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(Morales & Velando 2018; García-Campa et al. 2021; A. García-Antón, J. García-Campa, W. Müller and J. Morales unpublished data).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.