Simply because the best selections used today come from breeds with natural white plumage. A few decades ago, chickens that were not completely white, even with only a few coloured feathers, were observed to look ‘dirty’. Furthermore, ‘coloured’ chickens also have some coloured markings under the feathers. And this led to some rejection in the butcher’s shop because of the dirty feeling. Apart from the farms that continue to breed white breeds, today when people see a coloured chicken they tend to like it and even develop all kinds of fantasies about its origins.
