As a warm blooded creature, a passerine (songbird) has a body temperature that varies between 102 degrees Fahrenheit and 112.3 degrees Fahrenheit. During periods of extreme cold, its temperature may drop drastically. How does a bird control its body temperature so it won’t freeze when the ambient temperature plummets below the freezing mark? How does it find warm places to sleep?
A bird’s coat of feathers forms its primary defense against cold. Passerines’ feathers are so thick that very little skin is exposed to air, which prevents body heat from escaping. A system of muscles connected to each feather enables a bird to raise or lower its feathers. When the environmental temperature falls, a bird “fluffs” its feathers much the same as we “fluff” a down jacket increasing the air spaces. The more air spaces, the better the insulation.
Birds have other means of retaining heat. Sometimes a bird draws one leg up into its belly feathers. Ground feeding juncos, sparrows, and some finches lower their bodies to ground level to cover their feet with feathers. You also may see a bird with its head and bill tucked back into its scapulars, or feathers along its shoulder.
A bird can increase its body heat by stimulating muscle activity through shivering. A bird that shakes all over is revving its engine to stay warm.
Some birds – bluebirds and chickadees, for example- sleep in groups on extremely cold nights, transferring heat from one body to the next. In some instances, birds find tree cavities or nest boxes for this sort of group sleeping.
Some species – such as swifts, nighthawks, and hummingbirds – reduce their energy requirements by going into a state of torpor. This lowers their body temperature by several degree to a level near that of the air.
Most birds that come to our backyard feeders stock up on seed and suet shortly before retiring for the night. This fuel stokes their fire so to speak.