Maintaining a normal temperature range depends on the amount of heat they produce. How they conserve heat has an influence on how they maintain body heat. In nocturnal birds, such as owls and nighthawks, their body temperature is higher at night when they are most active. On cold, wintry days, most birds fluff up their feathers which creates air pockets helping to keep them warm. The more spaces, the better the insulation.
Some birds perch on one leg, drawing the other leg up to their breast for warmth.
To keep their metabolic rate high most backyard birds eat rich, energy foods such as seeds, insects, and suet. Most songbirds will fill a special storage pouch in the esophagus with food before dark and digest the food overnight. Some birds like hummingbirds and swifts lower their metabolic rate and body temperatures to conserve energy supplies. Almost all birds use shivering as a short term adjustment to the cold. It is the main way birds increase their heat production while they are at rest.