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).