According to this;
Bearded Vultures, naturally, have black and white plumage. Their feathers have no other colors, and the only natural colors they have are in their orange eyes, and the red ring around them (called the scleral ring). The color of the scleral ring actually comes from their blood, which they can force into the ring to make themselves more intimidating.
So where does the rusty orange and fiery red plumage come from?
It can’t come from their diet, like flamingos, as these guys eat bones and bone marrow.
It comes from the dirt. Or, more specifically, it comes from the iron oxide found in red dirt. These guys will take dust baths and preen the color into their feathers, very intentionally. Birds observed while in captivity have been seen doing this for almost an entire hour.
While there is no solid evidence to support most theories, the most commonly held belief for why they do this is status. Older birds tend to have brighter colors than younger birds, and females seem to have brighter colors than males. Observers believe that these birds just really like the color red; not only are they attracted to the red dirt, they also seem to like red wood and red leaves.
These grooming rituals are now instinct in the species; individuals have colored their feathers with no outside prompting.