Why Don't Ducks' Feet Freeze? Cold Feet on Ice Explained

How countercurrent heat exchange works

A duck’s core body temperature is around 40–42 °C (104–108 °F), much warmer than ice or winter water. If warm blood flowed straight into bare feet standing on ice, the bird would lose huge amounts of heat through them, like a radiator left on outdoors.

Instead, in the upper leg, the arteries heading down to the feet split into a network of small vessels that lie tightly packed against the veins returning to the body. In many waterbirds this bundle forms a net-like structure sometimes called a rete.

As warm arterial blood flows down, it passes heat to the cold venous blood flowing up beside it:

• Blood going down cools gradually, so it arrives at the feet already cold. Cold feet lose much less heat to the ice.
• Blood coming back is warmed on its way up, so it does not chill the duck’s body when it returns.

Because the two flows run in opposite directions, heat transfer works along the whole length of the exchanger. Engineers use the same principle in industrial heat exchangers and ventilation systems that recover warmth from outgoing air.

Feet that tolerate the cold

Duck feet are well suited to running cold. They are made mostly of bone, tendons and tough, scaly skin. The muscles that move the toes sit higher up the leg, inside the warm feathers, and pull on the toes through long tendons. With little muscle and few nerves in the feet, there is less living tissue that needs warmth.

The blood supply can also be adjusted. When it is very cold, ducks narrow the blood vessels to the feet to cut heat loss further, while still sending enough warm blood to keep the tissue from freezing. The result is a foot held just above freezing, perfectly able to paddle, walk and grip ice.

In hot weather the system works in reverse. The same bare feet become a cooling surface, and ducks and geese can send more blood to them to shed heat.

Why not just keep the feet warm?

It might seem simpler for a duck to keep its feet as warm as the rest of its body. The problem is cost. Heat flows from warm to cold, and the bigger the temperature difference, the faster it flows. A warm foot pressed against ice would drain body heat rapidly, and the duck would have to eat far more food to replace it, which is hard in winter, when days are short and food is scarce.

By letting the feet run cold, a duck keeps that temperature difference small and loses only a modest amount of heat through its legs, even when standing on ice for hours.

Behavior helps too

Waterbirds back up their plumbing with simple habits:

• Standing on one leg halves the area of bare skin touching the cold and lets the other foot warm up in the belly feathers.
• Sitting down on the feet covers both feet with a warm, insulated body. On ice, a resting mallard often looks as if it has no legs at all.
• Tucking the feet up into the flank feathers while floating on cold water, paddling with just one foot or simply drifting.
• Tucking the bill into the back feathers, since the bill is the other main bare surface.
• Seeking open water, which in winter is often warmer than the air. Flocks on a partly frozen lake gather in the last ice-free patches, where water stays close to 0 °C (32 °F) even on much colder nights.

Waterproof, insulating plumage keeps the rest of the body warm; see how do ducks stay dry? for how the feathers do their part.

Geese, gulls and other cold-footed animals

Ducks are not the only animals with this system. Geese such as the Canada goose graze on frozen fields and stand on ice all winter with the same countercurrent exchangers in their legs. Gulls, herons, swans and penguins use it as well, and wading birds standing in cold water have it in their long legs.

Mammals use the idea too. Arctic foxes and reindeer keep their lower legs cooler than the body, and whales and dolphins have heat exchangers in their flippers and flukes that let them live in near-freezing seas. Tunas and some sharks have related exchangers around their swimming muscles, but working the other way, to keep those muscles warmer than the sea.

Do ducks ever get frozen feet?

Healthy, well-fed ducks rarely suffer frostbite. Problems can arise in extreme cold when a bird is sick, weak or exhausted, or when ducks rest on wet slush that then freezes around their feet. Reports of ducks and swans frozen into ice do occur in hard winters, but they are uncommon.

People sometimes worry when they see ducks sitting on ice in a cold snap. In most cases the birds are fine: they are resting with their feet tucked under their bodies, saving energy until the next feeding session.