Ducks are designed to survive in an incredible range of climates. They do well in the heat of summer when they regularly take a cool dive into the water. In the winter, however, it’s a whole different story. In temperate climates, the water in winter is very cold and it’s a good absorber of heat. If a human falls into icy water in the winter, hypothermia can occur in a very short time.
How can a duck stay in the cold water in winter? The feathers of a duck are waterproofed and provide incredible insulation from the cold. The waterproofing on the feathers is more concentrated on the outer feathers that are in the water. The cold water never really makes contact with the skin where heat loss would be great.
However, a duck’s feet do make contact with the cold water. Then why don’t their feet freeze? To prevent that from being a problem, the duck’s feet have capillaries with a lace-like structure that weave among one another. This creates a counter current heat exchange mechanism. Warm blood flows down the legs from the body and meets the colder blood coming back up and heat is exchanged in the close capillaries. The warm arterial blood exchanges heat with the cooled blood, preserving the core temperature of the foot so that the duck can still function in a normal way even in icy water.
The devices built into ducks to allow them to survive in all kinds of conditions show great wisdom in design.
“countercurrent exchange,” Wikipedia
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