... You don’t see many birds in winter. Most have left your area. Those that stay are not as active. Activity uses energy that is needed to keep warm. The worst problems for birds in winter are getting enough heat and holding on to the heat once it is made. These are problems for all birds. But it is especially true for very small ones. They cannot find enough food. The weather stays so cold for so long that they cannot eat enough to keep alive. But birds have many ways of fighting the cold.

You shiver to keep warm. The heat that you make is made mostly in your muscles. The muscles make more heat when they are active. So one way of keeping warm is to move about, use your muscles. Another way is to shiver. When your body needs heat, the muscles tighten and loosen quickly. They become active. Just as you shiver to keep warm, so do birds.