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.