Ducks are warm blooded. This means their bodies try to keep a constant internal body temperature. Water birds, such as ducks, have a unique process that helps maintain a warm core.
The circulatory system of ducks helps keep their core body temperature from dropping through a special process in their upper legs. Some of the warmth of the arterial blood going to the feet is transferred to the blood returning in the veins. Therefore, the feet submerged in cold water have little heat to lose. Unlike some other birds, ducks' feet do not have a thick layer of fat, nor do they have feathers. However, through this "countercurrent heat exchange," ducks are able to safely paddle in frigid water.