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 on
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.
Keywords : nestling colouration, quality, honest signalling,
condition dependence, UV colouration, sex differences, carotenoids