Condition-dependence
We found a significant among-nest effect of body mass on yellow UV chroma (F 2,220.78 = 57.57; P < 0.001; Table 1). Broods in which the average nestling mass was higher had higher levels of UV chroma than broods with on average lower nestling body mass (Fig. 2a). However, we did not find a significant within-nest effect of body mass on UV chroma (F 1,205.99 = 1.76; P = 0.19), so that nestlings with lower than average body mass in their brood did not have lower UV chroma than their siblings (Fig. 3a; see also Fig. S3). Yellow UV chroma was significantly affected by the interaction between year and nestling sex (F 2,1553.2 = 5.10; P = 0.0062; Fig. S1). UV chroma was higher in females than in males in all the seasons (all Post-Hoc tests: P < 0.001). In females, UV chroma was higher in 2017 than in the other two years and it did not differ between 2018 and 2019 (P = 0.23). The same effect was found for males, but, unlike females, the difference between 2018 and 2019 was almost significant (P = 0.051). The rest of the interactions were not significant (all P > 0.11).
We did not find among-nest (F 1,225.43 = 0.22;P = 0.64) or within-nest effects (F 1,1161.01 = 1.98; P = 0.16) of body mass on carotenoid chroma (Table 1; Fig. 2b and 2c; see also Fig. S4). Carotenoid chroma significantly differed among years (F 2,218.77 = 8.99; P = 0.002) and sexes (F 1,1642.67 = 73.39; P < 0.001), being higher in 2017 than in the other two years (Post-hoc tests:P < 0.001), and being higher in males than in females. However, there was not a significant interaction effect between year and sex. The rest of the interactions were also not significant (allP > 0.63).
Interestingly, we found significant both among-nest (F 1,227.51 = 5.05; P = 0.026) and within-nest effects (F 1,1410.24 = 22.74; P< 0.001) of body mass on yellow brightness (Table 1). Thus, broods with higher than average body mass displayed brighter yellow colourations (Fig. 2c), and those nestlings with a higher body mass relative to the average body mass of their brood displayed brighter yellow colouration than their siblings (Fig. 3c; see also Fig. S5). Yellow brightness was significantly affected by the interaction between year and nestling sex (F 2,1622.30 = 6.23;P = 0.0020; Fig. S2). Yellow brightness was higher for male nestlings than for female nestlings in years 2017 and 2019, while there was not a significant difference in 2018 (Post-Hoc test: P = 0.28; Fig. S2). All other interactions were not significantly different (all P > 0.30).