As you sit in your cozy warm house, blanket on your lap, the outdoor temperature in single digits, do you ever look out your window at your backyard birds and wonder, “How are they not freezing out there?” I have, so I decided to investigate.
A small songbird needs to have an internal body temperature around 105-107 degrees Fahrenheit in order to maintain activity in cold weather. They do this by generating high metabolic heat. Here are some ways in which they accomplish this, along with other physiological abilities and behavioral strategies that help them survive our cold winter months.
Birds purposely shiver to keep warm. They contract their muscles, which increases their metabolism. High metabolism generates heat leading to a higher core temperature, even in cold environments.
Just like our fingers and toes, birds’ bills and legs are vulnerable to cold. Canada geese, for example, tuck in their bills and legs against their warm body feathers as a strategy to reduce heat loss in those areas just as we may tuck our hands into our pockets.
Birds fluff their feathers to keep warm. They raise their feathers to create air pockets that trap and warm air for insulation. Some birds, such as American goldfinches, grow more feathers in the winter giving them more to fluff.
How can birds stand on a frozen lake for hours at a time without freezing? It is due to countercurrent exchange. The arteries moving from the heart to the feet hold warm blood. The blood becomes cold when it enters the feet on the frozen lake. The cold blood from the feet going up the veins to the heart are located so closely to the arteries holding warm blood that the arteries warm the cold venous blood. This helps the body to retain heat and stay warm.
Birds can waterproof themselves. They rub their beaks against the uropygial (preen) gland near the tail base picking up natural oils, which they spread while preening. This creates a water-repellent, insulating layer keeping them warm and dry. Think of it as their natural version of Vaseline. Providing a heated bird bath can help keep the birds clean, thereby making it easier for them to spread their insulating oils when they preen.
Many birds huddle together at night to keep warm. Staying close to each other allows them to share body heat and conserve energy. Some birds, like nuthatches, survive by joining mixed flocks led by chickadees or titmice — not only for warmth but also because more eyes in a group make food easier to find and predators easier to spot.
Sheltering together in cavities, dense shrubs or brush piles protects birds from the wind and cold. It’s the season when your leftover Christmas tree would provide great shelter throughout the winter. What a wonderful way to keep the holiday spirit alive while helping birds.
Most of our backyard winter birds weigh between 10-30 grams, the weight of two to five nickels. To survive long cold nights, they gain significant weight in winter with fat making up 10% or more of their body weight for insulation and fuel. They may use 75-80% of their fat reserves in one night, needing to replace it daily. High-fat, high-energy foods like suet and black-oil sunflower seeds help to replenish this loss.
To ensure food is available to increase their fat intake, some birds like blue jays and red-bellied woodpeckers store food, known as caching. They use incredible spatial memory to recall thousands of hiding spots for later retrieval giving them a steady supply of food throughout the winter while I can’t remember where I put my glasses.
Some small birds, such as kinglets and chickadees, enter a deep sleep-like state, called torpor, where they lower their body temperature and metabolism to save energy. I’ve seen a hummingbird in a torpor state. It was hanging upside down and looked like it was dead, but it was still holding on to its perch. If you see a bird like this, you should simply leave it alone.
When comparing birds to humans, bird cells contain less water making them less prone to frostbite. While humans have more sweat glands in their feet than any other part of the body, bird feet have none, keeping them dry. Humans have countercurrent exchange, but it is not as specialized or efficient as birds’. Of course, birds have feathers to provide insulation; we have to buy down coats and blankets.
With my furnace turned up, I sit astounded at having learned about these remarkable physical abilities and coordinated strategies birds use to handle the most frigid temperatures. I help by providing them with high-fat food, shelter and a heated bird bath. I hope you, too, find our winter birds amazing and join me in lending a helping hand.
Lisa Jansen is a Wisconsin master naturalist, nature writer and award-winning photographer. She specializes in raising awareness of birds and butterflies of the Midwest.
