It is well-established that plumage colours are important for avian visual communication and are used to signal social information. Yet, little is known about the ability of birds to modify the expression of plumage colours with exogenous materials after feather development, a phenomenon also known as avian cosmetics. The deliberate staining of feathers with red soil in a social signalling context has so far only been described in the Bearded vulture. Here, for the first time feather painting...