If you were one of the many bird watchers who fell into the Polar Vortex and temperatures dropped to nearly 30 below you may have wondered how birds survive such brutally cold temperatures. I sure did, I spent much of the cold spell sitting in a cozy house, furnace on, wrapped in warm fleece, with favorite cup of coffee.
On days like these I am always astounded that there are any birds left alive, especially considering that most winter feeder visitors weigh in around 10–25 grams (the weight of 2-5 nickels)! But it turns out that birds employ many of the same strategies I was using inside my house plus a couple more to keep their motors running through cold snaps.
So how do they survive?
Bird Body Temperatures
Birds 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 105 degrees Fahrenheit (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.
What Wild Birds Do to Keep Warm
Birds have many physical and behavioral adaptations to keep warm, no matter what the low temperatures of their surroundings.
Physical Adaptations
- Feathers: Birds’ feathers provide remarkable insulation against the cold, and many 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 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.
Behavioral Adaptations
- Fluffing: Birds will fluff out their feathers to create air pockets for additional insulation in cold temperatures.
- 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.
- 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.
- Roosting: Many small birds, including bluebirds, 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.
Torpor
Many birds will enter torpor to conserve energy during cold winter nights. Torpor is a state of reduced metabolism when the body temperature is lowered, therefore requiring fewer calories to maintain the proper heat. Most birds can lower their body temperature by a few degrees, but torpid birds have lowered their body temperatures by as much as 50 degrees. Torpor can be a dangerous behavior, however, as the reduced temperature also leads to reduced reactions and greater vulnerability to predators. Hummingbirds, chickadees, swifts and other types of birds regularly use torpor as a way to survive cold temperatures.
Helping Keep Birds Warm
Even with all these adaptations to conserve heat and stay warm, many birds still succumb to frigid temperatures and bird mortality can be very high during severe winters. Birders who know how to keep wild birds warm in winter can help their backyard flocks have an edge over the cruelest weather.
- Offer Good Food: Choosing the best winter bird foods to offer means selecting seeds, suet, scraps and other items high in fat and calories to give birds plenty of energy to generate sufficient body heat.
- Keep Feeders Full: After a long, cold night birds will need ready access to food to replenish their energy reserves. Keep your birdfeeders full of nutritious seed no matter what the weather so the birds know where to go for a high energy meal.
- Offer Liquid Water: 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: Plant evergreen shrubs and coniferous trees that will provide suitable shelter throughout the winter, or build a brush pile to give birds a safe, sheltered place to roost. Adding a roost box to your yard is also helpful.
When temperatures start to dip, it isn’t necessary to worry about how birds keep warm; they have plenty of efficient adaptations to survive even the chilliest nights. Birders who understand those adaptations and help birds with even better food, shelter and other necessities, however, will be sure to enjoy warm and healthy winter backyard birds no matter how cold it is outside.
Winter Birds Myths and Facts:
When it comes to winter birds, it seems there are more myths than usual. Here are a few of the common ones I’ve heard. Hopefully, I can help debunk these winter birds’ myths once and for all with the correct winter bird’s facts.
Winter Birds Myth: Birds will freeze to death when temperatures drop far below zero. Birds are well equipped to survive the coldest of temperatures. They store fat during the short days of winter to keep themselves warm during the long nights. During those freezing nights, they fluff their feathers to trap heat and slow their metabolism to conserve energy. They also look for good places to roost, whether it’s a birdhouse, natural tree cavity, grass thicket, evergreen or shrub.
Winter Birds Myth: Birds’ feet will stick to metal bird feeders and suet cages. Most suet cages have a laminated covering, so you don’t have to worry about birds’ feet sticking to it. But in general, their feet can endure cold weather. Birds have a protective scale-like covering on their feet, and special veins and arteries that keep their feet warm.
Winter Birds Myth: Woodpeckers drill on house siding in winter for food or to create nesting cavities. Though there are cases where woodpeckers find food in wood siding (and may even nest inside the boards), nearly all the drilling in late winter is done to make a noise to court mates. This is their way of singing a song to declare territory.
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