Data from: Melanin-based sexual dichromatism in the Western Palearctic avifauna implies darker males and lighter females

Melanins are the most common pigments providing coloration in the plumage
and bare skin of birds and other vertebrates. Numerous species are
dichromatic in the adult or definitive plumage, but the direction of this
type of sexual dichromatism (i.e., whether one sex tends to be darker than
the other ones) has not been thoroughly investigated. Using color plates,
we analysed the presence of melanin-based color patches in 666 species
belonging to 69 families regularly breeding in the Western Palearctic.
Sexual dichromatism based on melanins in at least one integumentary part
involved 205 (30.7%) species. The body parts contributing more frequently
to dichromatism were the dorsal areas, head and breast, whereas the less
dichromatic body parts were the belly and the exposed integumentary parts
(i.e., bill and legs). Regarding the phylogenetic spread of dichromatisms,
37 (53.6%) families contained at least one species with melanin-based
sexual dimorphism in the definitive adult plumage. As for the direction of
the color difference, males are darker than females in a majority of
species, meaning that males tend to produce more eumelanin and females
tend to synthesize more pheomelanin. This survey has revealed the high
prevalence of melanins in the emergence of sexual dichromatism in birds,
at least in the Western Palearctic. Whether the described pattern is due
to sexual selection promoting more conspicuous males or to natural
selection for more cryptic females remains to be determined. Given that
pheomelanin synthesis concurrently consumes the antioxidant glutathione
but also reduces toxic cysteine, sex-biased physiological factors should
also be given consideration in the evolution of bird plumages.