Bird Body TemperaturesBirds are warm blooded animals that have a much higher metabolism, and thus higher body temperature, than humans. While the exact measurement varies for different bird species, the average bird’s body temperature is 40 degrees Celsius. Body temperature can fluctuate during the day depending on climate and activity, but it can be a challenge for birds to maintain such a high body heat when temperatures dip too severely. Smaller birds are particularly at risk, since they have a proportionally larger surface area on their bodies to lose heat but a smaller core volume to generate it.
Even the smallest birds, however, have several ways they can efficiently keep warm. Birds have many physical and behavioral adaptations to keep warm, no matter what the low temperatures of their surroundings.
- Feathers: Birds’ feathers provide remarkable insulation against the cold, and some bird species grow extra feathers as part of a late fall molt to give them thicker protection in the winter. The oil that coats birds’ feathers from their uropygial gland also provides insulation as well as waterproofing.
- Legs and Feet: Birds’ legs and feet are covered with specialized scales that minimize heat loss. Birds can also control the temperature of their legs and feet separately from their bodies by constricting blood flow to their extremities, thereby reducing heat loss even further.
- Fat Reserves: Even small birds can build up fat reserves to serve as insulation and extra energy for generating body heat. Many birds will gorge during the fall when food sources are abundant, giving them an extra fatty layer before winter arrives.
- Fluffing: Birds will fluff out their feathers to create air pockets for additional insulation in cold temperatures. This can make them look fat and puffy while they are toasty warm.
- Tucking: It is not unusual to see a bird standing on one leg or crouched to cover both legs with its feathers to shield them from the cold. Birds can also tuck their bills into their shoulder feathers for protection and to breathe air warmed from their body heat. Birds can not only tuck their feet in under their down, but they may also switch which foot is holding them on a branch. Bird feet are generally grabbing at rest, so it takes very little energy to stay attached to a branch
- Sunning: On sunny winter days, many birds will take advantage of solar heat by turning their backs to the sun (therefore exposing the largest surface of their bodies to the heat) and raising their feathers slightly. This allows the sun to heat the skin and feathers more efficiently. Wings may also be drooped or spread while sunning, and the tail may be spread as well.
- Shivering: Birds will shiver to raise their metabolic rate and generate more body heat as a short term solution to extreme cold. While shivering does require more calories, it is an effective way to stay warm in extreme conditions.
- Foraging: Birds may eat more, or selectively eat higher energy food during the winter. For small birds this may include storing up food during the warmer season.
- Roosting: Many small birds will gather in large flocks at night and crowd together in a small, tight space to share body heat like in dense shrubbery or trees. Even individual birds choose roost spots that may have residual heat from the day’s sunlight, such as close to the trunk of a tree or near any dark surface.