Adult bearded vultures often have a deep orange-red tinge on their plumage, which is variable from bird to bird and region to region. It was long thought that the colour could come from birds bathing in mud rich in iron oxide, but this fact was first described with certainty in captivity (as many other behaviours with this species), and then later confirmed in the wild.
Although the reasons for this behaviour are still unknown, various hypotheses exist, with one of them being that the orange colour is a signal of status.
In captivity bearded vultures regularly bath in the red mud, with the frequency of this behaviour depending on the weather and on how often mud is provided. Furthermore, the intensity of the colouration generally correlates with the age and sex of the bird, with older individuals and females displaying more intense pigmentation.
However, this behaviour seems to be innate - by providing a bowl for mud baths in the nest, we could observe 90-days old nestlings bathing in the mud. Also, bathing in the mud only happens in captivity if birds are not disturbed.
So we can use this behaviour as one indicator of the welfare of captive birds. Our observations suggest that in pairs, birds will take a mud bath only if the pairing process is developing well. If captive bearded vultures do not feel well in their aviary, are disturbed by the visitors (short security distance), or are threatened by their own partner, you will not see them bathing in the mud.
So to determine if pair bonding of a new couple has occurred, it is recommended to offer a bowl with mud rich in iron oxide. You can see this in the photo below, from the Academie de Fauconnerie du Puy du Fou, taken last month. Only a few days after being offered for the first time a mud bowl, the female started taking baths, confirming the good relations between the pair.
Do not be alarmed if suddenly a red chick appears during the rearing period: wet adult birds can colour their chicks by brooding – see photo!