Full feathered is fine. I believe if chickens are in a bad moult, that is another story. I haven't had full feathered chickens shaking in the cold but I have had moulting ones do so. I'll use heat on moulting ones. They get right under it and make it clear that they don't want to be messed with (like when on the nest) until warmed up. Then they go about their normal business. Just pay attention to the birds, their condition and keep them dry and draft free at night.