As temperatures dipped into the negative numbers last week, I started wondering—how do wild birds, some no bigger than my fist, manage to stay warm in such frigid conditions? Of course, some bird avoid the problem by migrating, but plenty of birds winter right here in Colorado. I already knew that birds eat more when the weather is cold; my need to constantly refill the bird feeder is proof enough. The suet feeder, with all that high-calorie fat, empties even faster. But could a higher metabolism be enough to carry such seemingly fragile puffballs through a Colorado winter? I decided to find out.
I was right about birds eating more in cold weather. Their metabolisms are much higher than ours, and can burn all that food to produce considerable amounts of heat. In fact, they start eating more long before winter arrives, storing up a layer of fat that they can draw on when needed.
Producing extra heat is a good start, but it’s just one strategy. I learned that feathers make excellent long underwear. Many species grow extra feathers at this time of year—their way of putting on a winter coat. Then, in addition to the familiar plumage that we use to ID different species, there is an under-layer of downy fluff. When birds get (aptly-named) goose bumps, those downy feathers stand up, creating a toasty air space next to the birds’ skin. If you’ve ever snuggled under a good quality down comforter, you know just how effective those feathers can be.
To go one step further, birds tuck their legs and/or heads into their feathers, minimizing exposure to the cold. This is just one example of a variety of common sense behaviors that help birds survive the winter. Huddling together in a hollow tree (or roosting box), getting out of the wind, or even something as simple as moving to the sunny side of the trunk are all effective strategies. This is one reason why birds appreciate our landscaping with dense shrubbery and evergreens.
When visiting wetlands, especially in the winter, I’ve always wondered how sandpipers, herons, and other wading birds manage to stand around in that frigid water. It turns out that there are a number of adaptations that keep heat from being lost. For one, the skin is covered with insulating scales. Then, birds have separate temperature controls for their bodies and their legs and feet. Their body can be warm while they allow their legs to be much cooler.
Even more impressive, the blood circulation to, say, a duck’s feet is arranged to recover as much heat as possible. The warm, downward-flowing vessels are right next to the cold, upward-flowing vessels, allowing the warm blood to give its heat to the cold blood, minimizing heat loss in the feet. Imagine walking on ice barefoot, as this Northern Shoveler is doing!
I thought that a restricted blood flow would injure the tissues in these extremities, but again, that’s not the case. Rather, bird legs operate on what is essentially a pulley and lever system. All the muscles (think drumsticks) are high up in the feathered portion of the bird. All that the legs contain are tendons, which don’t need much of a blood supply. (Our hands and feet operate in much the same way.)
There’s one more desperate measure a bird can take to make it through a really cold spell. They can deliberately let their body temperature drop, conserving energy. The technical term for this is “facultative hypothermia.” Of course, there’s a downside to this—as their metabolism slows, their reaction time slows as well, making it more difficult to evade a potential predator. Still, if you are going to die from the cold, why worry about getting eaten?
The extreme example of facultative hypothermia is torpor. In this case, the bird is basically hibernating. It may appear to be asleep, as does this House Finch, or even dead, but will revive as its environment warms. Nighthawks and hummingbirds are two kinds of birds that enter torpor when it gets too cold, an ability that helps when they migrate too early or too late.
There are a few things we can do to help birds survive the next arctic vortex, such as making sure the feeders are kept filled with high-energy foods such as suet and black-oil sunflower seeds, providing nest boxes or roosting boxes, and offering liquid water (my electric bird bath receives a steady stream of customers). Learn more from my previous post on “Helping Birds Through the Winter.”