Where do chickadees sleep in the winter?

Today I was watching the little black capped chickadees at my bird feeder, and I couldn't help but wonder how they can survive this cold and nasty weather. I know they have to eat a lot of sunflower seeds, and I knew that they had the capacity to store food, like squirrels do, and remember where they left it.  What I learned today is that chickadees also have the capacity to lower their body temperature when it's cold so that they use less energy. At night they puff up their feathers to stay warm, turn down their body temperature and crawl into little crevices and holes in tree bark until morning. Who knew?

http://www.startribune.com/lifestyle/homegarden/12593411.html
http://www.adn.com/2010/12/27/1621803/where-do-chickadees-catch-zs.html