Abstract
In many species, offspring display conspicuous adult-like colouration
already early in life, even though they might be very vulnerable to
predation at this stage. Yet, the signalling function of adult-like
traits in nestlings has been little explored to date. 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. Carotenoid
chroma was not affected by body mass. However, UV chroma and total
brightness showed an among-nest effect of body mass, 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 signal different aspects of
offspring quality and suggest that they might have a correlated
signalling value across life-history stages.