Colour measurements
We measured breast plumage colouration with a portable spectrophotometer (Jazz, OceanOptics©). For each nestling, we took three consecutive measurements relative to a white standard and perpendicular to the feather surface, using an external probe fitted with a plastic cylinder to standardize the measuring distance and exclude ambient light. We then obtained the reflectance spectra between 320-700 nm using CLR program v 1.1 (Montgomerie 2009). We excluded the first part of the spectrum (300-320 nm) in order to avoid noisy reflectance values.
We then calculated three objective colour parameters: i) total brightness (i.e., average reflectance between 320 and 700 nm; adapted from Jacot & Kempenaers 2007), ii) UV chroma (i.e., reflectance in the UV wave-band region of the spectrum divided by the total reflectance of the spectrum in the avian visual range (R320–400/R320–700); adapted from Johnsen et al 2003) and iii) carotenoid chroma (i.e., an estimation of the carotenoid content of yellow breast feathers (R700-R450)/R700), since carotenoids highly absorb in 450 nm; Shawkey and Hill 2005). For each colour parameter, we then calculated the mean of the three consecutive colour measurements sampled per nestling.
We measured the plumage colouration of 1837 nestlings (n2017 = 672; n 2018 = 639; n2019 = 526) of which 945 were males and 892 females. Due to the experiments performed in parallel studies, colour measurements were taken at the age of 12 days in 2017 and 2018, and 11 days in 2019. On the day of colour measurement, we also weighted each nestling to the nearest 0.01 g with an electronic Pesola spring balance.
In 2017, we provided blue tit females with extra lutein pigment prior to and during egg laying (for details, see García-Campa et al. 2020). However, nestlings of lutein supplemented mothers did not differ in colouration from control nestlings (one-way ANOVA test; total brightness: F 1,670 = 2.13; P = 0.15; UV chroma: F 1,669 = 0.29; P = 0.59; carotenoid chroma: F 1,670 = 0.02; P = 0.89). In the 2018 season, we reduced the yellow UV chroma of one blue tit parent (indistinctly males and females) at the nest on the second week of nestling age (García-Campa et al. 2022). Nonetheless, parental UV chroma did not have an effect on offspring colour parameters (one-way ANOVA test; total brightness: F 1,521 = 0.61;P = 0.44; UV chroma: F 1,521 = 1.20;P = 0.28; carotenoid chroma: F 1,521 = 0.78; P = 0.38). Additionally, in 2017 and 2018, we cross-fostered full clutches among nests at the end of incubation. Thus, among-nest differences in the relationship between colour and condition could be partly explained by the rearing effects of the foster nest.