Why don't ducks freeze in the winter?

Photo by Derek Ramsey from Wikimedia.

Ducks have several traits that let them live in snowy weather, as  long as they can find food and water. 

Ducks have a special blood system that helps keep them warm on cold ground.  Warm blood leaving the duck's heart heats up cold blood coming from the duck's feet.  The paths for the warm blood and the cold blood come very close to each other.  This is called countercurrent blood flow.  It keeps the duck warm in the middle and the head, where it matters most. 

Ducks also have a lot of fat and feathers to keep them warm, just like a jacket.  Most of a duck's organs (like the heart and lungs) and muscles are under the fat and feathers.