Pigment contribution to feather mass depends on melanin form and is restricted to approximately 25%

Feathers are lightweight keratinous structures that have promoted the evolutionary success of birds by facilitating flight. Complex feathers, however, are believed to have evolved in response to visual functions, meaning a relevant role of pigmentation in feather evolution. The most common pigments in birds are melanins, large polymers synthesized at feather follicles, which thus have the potential to contribute significantly to the mass of feathers and compromise their lightweight nature. This contribution has never been quantified. Here, we leveraged a melanin extraction method to measure the mass contribution of different melanin forms to feathers from 19 species of birds, mainly golden eagles <i>Aquila chrysaetos</i> and pied flycatchers <i>Ficedula hypoleuca</i>. Melanin contribution to feather mass averaged 22.3%, but the contribution of eumelanin, responsible for black/grey/dark brown colour phenotypes, was higher than that of pheomelanin, responsible for lighter phenotypes. Eumelanins with a lower content of indole-carboxylated subunits also contributed more to feather mass. Melanin forms do not exert additive effects and constitute approximately 25% of feather mass regardless of whether the pigment composition is mixed or contains a single form. Our findings introduce a novel metabolic cost for flight associated with different plumage phenotypes that may help understand the evolutionary predictors of bird colour diversity.