On a very cold, snowy walk one recent morning, I wondered where the birds were.
Just the day before, I’d seen all the usual suspects–chickadees, titmice, nuthatches, and juncoes. ¬†They’d eaten from the feeder, flitted in and out of ¬†bushes, and hopped about on the ground. ¬†But this day was much colder, and they were ¬†nowhere to be seen. ¬†Except for an occasional crow.
I assumed these little birds were keeping warm somewhere, taking the day off from their usual foraging. ¬†But where were they?
And were they going without food? ¬†I had to wonder–if ¬†birds must eat voraciously every day in order to maintain their body weight, did the day’s extreme cold mean they had to sacrifice nourishment in order to keep warm?
And while I didn’t know the answers to my questions, I was not worried. ¬† Birds have been navigating harsh winter weather for a very long time. ¬†Still, I was curious. Where do birds go on the coldest days?
So I decided to do a little research.
I learned, not surprisingly, that small birds are more vulnerable to cold than large ones. Hence, the spotting of crows on that cold day, but not little guys. ¬†Because of small birds’ proportionally larger surface area compared to their heat-generating core, keeping warm is more challenging for them.
Fortunately, small birds have many strategies for keeping warm on the coldest winter days. They do what they do on winter nights. ¬†Chickadees, titmice, nuthatches, and woodpeckers, for example, gather together in a tight space, like a cavity in a tree, the dense branches of an evergreen tree, an old nest, a chimney, a birdhouse, or even in the space between a building and close-growing bushes. ¬†In such close quarters, birds huddle together and share body heat.
Packing into a small space with their fellows is not the only tool small birds have for staying warm. Chickadees, for instance, can significantly slow their metabolism, so that their bodies burn fewer calories. ¬†Their body temperature can drop by as much as 14 degrees F during very cold spells! Many birds can even control the temperature of their legs and feet separately from the rest of their body! ¬†By constricting the flow of blood to their extremities, birds can maintain more heat in their core.
Feathers also play a key role in small birds’ winter survival. ¬†Their oily surface is insulating, and some species even grow additional feathers during the fall to add greater warmth in winter.
And birds¬†use their feathers in ways that increase the warmth they provide. ¬†For example, many birds fluff out their feathers, creating tiny air pockets that hold warm air close to the body, like a down jacket. ¬†On sunny days, birds turn their back to the sun and spread their wings and tail, maximizing the feathered surface area that receives the sun’s rays. ¬†And birds crouch, covering–and warming–their feet and legs with their feathers.
Many birds fatten themselves up for the winter, building fat reserves by gorging during the fall. ¬†During extreme cold, a last-ditch mechanism–that we share with birds–may kick in–shivering. Shivering occurs when the metabolic rate is elevated, generating more body heat. ¬†But shivering is only a short-term solution, since it uses up a great many calories.
And I learned that, indeed, many birds do go without food on the coldest days. ¬†Apparently, the energy little birds would expend in foraging on such cold days exceeds the energy they would derive from the food itself. ¬†So, even though their nutritional needs are greater in colder weather, on the coldest days it makes sense for them to hunker down until the temperature rises.–April Moore