The researchers studied greater flamingos gathering for group displays in the wetlands of southern Spain and saw that many applied reddish-orange pigments, called carotenoids, from glands near the base of their tails to their plumage, as well as tidying their feathers.

They rubbed cheeks against the preen glands and then transferred the oils that were secreted on to their nest, breast and back feathers. The more colourful birds started breeding earlier.

The most colourful birds were those seen reapplying the pigment most often, but breeding birds lost their plumage colour. The team, led by experts at the Doñana biological station in Spain, suggest this might be because the carotenoids perform physiological roles in the birds' bodies – for example in egg yolk in females and in secretions added to food for chicks.

The paper adds that flamingos with more coloured plumage started laying earlier. This might add to the survival chances of their offspring because their parents had gained control of the best breeding sites.

"The presence of carotenoids in preen oils has been previously only suggested, and here we confirm for the first time its presence in such oils," say the authors.

"Given that cosmetic colouration may be related to individual quality, our findings may have important implications for the theories of sexual selection and signalling, highlighting the key role of the manipulation of plumage colour by the birds themselves to improve signal efficiency."