I don't understand why you think the chickens got skinny in the winter if they weren't feed store bought food. I grew up in Illinios, we had rough winters. My grandparents didn't buy store bought food for their stock. They raised hogs, cows and chickens. They grew their feed in the summer, chopped up the corn stocks and cobs, stored that, stored their wheat, corn, soybean and oats in the silos. Our egg production never went down much during the winter and the chickens would forage in the barns instead of out in the fields during the wintery days. Cow manure, pig manure were the favorite things to go forage around in. My uncle would get so mad because he mucked out the barns first thing, put all the manure in the bins for composting and when he would come out from having breakfast, there would be all the chickens just making a mess!! That was farm life. When I started raising my own flocks in the 60's we bought bulk grains for our chickens. We started getting mixed feeds in the late 60's early 70's because we were then breeding for eggs for the hatchery. Chickens can survive and be productive without all the junk everyone want to put into their feeds if allowed to forage. If penned, yes they need a good diet. don't be afraid to feed your chickens meat scraps and bones, they love them and it is good protien.