Understanding that I don't actually have winter, just a couple cold days each year, so I have no first hand experience, here's what the biochemistry says.
Corn is basically fat and easy carbs, it breaks down rapidly and provides plenty of calories. But the WAY the body breaks down doesn't generate any heat to speak of. It just provides calories/energy the body can use to generate heat through other processes.
Protein, otoh, is much more difficult for the body to turn into energy - and the effort the body puts into that generates more heat than is generated when the body converts a similar number of calories of corn into energy. Its less efficient a process, and that inefficiency is expressed as heat.
So, on a calorie : calorie basis, if you are concerned about your birds making heat in winter, you are better off offering 100 calories worth of protein than 100 calories worth of corn. That's the science.
Practically??? Even with US feed bags, you can't easily compare caloric values in your feed. The typical recommend is anywhere from 12 - 13.8 MJ/kg, but that number simply isn't present on most labels, and some specialty feeds can be significantly lower (10.5 "maintenance" for breeder Cx) or over 14.5 MJ/kg at the other end of the pendulum.
What would I do?
I would feed a nutritionally complete feed in the 20% protein range all the time, for reasons other than "winter", and not diminish their feed by the addition of low nutrition, high energy simple carbs like corn - which favors fat production (not a good thing, think about where a bird puts on fat!) and disfavors maintaining muscle mass, feather production, intestinal health, and all the rest.
