Data for: Offspring plumage colouration as a condition-dependent signal in the Blue Tit

In many species, offspring display conspicuous colouration already early
in life, even though they might be very vulnerable to predation at this
stage. However, most attention has been drawn to the conspicuous plumage
displayed by adult individuals in a sexual context, while other signalling
functions have been explored much less. Here, we investigated whether the
yellow breast plumage of blue tit (Cyanistes caeruleus) nestlings shows
patterns of condition-dependence and hence signals individual quality, as
has been described for adult birds. During three consecutive breeding
seasons, we, therefore, explored the association between nestling body
mass and three colour components of the yellow breast plumage (i.e., UV
chroma, carotenoid chroma and total brightness), considering both within
and among nest effects. Variation in carotenoid chroma was not related to
body mass. However, UV chroma and total brightness varied with body mass
on an among-nest level, suggesting that they might signal aspects of
genetic quality or parental rearing capacity. Interestingly, we also found
a within-nest effect of body mass on total brightness, suggesting that
this is a good candidate for a condition-dependent signal within the
family. Thus, other family members could rely on brightness to adjust
their behavioural strategies, such as feeding behaviour in parents. Our
study thus reveals that certain colour components of the yellow breast
plumage might signal different aspects of offspring quality and they might
have a correlated signalling value across life-history stages.