Associations between colour parameters
At the individual level, yellow UV chroma of nestlings was positively correlated with yellow brightness (r = 0.34; P < 0.001; n = 1834), and negatively with carotenoid chroma (r = - 0.54; P < 0.001; n = 1835). In contrast, there was no relationship between carotenoid chroma and brightness (r = - 0.010; P = 0.70; n = 1834).
When running the correlations of colour parameters at the nest level, mean yellow UV chroma was positively correlated with mean yellow brightness (r = 0.54; P < 0.001; n = 234; Figure 1a), and negatively with carotenoid chroma (r = -0.52; P< 0.001; n = 234; Fig. 1b). Moreover, the relationship between carotenoid chroma and brightness was slightly significant and negative (r = -0.14; P = 0.030; n = 234; Fig. 1c).