I noticed that my flock lost quite a bit of weight after our temperatures were between 0 to -13 degrees for about 5 days. I figured I was doing a good job of letting them plump up, but after the cold snap, it was like they hadn't eaten which I know wasn't the case. My birds choose not to go out...
You should probably have an area that your birds can have that has protection both from ground predators and predators from the sky. I have an enclosed area and I used to just let my chickens free range all day everyday anyway. As winter approached I had an increasing number of predator...