How Birds Keep Warm in Winter
It always amazes me how such small creatures don’t freeze in the winter. Just looking out the window on a cold, blustery day in the winter makes me shiver. But birds have many awesome adaptations that allow them to survive even in the most frigid conditions, and understanding what birds need to keep warm can easily help these feathered friends.
Birds are warm-blooded animals that have a much higher metabolism and higher body temperature ( on average,40 degrees Celsius) than humans. Their body temperature fluctuates depending on the climate and activity level. When outside temperatures plummet, its a real challenge for birds to maintain their high body temperature.
So how do birds keep warm?
Birds have many physical and behavioral adaptations to keep them warm on the coldest of days.
- Feathers: A birds feathers provide insulation from the cold. Many birds grow extra feathers during their fall moult. Feathers are covered will oil during preening, helping further insulate the feathers and waterproofing them too.
- Fat Reserves: Birds build up fat reserves to serve as insulation and extra energy for generating body heat. Many birds will gorge themselves during the fall when food sources are abundant, giving them an extra fatty layer before winter arrives.
- Legs & Feet: A birds’ feet are made up of mainly bone, tendon, and scaly skin. Unlike humans, birds don’t have sweat glands in their skin to produce any moisture to freeze. Certain birds also have a heat exchange system in their legs. It is a specialized circulatory pattern to reduce heat lost through the feet when standing in cold water. Blood is diverted into a heat exchanger in birds’ legs by sphincter muscles to conserve heat. Arterial blood arriving from the heat exchanger in the leg is as warm as the body core temperature and the venous blood from the feet is nearly as cold as the surrounding air. Similar to a radiator, the heat flows from the arteries into the veins along the length of the heat exchange, ensuring that the venous blood will be warmed before returning to the rest of the body.
Birds will fluff up their feathers to create air pockets for additional insulation in cold temperatures. When fluffed up, feathers act as a buffer from the winds and wet. Sometimes, you will 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. This behavior is known as tucking.
Birds 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. Many small birds, including chickadees and titmice, will gather in large flocks at night and crowd together in a small, tight space to share body heat. They can roost in shrubbery or trees, and empty birdhouses and bird roost boxes are also popular locations to conserve heat. 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.
On sunny winter days, many birds take advantage of solar heat by turning their backs to the sun and raising their feathers slightly. This allows the sun to heat the skin and feathers more efficiently.
To meet the metabolic demands, many birds save enough energy to survive cold nights by lowering their internal thermostat at night, becoming hypothermic. This reduced physiological state is an evolutionary adaptation that is referred to as torpor.
Torpor is a type of deep sleep where an animal lowers its metabolic rate by as much as 95%. By doing so, a torpid hummingbird consumes up to 50 times less energy when torpid than when awake. This lowered metabolic rate also causes a cooled body temperature. A hummingbird’s night time body temperature is maintained at a hypothermic threshold that is barely sufficient to maintain life. This threshold is known as their set point and it is far below the normal daytime body temperature of 104°F or 40°C recorded for other similarly-sized birds.
Let’s Do Our Part
Birders can help their backyard flocks have an edge over the cruelest weather. Offer good food by choosing the best winter bird foods (seeds, suet, scraps and other items high in fat and calories to give birds plenty of energy to generate sufficient body heat). Keep those wild bird feeders full! After a long, cold night birds will need ready access to food to replenish their energy reserves. Birds can melt snow to drink if necessary, but doing so will use precious energy that is needed to maintain body heat. If the birds can drink from a liquid birdbath even in freezing temperatures, they will have a better chance at survival.Provide shelter by planting evergreen shrubs and coniferous trees. Build a brush pile to give birds a safe, sheltered place to roost. Adding a roost box to your yard is also helpful.
Birds know how to keep warm; they have efficient adaptations to survive even the chilliest nights. Birders who help birds with even better food, shelter and other necessities will be sure to enjoy warm and healthy winter backyard birds, no matter how cold it is outside.
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