Trumpeter swans have incredible adaptations that allow them to survive in frigid conditions. One adaptation in particular allows swans (and other waterfowl) to stand in freezing cold water without the risk of frostbite or hypothermia.
How is it possible? Cold feet.
Veins and arteries are the vessels that transport blood throughout an animal's body. The heart pumps oxygenated blood into arteries, and veins transport deoxygenated blood back to the heart. In waterfowl, arteries and veins are wrapped around each other, allowing for what scientists call countercurrent heat exchange. Here is how it works: warm blood leaving the animal's core is slowly cooled by blood returning from extremities. Likewise, returning blood is slowly warmed as it re-enters the core. This allows the animal to keep its feet cold and its core warm.
Here is why cold feet matter: the larger the temperature gradient between two objects, the faster heat is lost. By keeping feet colder, swans reduce the amount of heat they lose to a cold environment.
Trumpeter swans are a conservation success story. Once hunted to near extinction, reintroduction programs have helped trumpeter swans make a comeback in North America.