Surviving the Cold by Rick Hollis
A question that has been popping up a lot on various birding listserves is how do birds survive when it is so cold. A couple of methods have to do with not letting their body cool off too much. There are several ways birds keep their feet from freezing off in really cold weather.
One way is behavioral – instead of standing on their feet, once on the snow they pull the feet up into their feathers and sit on their feathers. Look at the picture of the Dark-eyed Junco. See how it is resting on its belly on the snow.
The second way is a technique known as counter-current heat exchange. The warm blood from their bodies travels in arteries that pass very close to their veins as it travels from their bodies to their feet. As is passes next to the cool blood in the veins, heat is exchanged. The veins entering the body warm from heat they get from the arteries. They do not cool the body down as much as you think. Arteries entering the feet are cooled before they get to the feet, therefore not losing as much heat as they would otherwise. The diagram shows this.
Sometime I will address other ways birds survive the cold