Why do older African gray parrots sometimes grow red feathers among their gray body feathers?

In the African gray parrot, the appearance of red feathers among the gray body plumage is a natural occurrence in older birds. While the species is characterized by its scalloped gray feathers and bright red tail, these random red feathers may emerge as the bird ages.1

It is worth noting that this trait has been selectively bred by South African breeders over many generations, resulting in the development of all-red birds that are highly valued in the pet trade.1