It gets down to -25 C here in Bulgaria during winter with snow easily a metre deep, we put extra hay in the hen house/coop and spread some around the ground for when they come out so their little tootsies do not get cold (they refuse to wear slippers!), we have put vaseline on their "exposed bits" previously but was not sure if it actually had any effect. Only one slight case of frostbite but nothing serious that was down to lack of ventilation in the coop which we soon rectified. I think sometimes we worry too much as they are adaptable and hardy birds.