This question is partially prompted by this question and the two "how to cook turkey" links therein. The question and the links seem to implicitly assume you cook your turkey on its back, with the breast upwards.
I rarely cook whole turkeys, but I quite frequently cook whole chickens and sometimes pheasant. As I really dislike dry breast meat I always cook them breast down, so the breast is sitting in the juices and so does not dry out. From the links it seems that getting the breast cooked may be an issue, but I think sitting in the juices that are close to 100 °C ensures the breast is cooked because of the higher heat transfer through liquids than the air. I tend to check it is cooked by looking for liquid blood and the only places I ever find it is around the joint capsules of the leg, principally the hip but sometimes the knee.
What are the considerations here? Why do most people cook turkeys at least breast up? What about smaller birds, how do people usually cook them? Does wild/farmed make a difference, game tending to be far leaner?