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How do the birds’ feet keep from freezing off?  Some birds live in Minnesota during the winter, where actual temperatures might still get down below  – 20 F, and wind chills can drop to -50 or lower. Birds’ feet are alive, which means that they need to keep at least some blood flowing to nourish the cells and and avoid bone destruction, despite lack of insulation.  There are a number of descriptions online for the adaptation they have evolved. Our expert bird consultant, Chase Davies, contributed this explanation:

In this kind of circulatory system, blood leaves the bird’s main body at a warm temperature through the arteries to warm their feet, and venous blood returning from the feet comes back at a lower temperature.

The crux of the matter is that the arteries and veins lie close together in skinny bird legs. So the cold blood returning from the cold feet is warmed by the arterial blood flowing to the feet – and similarly, the arterial blood is cooled by the blood returning from the feet toward the body.

This same sort of “heat exchange” occurs and is easily observed in humans at the wrist, the ankles and less obviously in various other places – it corresponds to why our nose, toes and fingers lose feeling when we are out in the cold with ineffective insulation of boots, socks, mittens, gloves, etc.

Rete mirable is an appropriate name to label a simple diagram of this. Counter current exchange system is a more engineering style of term and is also descriptively accurate.
In the following diagram – NOTE that there are red and blue arrows (not very obvious) showing the direction of flow of red arterial hot blood and cold blue venous blood; the change in color is the change in temperature NOT an interchange of fluids.

rete mirabile

It is also true but not necessarily a part of this conversation that bird feet do not seem to freeze to hard objects. The observation that they are not moist so there is no moisture to freeze  is one aspect of the interpretation. There are many instances where creatures remain living when frozen, but I digress… we would have to start with a definition of freezing which is not always at 32 degrees Fahrenheit depending on the solution in question, and consider crystal size in the solidified fluid.

There are truthful stories of waterfowl becoming trapped when the surface of the water on which they floated while sleeping through a night froze them in solidly. They could not pull their feet through the little opening the leg formed in the ice layer. If the ice melted soon they could swim or fly away. If the lake or pond was frozen for the winter, well, not so fortunate. And think three times before going out to rescue the bird……. Call professional help: fire department, police, do not send your Saint Bernard dog.

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