This vulture's diet is up to 90% bone, and it dyes its own feathers red

The bearded vulture's diet is up to 90% bone and bone marrow, the highest reliance on bone of any vertebrate, digested by stomach acid roughly as strong as battery acid. Bones too large to swallow are carried aloft and dropped onto rocks to shatter them, and adults deliberately rub iron-rich mud into their pale feathers to dye them rusty orange.

— Antoni Margalida et al., Bearded vulture bone-eating ecology and cosmetic plumage colouration — ornithological field studies