Molecular and Evolutionary Basis of Avian Plumage Colour

The rapid growth of avian genomic resources has created a field rich in genomic data but still in need of integrative synthesis. Here, we review evidence linking four colour-producing systems-melanin, carotenoids, psittacofulvins and structural colouration-to feather development and evolutionary change. Melanin output is shaped by the MC1R-ASIP-MITF regulatory axis, melanogenic enzymes and the local availability of substrates such as cysteine. Carotenoid colouration depends on dietary uptake, SCARB1-mediated transport, BCO2-regulated precursor availability and, in many red species, CYP2J19-mediated ketolation acting with BDH1L; however, CYP2J19-independent red routes demonstrate that similar colour phenotypes can evolve through different molecular solutions. Structural colours arise from feather nanostructures and may interact developmentally with pigment deposition, whereas psittacofulvins represent an independent pigment system in parrots. We also discuss sex-biased gene expression, feather-region specificity, environmental sensitivity and lineage-specific regulatory change. Overall, avian plumage colour evolution is shaped by conserved biochemical and developmental constraints, yet similar colour phenotypes can arise through different regulatory, metabolic or structural routes.