The orange color of the bearded vulture does not seem to have an antibacterial function

The bearded vulture visit regularly the Ferruginous springs for aesthetic purposes for their orange plumage coloration. Different hypotheses have been proposed to explain this deliberate application of cosmetic coloration: (1) to indicate the status of individual dominance; (2) to exploit the antibacterial effect of iron oxides in order to reduce the degradation of feathers by bacteria and, in parallel (3) so that the birds that hatch transfer this protection to the developing embryos to increase the success of hatching. In this article, optimize the antibacterial hypothesis using three experimental approaches: (a) by applying feather-degrading bacteria to the bearded vulture feathers dyed and not stained; (b) evaluating the antibacterial activity of ochre; and (c) by comparing the reproductive success of individuals Orange pale vs. Our results suggest that the in vitro addition of Bacillus licheniform s to naturally stained Bearded Vulture feathers did not retard feather degradation compared to controls. Iron particles from red soil (ochre) or iron salts had no antibacterial effect on the growth of three species of bacteria (Escherichia coli, Kocuria rhizophila and Bacillus licheniformis), incubated either in the dark or under visible light. Finally, breeding success did not differ between territories occupied by pale individuals versus orange ones. These results run counter to the hypothesis that iron oxides have an antibacterial role in Bearded Vultures. The use of red soils by Bearded Vultures may function as a territorial status signal, but may also be involved in other processes , such as pair formation and the long-term maintenance of the pair bond.

Margalida, A., Braun, M.S., Negro, J.J., Schulze-Hagen, K. & Wink, M. 2019. Cosmetic colouring by Bearded Vultures Gypaetus barbatus: still no evidence for an antibacterial function. PeerJ 7:e6783