The extraordinary variety of bird colours has fascinated scientists since Darwin, but research has mainly been focused on plumage colouration. As with other vertebrates, some birds have developed black skin, but this evolutionary mechanism and its drivers have received little attention in the scientific community.
In this study, researchers examined specimens of more than 2,200 species across all bird families, categorizing species by skin colour, and found black skin on the head to be a widespread trait, present in at least 138 genera. The trait appeared to have evolved more than 100 times independently, but has been frequently lost, indicating a costly specialization only evolved when serving a particular function.
Lumping GBIF-mediated occurrences of black-skinned bird species together, the authors modelled the predicted distribution of the bird trait, which followed the so-called Gloger's rule with skin melanization increasing towards the equator. The best predictor of black skin in birds was exposure to UV radiation, while the authors found little support for hypotheses related to thermal regulation and bacterial protection.
In addition, black skin was most common in species with thin, very bright or no plumage and even more common in hatchlings. These findings together suggest that melanin-rich darkening of the skin evolved as a protection against damaging UV radiation.