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At a feeding station on the Canary Islands off the coast of Morocco, scientists presented Egyptian Vultures with two pools of water: one pure, the other filled with muddy iron-red water. While most of the endangered birds wander about, three crowd around the mucky basin. An adult female dips her head into the mud, shaking it side to side as she raises it up. Then she steps into the pool, repeating the movement in earnest, tilting her head and rooting around before preening the red goop into her wings. With the white feathers on her face and chest temporarily dyed deep reddish-brown, she looks at her neighbors and calls a few times.

Exactly what she’s trying to convey with the mud mask is a mystery that scientists are trying to solve. Researchers from Spain’s Doñana Biological Station recorded this scene and several other instances of the vultures deliberately staining their feathers—a rare behavior known as “avian cosmetics.” Of the 91 birds that visited the feeding station, 18 painted themselves with mud, ecologist Thijs van Overveld and his colleagues report in the journal . The dabblers included adults and juveniles of both sexes, and the baths ranged from a single hesitant pass to a full-fledged beauty routine. Three birds returned for a second round; van Overveld’s team spotted only one individual take a similar bath in the clear water.

While plumage color is important for everything from camouflage to establishing dominance in the bird world, only a handful of avian species have been observed applying makeup. “We know that birds do this—they put stuff on their feathers,” says , an ornithologist at Monash University in Australia who was not involved in the Egyptian Vulture study, “But when we talk about cosmetics, we’re usually trying to imply that they’re trying to change their appearance.” As with the Egyptian Vulture, in most cases, no one knows for certain why birds paint themselves. “All we have now are tantalizing links,” Delhey says, referencing correlations between when a bird displays this behavior and its current situation.

Before we delve into some of those intriguing examples, let’s define a “cosmetic” as it applies to birds. The substance has to provide a purely visual change (not, say, be used to stave off parasites). And there has to be some sort of interaction at play—whether the made-up bird is trying to attract a mate, or evade a predator. “Just getting yourself dirty [in a way] that nobody cares about or changing your color and nobody notices doesn’t count,” says , a reproductive biologist at Queens University in Canada. While Montgomerie argues that the substance has to come from the environment, Delhey counts some chemicals produced by the bird as cosmetics. Even with this expanded definition, has found compelling evidence of only around a dozen bird species using makeup.

The apparent reason for the Rock Ptarmigans’ use of cosmetics—camouflage—may be the most clear-cut, to humans, anyway. During 17 years of fieldwork in northern Canada, Montgomerie and his colleagues noticed that in late May and early June, after the birds mated and the snow melted, covered their immaculate white breeding plumage with dirt, disappearing against the rocky terrain. If a birds’ eggs were eaten by predators, it would shed its disguise, quickly cleaning itself to put its breeding plumage back on display in hopes of a second chance to nest that season.

The case of the Bearded Vulture, a close cousin to the Egyptian Vulture, was more difficult to parse. Naturalists had been long puzzled by seeing both white birds and red birds. Then, in the 1990s, scientists documented individuals seemingly intentionally applying reddish muds or soils to their feathers. There weren’t any obvious from the behavior, and camouflage was an unlikely goal, since adults have few predators. A close examination revealed that the most vividly dyed birds were older, more dominant, and took their mud bath out of sight—all of which led that the practice marked high-status birds.

Greater Flamingos produce their own makeup, which they appear to apply in order to attract a mate. The vibrant waders obtain their trademark color from their diet: They store the pigment from tiny crustaceans they eat in their feathers. But there’s more to their pink plumage. The pigments also travel to the birds’ uropygial gland, which produces preen oil, researchers . They noticed that between October and April, when flamingos are wooing mates, the birds rub their cheeks against the glands near their tails, then daub it onto their back feathers, enhancing the rosy hue. Removing the substance from feathers, the scientists found, reduced the intensity of the color. They also discovered that pinker birds nested earlier, which usually results in more chicks.

Japanese Crested Ibises also secrete a substance that they use to . In January and February, patches of skin around the faces of the white birds begin oozing a black tar-like substance. The birds rub these patches on their upper back and wings to transfer the goo, then rub their heads and necks into it, turning their snowy-white feathers a dark grey. As with the Egyptian Vulture, scientists don’t know how, or even if, the dyed plumage benefits the birds.

“To be certain that something is working as a cosmetic, I think we need to do experiments,” Delhey says. For Egyptian Vultures, that could entail painting the birds different hues to see what happens, or providing pools of varied colors, and closely watching the birds’ responses—both the ones that are dolled up, and those that aren’t. But for now, the scavengers are keeping their beauty secrets to themselves.