I live in a an area with a mild winter. We might get to low 30s or high 20s for a little bit some nights. Most of the time, it's probably 45 or less in the day time.
The first protection I give them is to wrap tarp around their pen for pretty much the entire winter. They get light from overhead or from the uncovered side which is exposed in the day time but covered at night. This just prevents cold winds from blowing through the pen.
There are 2y times I'm really concerned. First, when it's both cold and wet (rainy days). For that time, I usually just make sure the birds can stay dry or if it's several days expected of rain/ice/snow, then I bring them in to the garage (which I know you couldn't do).
Secondly, when it goes to low 20s or teens, but if it's not a wet night/day, then I put them inside a smaller container in their pen. All of them being in closer quarters for a day or so, will keep them warmer together even if outside was colder. Think of it like how a tent keeps a warmer inside temperature even though outside is colder.