Here, we investigated the plumage colouration in pre-fledging nestlings of the collared flycatcher (<i>Ficedula albicollis</i>) in relation to the rearing environment and parental colour ornamentation. We collected reflectance spectra from two different nestling plumage parts with different proximal backgrounds (i.e. eu- and pheomelanin-based brown, and structural–porphyrin-based yellow), and we also applied molecular-based sex-determination. The focal plumage parts are still present at yearling age, so their colour could have importance in sexual competition, with potential fitness consequences. We found that the colouration was less developed in nestlings reared under supposedly unfavourable environmental conditions (higher mismatch with food peak, larger brood size, inexperienced father), and when parental colouration was less exaggerated. Overall, the brown plumage part of nestlings under presumably poor conditions was less brownish with higher intensity, and the yellow part was less yellowish with lower intensity. The correlations suggest a certain degree of sex-dependent sensitivity in colour development. Based on the findings, the expression of plumage colouration in nestlings of the collared flycatcher could have the potential to be an honest indicator of certain aspects of individual quality.