Cross-fostering experiments to compare carotenoid- and melanin-based plumage traits and long-term parental effects in post-moulted great tits

A current hot topic in the study of plumage coloration refers to whether melanin- and carotenoid-based plumage traits differ in the factors which determine ornament expression. However, approaches and results are mixed and controversial. The present study aims to apply a cross-fostering experiment in the great tit (Parus major) to compare melanin- and carotenoid-based traits of both genetic and foster parents with those of their offspring after completing post-juvenile moult. The size of the melanin-based black tie area of yearling great tits was significantly correlated to that of their genetic father. Conversely, the chroma of the carotenoid-based yellow breast colouration of yearling offspring was positively correlated to the mid-parent value of the foster parents and to the value of each foster parent separately. Our results suggest that melanin-based traits are mainly influenced by genetic factors, while carotenoid-based colouration is environmentally dependent, supporting the view that different factors determine ornament expression of the two traits. Our results also emphasize the long-term parental effect of carotenoid-based plumage colouration.