My feet get uncomfortably cold very easily. If I spend even a short time outside in freezing conditions, even while wearing insulated boots, my toes go numb and my feet start to ache. I think it’s a genetic thing. My parents used to complain about their cold feet while standing around various ice rinks while I was playing hockey as a kid. And now I have the same problem.
Yet ducks, geese and a number of other birds are apparently unbothered by the cold as they stand on ice-covered ponds in their bare feet. What do they have that people don’t have? It’s one of the most common questions I get about birds at this time of year. Why don’t they rock back and forth, stomp their feet, or use any number of other strategies that humans employ to keep their feet warm?
The answer, according to University of Rhode Island ecologist Scott McWilliams — my go-to guy for all bird physiology questions — has a lot to do with blood circulation. The birds have what McWilliams calls a counter-current heat exchange system between the veins and arteries in their legs. As warm arterial blood flows down their legs toward their feet, it passes near the cold blood in their veins that is returning from their feet. The blood going down to their feet warms up the blood that’s going back up, and as it does so, the blood going down drops in temperature. As a result, the blood that flows through their feet is quite cool — just warm enough to avoid frostbite. By reducing the difference in temperature between their feet and the ice, the ducks lose little heat through their feet.
“It’s a common solution to the problem of keeping your body core warm and not having to expend a ton more energy trying to keep your entire body warm, including your extremities,” McWilliams said.
Ducks also have downy, waterproof feathers and a thick layer of body fat to keep them warm in freezing conditions. If the weather gets extremely cold and the birds feel the chill, they have several additional options. They can stand around on one foot while tucking the other foot in their feathers to keep it warm, or they can fluff up their feathers to trap more of their body heat, which provides an insulating blanket around their bodies.
Like almost all other birds that live in a cold climate for at least part of the year, ducks can also slow their metabolism to conserve energy. For tiny birds like kinglets, this strategy can save as much as 20 percent of their daily energy budget.
Birds also have something called brown fat, which is designed to produce heat through a biochemical process, much like humans do by shivering. And then there’s McWilliams’ favorite strategy — huddling. Many small birds will gather together in tight groups during chilly nights to share their body heat.
While I understand how these physiological adaptations enable birds to survive most winters unscathed, it still amazes me that the adaptations do the trick when temperatures remain well below freezing for weeks at a time, like that two-week stretch around New Years. It’s especially impressive that tiny birds like chickadees make it through such cold snaps.
Still, if birds can spend the whole winter outside, why can’t I last for more than an hour or two? Maybe I need to grow some downy feathers. Or brown fat.