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.

We used R 4.1.0 (R Core Team 2020) for statistical analyses. First, to explore how the colour parameters were inter-related, we performed correlations between UV chroma, carotenoid chroma and brightness both at the individual level and at the nest level (the latter using mean values of colour parameters). Second, we fitted three linear mixed models with a normal distribution of errors using the lmer function in the “lme4” package (Bates et al. 2015) to determine the relationships between body mass and each of the three colour parameters. We assumed normality in all cases after checking the residual plots, given also the robustness of mixed models to violations of normality assumptions (Schielzeth et al. 2020). The models included as fixed effects the average body mass of the brood (= among-nest effect), the deviation from the average body mass of the brood (= within-nest effect), and their interaction. We included in addition year (2017, 2018 and 2019), nestling sex, brood size, and the interactions between year and nestling sex, average body mass and year, average body mass and brood size, and nestling sex with the deviation from the average body mass. Backward elimination for non-significant interactions (α = 0.05) was used to build the minimal models. We also included nest ID as a random intercept, and the interaction between nest ID and the deviation from the average body mass (= within-nest effect) as a random slope.

The current database includes the variables used in the analysis performed in the manuscript.