If you want to keep chickens warm when it’s cold, give them corn. As they digest the corn the heat generated (heat of digestion) will help keep them warm.
If your birds are indoors and you give them corn, make sure you also make grits available at all times. The grits will help to grind the corn in the gizzard. A better option though is to cover the sides with tarpauline or nylon whenever it is cold because if you give corn, you'll spend a lot if they're many.