Conclusions
We show that yellow breast feathers could function as a condition-dependent signalling trait in nestling blue tits, given the observed associations with body mass. While total brightness and UV chroma (but not carotenoid chroma) seem to reflect genetic or other common environmental effects (=among-nest effects), total brightness could also act as an honest signal during intra-family interactions (=within-nest effect). Intriguingly, plumage colour was a sexually dimorphic trait in nestlings, in contrast to the situation in adults. This is somewhat counterintuitive as the forces of sexual selection are supposed to be greater in adults. This urges further studies to identify possible diverging selective pressures for males and females in the nestling and post-fledgling periods.