How do ducks keep their feet from freezing in cold water?

John Wyman from Stoney Creek, Ont. asks: I understand how feathers protect water birds from almost freezing water, but how do they keep their feet from freezing?

Biology professor Erica Nol from Trent University in Peterborough, Ont., says ducks and other water birds adapt to cold weather by using a counter-current heat exchange system between the arteries and veins in their legs. It differs from how mammals keep their body temperature about the same throughout the body. 

In ducks, veins that take blood from the extremities back to the heart, surround the arteries that take blood from the heart and pump it away to the extremities.

As the warm blood comes from the heart, it goes through the body and to the extremities, particularly the legs, and encounters veins. The warmth from the blood passes through to the surrounding vein and brings warm blood back to the body cavity.

As the blood enters the legs, it cools quite significantly. The feathered part of bird can be 25 C while the legs dip to 1 C, Nol says. The bird therefore doesn't lose much heat to the environment.

The birds' legs also have tendons and bones instead of muscles and nerves like in mammals, which need more oxygenated blood.