By Victoria Gill
Science and nature reporter, BBC News
Flamingos in the wild use pigments as "cosmetics" to enhance the colour of their plumage, according to scientists.
Researchers studying greater flamingos in the wetlands of southern Spain found that the birds rubbed pigmented secretions onto their feathers.
They produce the pigments in glands near the base of their tails.
The scientists describe in the journal Behavioural Ecology and Sociobiology how the birds use the pigments to signal to potential mates.
The researchers identified pigments called carotenoids in the mixture of wax oil that the birds secrete in their preen glands.
They noticed that, as well as smoothing and tidying their feathers, many birds deliberately rubbed their cheeks against the preen gland and then immediately onto their neck, breast and back feathers.
Reddish-orange carotenoid pigments in the oils then brightened the signature pink hue of the birds' feathers.
Juan Amat from the Donana Biological Station in Spain led the study.
He said that the birds appeared to "manipulate the colour of their plumage" as a signal of their quality.
Since it takes time and energy to apply the pigments, being more colourful could be a powerful visual signal of a healthy, well-nourished flamingo with time to take care of its appearance.
"The rubbing is time-consuming," Dr Amat told BBC News. "And the more frequently the birds practise it, the more coloured they appear.
"If the birds stop the rubbing, [their] plumage colour fades in a few days because carotenoids bleach quickly in the sunlight."
This means that, like with make-up, frequent reapplication is necessary to stay colourful.
The scientists have not directly investigated how this cosmetic use benefits the birds, but they think that the more colourful flamingos might have more success in finding a mate.
"We found that the more coloured birds started breeding earlier than paler ones," explained Dr Amat.
"So by mating to a colourful bird an individual may increase its reproductive success, as from previous studies we know that the first pairs to start breeding gain access to the best breeding sites."
And there is more to the story of cosmetic-using flamingos than this one study, according to Dr Amat.
"We have data indicating that females make-up much more often than males - just like in humans," he told BBC News.
"Also, we know that flamingos apply make-up more often in better habitats, and that the more coloured birds obtain food of better quality."
Professor Matthew Anderson from Saint Joseph's University in Philadelphia, US, said the study provided "convincing evidence that plumage colouration may be involved in mate choice in these birds".
He added: "The beautiful pink plumage of flamingos has long captured the attention of the general public. It [now] appears that flamingos may be paying as much attention to their vibrant colouration as we are."