A unique heat-exchange system in the birds’ legs known as counter-current circulation makes it bearable for this mallard to stand on ice.
How do ducks and geese rest, seemingly comfortable, in below-freezing weather on ice- or snow-covered ground in winter? How do they keep warm? And especially, how do they keep those bare feet and legs from suffering frostbite?
Well, it’s in the blood, the feathers and skin. Waterfowl have evolved physical and metabolic mechanisms that allow them to withstand, within limits, very cold temperatures.
The most obvious physical shield waterfowl use to stave off cold is a coat of insulating feathers. Feathers, of course, are lifeless, do not contain skin or blood vessels, and thus do not dissipate body heat into the environment. Instead, feathers actually retain body heat by trapping warm air near the skin. In addition to the flight feathers on their wings and the contour feathers covering their bodies, waterfowl possess thick layers of down, especially on their breasts and bellies. Down feathers are specialized, short fluffy plumes whose chief function is heat conservation. Anyone who has ever worn a down coat—especially one of prime goose down or eiderdown—in winter knows just how effective these types of feathers can be as insulators.
Beneath the feathers is the skin, which also plays an important role in regulating heat loss. The skin is suffused with blood vessels that carry warm blood from inside the body to the outer surfaces. Additionally, the dermis, or layer of skin immediately below the thin outer layer of skin, is a fat-storage site.
Dermal fat storage is prominent in birds such as ducks that live in cold aquatic environments. Fat reserves serve two purposes: They provide additional insulation, and they serve as an energy pool that can be drown upon during cold weather when metabolic demands are high and food shortages are most likely to occur.
Ever wonder how a mallard can stand comfortably on ice?
A unique heat-exchange system in the birds’ legs known as counter-current circulation makes this possible. The large, flat feet of waterfowl are natural radiators, so to minimize heat loss, the arteries and veins in the birds’ legs work in tandem to retain heat. Arteries supplying blood to the feet pass alongside the veins removing blood. The warm arterial blood flowing to the feet is cooled by venous blood flowing back to the body where it is warmed again. Consequently, very little of a duck’s body heat is lost through its extremities. Thus, while the core body temperature of a duck standing on ice is near 100 degrees Fahrenheit, the temperature of the bird’s feet may be just above freezing.
To further conserve heat in cold weather, waterfowl reduce the volume of blood flowing to their feet by constricting blood vessels in their legs. Experiments have shown that waterfowl gradually reduce blood flow to their feet as the air temperature drops to 32 degrees Fahrenheit (the freezing point). When temperatures fall below freezing, however, waterfowl again increase blood flow to their feet to prevent tissue damage. The birds also protect their feet by drawing them into their flank feathers and close to their body. To further minimize exposure in bitter cold weather, waterfowl often stand on one leg at time, tucking the other leg into their body feathers to protect it from the elements.
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