Yes, duck boots can very effectively warm your feet! There are three reasons for this. The first reason is that rubber soles prevent water from penetrating the shoe and your feet always stay dry in them, which enhances the feeling of warmth.  Another reason is that these boots are not breathable, so they do not let in cold air, but they retain the body heat of the feet created by body movements. Some models of duck boots, especially those designed to be used in winter conditions, have insulation from i.e., micro-fleece lining. It additionally warms your feet.