I've been raising cochins for several years now and this looks atypical to me. At least I have never seen it in any of my birds. (Looks like these two little cockerals, BTW.)
They look healthy the best I can tell. Cochins can be pretty docile compared with some other breeds, and may end up at the bottom of the pecking order. Do you think they are getting enough to eat? Or have you noticed the others picking at them? If the others are all the same breed they may do this just because these five are "different". Make sure you are using a good start and grow with enough protein to grow in full, healthy feathers.
Good luck!