At times the birds will shiver to help stay warm. When it is cold, one of the places that some of the birds will find to stay warm at night are in tree cavities. The larger trees that are alive that have cavities, will hold the heat from the day longer then dead tree cavities.
Some smaller birds will group together in; shrubs, evergreen trees, logs, or nest boxes. By sharing their heat it will help them to save a lot of their metabolic energy.
The birds here are Carolina Chickadee's