Soaring high among the mountains from Europe to China and to Africa, the bearded vulture (Gypaetus barbatus) commands attention at any lunch party. It likes to gate crash into the frenzy around carrion, pushing other scavengers aside with wings that could stretch the height of Michael Jordan, only to ignore the carrion’s juicy flesh or brains, and fly away with a large piece of bone in its talons.
The bearded vulture is one of the largest birds of prey, yet it eats mostly bone marrow. It is also the only bird known to decorate itself.
Adult bearded vultures sport a coat of snow-white feathers on their neck, shoulder and chest. On their white feathery canvas, the vultures paint a shade of rusty-red by bathing in soils or water rich in red iron oxide deposits.For years, scientists questioned the origins of the bearded vultures’ red paint. Field studies, including an intensive 3-year radio-tracking study, failed to discover the origin of the red coloration. Scientists suggested that the red stains might have been caused by the birds haphazardly resting near iron deposits. Yet the coloring seemed intentional, as captive birds that were offered damp red soils quickly pounced on the soils and bathed in it, dusting their bellies and necks red like those of wild birds. The birds would use beak and talons to spread the red mud from their chest to their shoulders and upper back. Bearded vultures clearly likes donning a shade of red.
Finally, in 1995 and then 1998, wild bearded vultures were seen bathing in pools thick with iron deposits in the French and Spanish Pyranees. We now know that bearded vultures deliberately dust themselves red. But why?
Camouflage is unlikely, since the vultures sit atop the food chain (and nobody bothers to hide from bones); iron oxides do not seem to improve feather durability, as colored and white feathers wear down just the same. Scientist settled on two possible functions to the bearded vulture’s iron oxide decoration. They just cannot agree which is correct: are the iron oxides cosmetics or prophylactics?
In 1999, Juan Jose Negro, ecologist at the Doñana Biological Station in Spain, reasoned that bearded vultures might use iron oxides to advertise their strengths to other vultures. Suitable iron oxide deposits are likely rare, as suggested by the scarce observation of wild vultures bathing in iron oxides deposits. Hence, bearded vultures might have to expend much energy and time to find iron oxides, making iron oxides a cosmetic that only a strong or healthy bird could afford. If so, sporting a coat of red iron oxides would be no less a stamp of strength than a Rolls-Royce a mark of wealth.Advertising strength could be used to establish dominance: bearded vultures fighting over bones (and the fatty marrow within) would stare down their challengers and swing their heads while the neck and head feathers erect like sentinels. Coloring seems to intensify with age, size and dominance. Females, often larger than males, also sport more intense iron oxide colors and dominate matings; among males, those with paler red reportedly mate less.
Three years after Negro’s publication, Raphael Arlettaz, ecologist at University of Bern in Switzerland, offered an alternative hypothesis to explain the vulture’s red coloration: the iron oxides are not mere cosmetics, but prophylactics instead. Chlorine dioxide and ozone kills bacteria, prompting Arlettaz to suggest that iron oxides may also do the same. Bearded vultures likely contact lots of bacteria in their professional lives as scavengers, and these bacteria threaten the vultures’ nestlings and eggs. Arlettaz suggests iron oxides may help nestlings and eggs resist the bacterial onslaught their parents bring home.
Like other birds, bearded vultures cannot produce carotenoids, antioxidant compounds that protect cells from free radicals in the body. Many bird species acquire carotenoids through their diets, whereas the bearded vulture’s diet of bone marrow, though fatty and delicious, contains no carotenoids. Because iron atoms form parts of antioxidant enzymes, Arlettaz speculates that bearded vultures use iron oxides in place of carotenoids. Citing his colleague’s observation that captive vultures would return to their nests after a bath in iron oxide-rich water, and “rub their feathers impregnated with pigments on to their eggs or offspring”, Arlettaz suggests that bearded vulture parents may pass the iron oxides to their children. If so, then that females sporting brighter red than males may simply reflect the former’s greater need to protect their children with iron oxides.
Negro responded to Arlettaz’s ‘prophylactic hypothesis’ with several counter-arguments. Negro noted the lack of evidence that iron oxides kill bacteria (on the contrary, many microbes fight for iron); other vultures that also lack carotenoids like bearded vultures do have not evolved to use iron oxides as an alternative; and juvenile vultures would start staining themselves with iron oxides years before they would bear nestlings.The academic discussion on the matter seemed to have ended in 2002 with no subsequent papers on iron oxides use among bearded vultures. Both Negro and Arlettaz have moved on to other projects, but they hope that someday new data would provide a conclusive verdict on why bearded vultures color themselves red with iron oxides.
Negro JJ, Margalida A, Hiraldo F, & Heredia R (1999). The function of the cosmetic coloration of bearded vultures: when art imitates life. Animal behaviour, 58 (5) PMID: 10564620
Arlettaz, R., Christe, P., Surai, P.F., and Pape Møller, A. (2002). Deliberate rusty staining of plumage in the bearded vulture: does function precede art? Animal Behaviour 64, F1–F3.
Negro, J.J., Margalida, A., Torres, M.J., Grande, J.M., Hiraldo, F., and Heredia, R. (2002). Iron oxides in the plumage of bearded vultures. Medicine or cosmetics? Animal Behaviour 64, F5–F7.