Melanin-based color variation in the Bearded Vulture suggests a thermoregulatory function

Abstract

We document a case of intraspecific variation in plumage colouration in Bearded Vultures that may have arisen as a functional response to climate conditions. Two subspecies, Gypaetus barbatus barbatus (Eurasia and North Africa populations) and Gypaetus barbatus meridionalis (Eastern and Southern Africa), have been described on the basis of plumage colour differences. The plumage of G. b. barbatus tends to be darker in comparison with that of G. b. meridionalis. The plumage of the two subspecies differ in the feathering of the tarsi (more abundant in G. b. barbatus) and presence/absence of dark ear tufts (only present in G. b. barbatus, and this being the most useful character to distinguish adult specimens of both subspecies). When exposing skins under the sun or to electric bulbs in a cold room, temperature increases were significantly higher in the black ear tufts than in the frontal region of the head, suggesting that the melanized ear patch of G. b. barbatus serves, at least, to heat up the air entering the ear channel and perhaps also the underlying skin. In addition, G. b. barbatus, which inhabits mountainous regions with harsh and long winters, would benefit more from feathered tarsi to prevent heat loss through the legs.