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.