By LESLIE BISHOP, guest columnist
This winter has been confusing. One day I take a hike and wear only a light sweater, and then the next day I must bundle up in heavy coat and mittens.
Today I sit in my warm sunroom and watch birds at my feeder. The temperature is dropping and snow is expected.
Most of the birds visiting the feeder are small, weighing between 10 and 25 grams (which is equal to 2 to 5 nickels), and they seem vulnerable to changing weather conditions.
How do these tiny songbirds manage to survive the winter? The first line of defense against the cold is the most obvious: their coat of feathers. A bird’s marvelous coat of waterproof feathers prevents the loss of that internal heat.
Birds have several types of feathers: wing feathers designed for efficient aerodynamics, sturdy tail feathers to help steer during flight, and body feathers designed to keep the bird warm and dry. The top part of the body feathers has tightly interlocking microstructures that create a smooth surface as a barrier to cold. At the base of each body feather is down that traps warm air close to the body.
Hidden underneath the body feathers are semiplume feathers and downy feathers that are loosely structured for additional insulation. On very cold days, you may see a bird with feathers puffed up: this behavior creates additional air pockets to trap even more warmth.
Even with a warm feather coat, birds would become dangerously cold if their feathers got wet. Birds must groom (preen) the feathers frequently to keep them smooth, free of debris and oriented correctly. During preening, a bird will apply a waxy substance from its uropygial gland to each feather for waterproofing.
Even the thickest coat of feathers, however, will not keep a bird warm unless its body generates enough heat. Birds, like mammals, are endothermic, which means that they generate heat internally using their metabolism.
The challenge of endothermy is that it requires a lot of energy to generate heat. A drop of 200 can double a small bird’s metabolic rate. Thus, birds in winter need to stock up on foods high in fats, like the black-oil sunflower seeds and suet at my feeder.
Birds seem to know when a cold front is moving in. As air pressure changes, they will feed voraciously and in larger flocks. Nighttime poses extra challenges for birds during very cold weather, and they must rely on the energy store from food eaten during the day.
Many small birds like White Breasted Nuthatches will collect extra food during mild weather and cache (store) it for harsher days. They will methodically poke seeds under loose tree bark, and then camouflage their cache with pieces of moss or lichen. As the temperature drops, they return to their hiding places to retrieve their extra food.
Birds also use behavioral strategies to minimize the loss of the heat they produce. They adjust their position to minimize exposure to cold; for example, birds will move to the leeward side of a tree to avoid the wind. Some birds, like chickadees, will roost in clusters to generate group heat. Many birds roost in tree cavities, thick hedges or evergreen boughs for further protection from the wind and cold.
As the snow falls, I watch the cardinals vie for space on the feeders, the blue jays attempt to open peanuts, the mourning doves peck at the millet on the ground, and the woodpeckers cling precariously to the suet cage.
Each bird is a marvel to me with its unique behaviors and adaptations for surviving in winter. I never get tired of watching!
Leslie Bishop is a Brown County resident and retired biology professor from Earlham College. She is a volunteer interpretive naturalist at Brown County State Park. She can be reached through the newspaper at [email protected].
