A Vulture with Style
- Bearded vultures are the only birds known to wear cosmetics.
- They deliberately dye their feathers with red soil.
- 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 necks, shoulders and chests.
- On this 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 vultures’ red paint.
- Field studies, including an intensive three-year radio-tracking study, failed to uncover the origins of the red colouration.
- Researchers suggested that the red stains might have been caused by the birds randomly resting near iron deposits.
- Yet the colouring seemed intentional, as captive birds that were given access to damp red soil quickly pounced on it, dusting their bellies and necks red like their wild kin.
- The birds would use the beak and talons to spread the red mud from their chest to their shoulders and upper back.
- Bearded vultures clearly like to put on 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 Pyrenees.
- Vultures sit atop the food chain so there's no need to hide from predators, and since they eat bones, there's no creeping up on prey either.
- The iron oxides also do not seem to improve feather durability, as coloured and white feathers wear down just the same.
- So scientists settled on two other possible functions and 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 strength to other vultures.
- Females, often larger than males, also sport more intense iron oxide colours and dominate matings; among males, paler suitors reportedly mate less frequently.
- Three years after Negro’s publication, Raphael Arlettaz, ecologist at University of Bern in Switzerland, offered an alternative hypothesis to explain the vultures' red colouration: 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 probably come into contact with lots of bacteria in their 'professional' lives as scavengers, and these bacteria threaten the vultures’ nestlings and eggs.
- So Arlettaz suggested the iron oxides could be a weapon against the bacterial onslaught the vulture parents bring home.
- Negro responded to this ‘prophylactic hypothesis' with several counter-arguments. He noted the lack of evidence that iron oxides kill bacteria
- The debate on the matter seems to have died down since, and both Negro and Arlettaz have moved on to other projects. Until it resurfaces, the exact reasons for the vultures' passion for decoration remain a mystery.
