Throughout winter, many Alaskans participate in various outdoor activities like skiing, skating, snowshoeing, ice fishing and snowmachining. A common problem for all participants is keeping their feet warm. There are a variety of possible solutions for cold feet: waterproof and breathable boots, extra socks, highly insulated boots and even heat-generating chemical or electric warmers.
But what about the waterfowl that remain along the rivers and streams all winter? Do they make Bunny Boots for ducks?
There are several species of ducks, mergansers and even swans that frequent the Kenai River watershed all winter long. They can usually be found in or near open water sections because that is where they can feed and it provides them protection from terrestrial predators like fox, coyotes, wolves and lynx. Nonswimming ducks can be found standing or lying on ice for long periods of time and they do not freeze. What’s their secret?
Waterfowl actually allow their feet to get very cold, keeping them just above freezing temperatures. Since their feet contain mostly bones and tendons surrounded by thick skin, there isn’t a critical need for the feet to stay any warmer. All of the muscles moving the feet are in the upper legs, and they are maintained at normal body core temperatures of 102 to 106 degrees Fahrenheit.
If we immersed our feet in icy water for any length of time, our body core temperatures would drop drastically. Waterfowl do it all day long and are easily able to keep their core temperatures fairly steady. How?
Physiologically, waterfowl have a special blood vessel system at the start of their legs called a “counter current exchanger” that warms the blood returning from the feet. Cold venous blood coming from the feet runs very close to warm arterial vessels carrying blood toward the feet. The blood leaving the body is cooled and the returning blood is warmed. This way, very little heat is actually lost to the outside world from the feet.
While it might appear that their feet could actually freeze, the supplying arterial blood vessels can be dilated or constricted to allow just enough extra warm blood into the feet to keep them from actually freezing.
There are also several behaviors that these birds can use to help keep their feet warmer or to reduce body heat loss. One method is to stand on only one foot. When on one foot, the other can be tucked up close to the body and kept warm within the insulating feathers. If you observe this one-foot stance behavior, watch to see that they will periodically switch feet.
Another common behavior is for the waterfowl to rest completely on the ice with both feet tucked up into the breast feathers. Their breast plumage provides amazing insulation, and they are able to maintain their body temperatures while sleeping on the ice. Their heads and beaks can also be tucked under a wing or buried in the feathers to preheat the air they take in and to retain some of the heat they lose during respiration.
This superb insulating quality of waterfowl breast feathers is well known and used in popular winter clothing filled with duck or goose down. Perhaps the very first down jackets were made by a number of Alaska Native populations. Dozens of waterfowl breast skins were sewn together to make very warm winter coats. Examples of these bird-skin coats can be seen in a number of Alaska museums.
In the case of waterfowl, they really do have “cold feet and a warm heart.”
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the biology of the Kenai River watershed.