DOI: 10.5061/dryad.9ghx3ffn4
title: Data for: Offspring plumage colouration as a condition-dependent signal in the Blue Tit

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