Key is ventilation, and the right amount of space. IF you have a tiny little doll house coop, it's not possible to provide the right kind of ventilation in the right place. And the size of the coop in relation to the number of birds becomes a huge issue. Many doll house coops are not only small regarding the number of sq. feet, but they are small in that they don't have adequate height. The perch is too close to the floor, not allowing adequate depth of bedding. The bedding serves 2 functions: It absorbs the moisture from the feces, and hopefully, the feces gets mixed into the bedding instead of laying on top of it where the birds step in it, and then track it into the nest boxes. It also acts as an insulating layer. If there are only a couple of inches of bedding, that makes for a cold coop as there is not enough mass there to hold any heat. The perch is also too close to the ceiling. Even if there is good ventilation, it will be up high. In a short coop, it may be at the level of the bird's heads as they roost. but if the doll house coop perch is less than 18" from the ceiling, the birds combs are very close to that ceiling, and their moisture from respiration collects at ceiling height. It can condense on the ceiling and rain down on them b/c of the heat provided by the bird's metabolism. Those perches also tend to be too close to the back wall. Know what happens when your body touches a cold surface? Ever tried to sleep on the ground when camping... without adequate bedding under you? That cold ground sucks all the heat out of your body. In a small coop, if the birds touch the back wall, or they are crowded to the point that they are touching the side walls, that can make them colder.
