What you seem to be saying is that chickens are designed to be living within a specific temperature zone and that are not capable of and have not adapted to live outside of that zone.
No, that is not what I am saying. It is so obviously untrue.
Chickens obviously do cope with temperatures outside what would seem to be their comfort zone.
So far, the only data to be presented reagarding any physiological changes in chickens as an adaptation to a specific environmennt is the link Perris supplied which if one reads looks to me to be changes related to altitude rather than temperature.
I am saying, that until there is some measurable physiological change in chickens reputed to be cold hardy what would seem to be most likely is the chicken, because it hasn't had a choice in the matter copes. It either copes or dies.
Coping doesn't mean everything is just fine and the chicken wouldn't rather live in more suitable conditions.
I do have a friend in Finland who works with Landrace breeds and they forage in the snow and live outside and have what I think might best be described as accilmatised to their conditions, as in hardened off to them.
There is a world of difference between aclimatising to an environment and undergoing physiological changes that enable one to thrive.
The Husky dog can live comfortably in sub zero temperatures because it's fur is different to other dogs who live in more temperate climates.
