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The robin has such spindly legs that the blood must surely freeze on the way down to its feet in cold temperatures?

Sent in by Caroline Morris

Birds have developed several ways to cope with the cold. The feathers are great insulation and it is important they are kept in good condition. In cold weather, birds can fluff their feathers up which traps more air and creates better insulation. When it is too hot, then birds can lay their feathers flat.

The feet are a problem though because they are generally not feathered. To cope with this, birds have evolved ways of reducing the heat loss. Birds' feet don't need to be kept at the same temperature as the rest of the body. In some cases, they can be kept just above freezing. This is done by adjusting the blood flow so just enough warmth is given to the feet to stop them freezing. The small surface area of songbirds' feet also helps reduce heat loss. Birds with webbed feet have a larger surface area so more heat could be lost.

It is most important to keep the body warm because this is where the vital organs are. To stop cold blood from the legs returning to the body and cooling the bird, they have evolved a heat exchange system in the veins and arteries. Cold blood returning to the body from the feet passes very close to warm blood coming from the body so the cold blood is warmed before it returns to the heart.

Birds that spend long periods standing on snow or ice may stand on one leg to reduce the surface area in contact with the cold. Penguins in the Antarctic probably have to cope with the coldest weather can squat down over their feet to keep them warm.

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