You really don't need to heat anything. But I'm curious - why do you want them in as small a coop as possible? Do you not want to take up much space in your yard? Do you not want to clean a larger coop? Think of it this way. In the summer you can go outside, "free range" if you will, enjoy what you're doing and have plenty of space. Now imagine yourself in the middle of a 3-day blizzard with raging winds and sub-zero temperatures. BUT - your keeper has decided that you only need minimal space and locks you in a closet for the winter. Or, you get lucky and get locked in the bathroom - with 3 other people. Not just during the blizzard, but for most of the winter because you don't really like being out in the snow and cold. You know what MN winters are like. You really should give your birds a large enough coop that they'll be comfortable in it should they choose to be inside for days or weeks on end.
