Egyptian vultures, it turns out, love to stick on a bit of slap. Researchers have found that the birds apply red mud to their faces and white feathers as a form of make-up. This is only the second species of bird known to use external coloring for social communication.

Published in Ecology, the study examines the population of Egyptian vultures living on the Canary Islands, off the west coast of Africa. They found that the birds, usually adorned with a bare yellow face and white feathers, will smear themselves with red mud, putting their heads right into the pool and moving it from side to side to get maximum coverage.

While the use of cosmetics that are applied to the feathers and skin enhance coloration is actually fairly common in the birding world, such as red knots that apply “uropygial-oil” to their feathers to really bring out their coloration, using soil or mud to add coloration to feathers is altogether different, and a much rarer behavior.

The rock ptarmigan, for example, will cover itself in soil and dirt as the snow starts to melt in spring in order to camouflage its bright white winter plumage. But the use of coloring feathers for a purely social purpose has only ever been described in one other species of bird to date, the bearded vulture. These large creatures that live across much of central Asia do a very similar thing as the Egyptian vultures, in daubing their head and chests in earthy red mud.

The bearded vultures do this as a social signaling. Done in absolute secrecy – only a few people have actually observed a wild bearded vulture applying mud – it is thought to be used as a social status to indicate dominance. But this doesn’t quite explain what it might mean in the Egyptian vultures.

The Egyptian vultures, for example, painted themselves out in the open, in full view of all other birds. Not only that, but not all of them did so. In the bearded vultures, both males and females cover themselves in mud, while in the Canary Islands, only some of the Egyptian vultures decided to take the plunge. This also seems to rule out the use of the mud as some form of self-medication, as you would expect that if they were doing it to get rid of bacteria or viruses, for example, more members of the population would be doing so.

The researchers suggest that each species has different reasons behind the mud bath behavior and that the Egyptian vultures may be doing it in some sort of pair-bonding ritual. Currently, this is just an idea, and they suggest that more observations should be carried out to ascertain exactly which birds are doing, where, and at what point in the year.