Differences between the sexes
We also found a consistent effect of nestling sex on the three colour parameters analysed, in accordance with previous studies (Johnsen et al. 2003; 2005; Jacot and Kempenaers 2007). Females expressed higher mean values for yellow UV chroma than males, whereas we detected the opposite pattern for carotenoid chroma and brightness (even though UV chroma and brightness are strongly and positively associated). While the blue tit was one of the first species in which a sexual dimorphism in crown UV-based plumage colour was documented, this has not been found in adult yellow breast feathers (Hunt et al. 1998). It is somehow puzzling that the latter trait is dimorphic only in nestlings and juveniles- since yellow body feathers are moulted a few months after fledging (Schoppe 1977; Cramp and Perrins 1993). Thus, parents could potentially rely on both carotenoid-chroma and total brightness to discriminate offspring sex while adjusting their feeding strategies. Indeed, in other study populations, blue tit males and females receive different food items (GarcĂ­a-Navas et al. 2014) or the total amount of investment (Dickens and Hartley 2007). In addition, fledging yellow plumage could play a signalling role in family flocks that are formed immediately after fledging (Stenning 2017), and during social interactions within flocks (Tschirren et al. 2005). These sex-specific patterns clearly need further study.