Some birds, such a ptarmigan, grouse and common redpolls, use the insulative properties of snow to protect them from cold air. Shivering is another technique birds use to keep warm but it has a high energy price tag, because birds must eat large quantities of high-energy food to pay the debt. The benefit for the birds is that they can convert food into heat energy four to five times faster when shivering. They could not maintain the shivering if they did not have the food available. Birds' ability to increase their metabolic rate is believed to be triggered by the diminished light of winter.
Some birds use an opposite strategy, reducing their body temperature during periods of inactivity.
Susan Sharbough of the institute of Arctic Biology in Fairbanks studied black- capped chickadees and found that when chickadees are resting, they can reduce their body temperature and, therefore, slow down their metabolism. This abbreviated type of hibernation allows the birds to get through the long winter nights with sufficient reserves to fuel morning foraging activities. Most birds' legs are covered with scales, which do not protect them from the cold. Those staying the winter in cold climates have shorter legs than their relatives living in warmer regions and this structural strategy helps them conserve heat. Even with these adaptations for cold weather, mortality Is high for winter birds.