It's 10 degrees this morning, and soon the birds will arrive at the feeders. It amazes me that birds, who weigh so little, can tough out the winter here in Wisconsin. How do they survive?
Some birds grow extra feathers to keep them warm, doubling the amount they have in summer. Chickadees have about 2,000 feathers in winter. Other birds roost together in cavities or nest boxes to share warmth. Fluffing out their feathers allows them to trap air between them, keeping them even warmer.
When it gets close to zero, you may notice birds holding their feet up in their feathers to warm them. The way their feet are designed also helps them stay warm. Warm arterial blood descending from the heart meets a network of veins carrying chilled blood up from the feet. They are so close to each other that some of the warmth passes into the veins to warm the returning blood, while the rest keeps the feet warm.
Birds also shiver a lot in the winter. Stan Tekiela, a Minnesota naturalist, writes that shivering helps them produce heat at five times their basal rate, allowing birds to maintain a normal body temperature of 106-109 degrees when the air temperature drops to 70 degrees below zero.
Other birds take more extreme measures. As the temperature drops, they allow their body temperature to fall 10-12 degrees and enter torpor, a state in which they lose consciousness and can survive a temperature difference of 100 degrees between their bodies and the air. Hummingbirds, who migrate south for the winter, enter torpor at night to survive the cold at higher elevations. When morning arrives, they start shivering to raise their body temperature, and regain consciousness.
Birds must find food quickly on cold mornings to replenish the fat they lost the night before, or they will die. You can help them by putting out high-fat foods such as black oil sunflower seeds and suet. Making sure they have accessible food is especially important in snowstorms and blizzards. If you can, use something to block the snow and create a haven where they can come and eat.
photo by Yuri Timofeyev
