Discussion
We hypothesized that offspring and adults not only display similar colourful traits but that these traits might also have a correlated signalling value across life-history stages. Indeed, as has already been suggested (Johnsen et al. 2003; Jacot & Kempenaers 2006; Morales & Velando 2018), we found that certain colour components of nestling yellow breast feathers reflected body mass in the 3-year period. This pattern was particularly relevant for UV chroma and total brightness. Hence, these traits could act as condition-dependent signals beyond a sexual selection framework, around which most previous work has focussed (e.g., Senar et al. 2002; Hidalgo-Garcia 2006; Doutrelant et al. 2008, 2012; Ferns and Hinsley 2008; del Cerro et al. 2010; GarcĂ­a-Navas et al. 2012; Midamegbe et al. 2013; Ferrer et al. 2015). Furthermore, our approach allowed us to explore the potential of the three colour components as quality signals both within nests and among nests contexts.