Mottled also known as Progressive-Pied under other species is not yet an established mutation under Cockatiels. This phenomena has appeared in breeders collections all around the world and can be considered a collectors item. 

The pattern does not remain the same and changes each time a bird goes through a molt, it is believed that this mutation is linked to the early cell death of melanocytes and therefore as a bird ages the pied marking will progress throughout the plumage. 

This mutation has not reproduced itself yet and breeders are trying to establish this phenomena as a mutation at current date.