Migrate in winter - Birds

Why do birds migrate? They are not directly affected by the cold because of their feather covering and warm blood. But in winter getting food is not easy. So they migrate. In cold countries snow lies thick on the ground in winter and even lakes and rives are frozen over. The winter is such that birds will not be able to catch either insect of fish. If they do not migrate they will perish. During the cold season night are long, days are short and the time available for search food is short.