here's a random idea/thought....
when i bred rabbits i kept them in large groups (bear with me, it'll make sense i promise!)
occasionally there would be the odd scuffle or scrap. with the black rabbits, and ONLY the black rabbits, where they had had a small injury or scratch the fur would come back white...
now being a cockerel could he have been scrapping and are minorcas black?
maybe the black bunny theory could apply to birds too?