Members of the Anatidae family, ducks are aquatic birds that come in a variety of species. Generally monogamous, ducks can be found in a number of aquatic habitats ranging from subarctic to tropical. The most common of the types of water fowl, ducks have an oil gland under their feathers which allows the water to roll off their back without drenching the feathers and the skin underneath. Repelling water keeps ducks warm in even the coldest water. Another mechanism they have to keep warm is the lack of feeling in their feet. Ducks have no nerver tissue in their feet, so if the water is boiling hot or freezing cold, ducks would never know the difference.