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Why don’t birds’ feet get cold in the winter?

From the But Why? column of Iowa Outdoors magazine (March/April 2017)
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Mikayla, 10, in Marshall County, asks: Why don’t birds’ feet get cold in the winter?

Actually, birds do get cold feet, and that is why they don’t freeze. It’s all about heat exchange in their circulatory systems.

As colder blood returns from the feet, it passes warm arterial blood coming from the heart. Heat is exchanged between the two by blood flowing in opposite directions. Blood flowing down is cooled, and blood flowing to the heart is pre-warmed to maintain core heat.

The temperature of the feet is close to the temperature of the snow, so very little heat transfers between the two. The feet are just warm enough to avoid frostbite, and the bird’s circulation is fast enough that blood doesn’t remain long enough in the feet to freeze.

In severe cold, a bird may stand on one foot, with the other tucked under feathers. This further reduces heat loss by keeping one foot warm, and the surface area in contact with ice or snow is cut in half. 

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Why don't birds feet get cold in the winter? | Iowa Outdoors magazine

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