For the most part, it’s the insect-eating birds that migrate south. Those that stay behind are either seed eaters or insect eaters that have perfected techniques, which allow them to extract morsels hidden behind and between the bark of trees, as do woodpeckers and nuthatches.
It’s not difficult to observe birds preening themselves with their bills and feet to carefully clean, rearrange and oil their feathers. They do so, in part, to maintain flight capabilities, but in winter, the process is essential for heat regulation. Birds have a “preen gland” located on their rumps just below the upper tail feathers. Oil squeezed from this gland is rubbed over the body as a waterproofing agent.
Birds have over 25% more feathers in winter than during the summer months. Growing beneath the large, outer flight feathers are tiny, tuft-like, down feathers that provide one of the world’s most effective heat traps. It’s the same stuff humans have adapted for use in hats, coats and other cold-weather apparel.
When fluffed and preened into position, these feathers trap a layer of warm air next to the bird’s body that prohibits the loss of body heat. At night, or when it’s really cold during daylight hours, birds tuck their heads back under their body feathers into this warm-air source. This head tucking allows them to breathe pre-warmed air and further cut down on energy expenditure.
But what about their bare legs and feet? Well, you’ve no doubt observed a bird standing one-legged on a bare branch. The missing appendage was lifted up beneath the lower feathers into that warm-air zone. The exposed foot is protected by a physical adaptation ornithologists call the “counter-current heat exchange system.” In this system, leg arteries and veins are placed side by side so that heat in the arteries coming directly from the heart warms the chilled blood in the veins and keeps the lower extremities unfrozen. Beaver tails, whale fins and many other types of exposed animal limbs are protected in this fashion.
Making it through the night is the most challenging task facing birds during the winter months. Like humans, birds shiver involuntarily as a warming reflex. When all else fails, they huddle and snuggle together. Finches, sparrows, crows, jays and doves roost in the dense conifers to reduce heat loss. Species such as brown creepers, white-breasted nuthatches, winter wrens and bluebirds sometimes join one another in bird boxes or tree cavities.
There are birds in other parts of the world that actually hibernate, like woodchucks, snakes and other animals. Here in our region, the chickadee is the bird that comes closest to utilizing this technique. This process — which has been called “controlled hypothermia” and “overnight hibernation” — reduces the rate of heat loss from the bird by reducing the temperature difference between a bird’s body and the surrounding air. Shivering is stopped, so that body temperature drops until a level of hypothermia is reached. On a cold night, a chickadee can allow its temperature to drop up to 12 degrees, resulting in a large overnight energy savings. The only problem is waking up quickly enough from this torpid state when a predator happens along.