Condition-dependence: nestling colour as an honest signal
of quality
While the colour expression of the nestlings’ yellow breast feathers
showed condition-dependency, this effect differed for the three colour
parameters under study. Furthermore, the contribution of among-nest
effects (which encompass a combination of genetic effects, parental
quality effects or other common environmental effects) and within-nest
effects (reflecting the relative differences in body mass among all the
nestlings raised in the same brood) also varied between the colour
parameters.
We found a significant among-nest effect of body mass on yellow UV
chroma. Broods with higher mean body mass also had higher mean UV
chroma. This effect was independent of brood size. Interestingly, UV
chroma could reflect genetic quality (Charmantier et al. 2017) or
parental rearing capacity (Senar et al. 2002), which we cannot separate
in our study. However, we did not find differences in UV chroma between
nestlings of the same brood (within-nest effect). This suggests that, at
the intra-brood level, family members might not use UV chroma as a
reliable signal of body mass. This was unexpected since we have
experimentally demonstrated in the study population that chicks with
reduced UV chroma gain less body mass (Morales & Velando 2018), and
that this trait is used as a signal during intra-family interactions
(Morales & Velando 2018; García-Campa et al. 2021; A. García-Antón, J.
García-Campa, W. Müller and J. Morales unpublished data).
In addition, our findings show that there were among-nest effects of
nestling body mass on yellow brightness. Unlike UV chroma, it is less
likely that brightness functions as a signal of genetic quality, since
it shows low heritability (Charmantier et al. 2017). However, it may
reflect parental quality effects or other (common) environmental
effects. Surprisingly, there were neither among- nor within-nest effects
of nestling body mass on carotenoid chroma, in contrast to a number of
previous studies supporting that this colour component is
condition-dependent in nestling blue tits (Johnsen et al. 2003; 2005;
Jacot and Kempenaers 2007, Delhey et al. 2006; 2010). One possibility is
that differences in the calculation of carotenoid chroma used across
studies ((RUV peak –
R450)/RUV peak see in Bleiweiss 2004;
Jacot and Kempenaers 2007; and
R300–400/R300–700 see in Johnsen et
al. 2005) explains this inconsistency. Besides, since this colour
parameter is strongly dependent on dietary carotenoid availability, it
might contain a strong environmental component, which is, however, not
captured at the nest level.