|MadSci Network: Zoology|
Ducks and other arctic or sub-arctic birds have an efficient system of heat regulation to their extremities. The warm blood flowing from the heart can (when necessary) pass directly alongside the cold blood returning to the body. Over the length of the leg, this cools the outgoing blood and warms the incoming blood so very little heat is lost through the extremity. This system, called countercurrent exchange, is not always in effect, however; sometimes warm blood will flow to the feet, providing enough heat to keep it from freezing.
Since frostbite is caused by the freezing of water in cell tissues, swimming birds are not in any particular danger -- if the water outside the skin isn't frozen, it's unlikely that the water inside the tissues could be. Outside the water, many birds keep their feet warm by sitting on them, and some arctic birds even have feathers on their feet! But the primary mechanism that helps birds' feet survive cold weather is the heat-regulating system mentioned above.
When mammals get frostbite, the flow of warm blood to the extremity is shut off to prevent heat loss. Because of the bird's more efficient circulation, they can continue to send warm blood to the extremity without a significant loss of body heat.
I couldn't find any web sites that address this, but there are some 'strange but true' bird facts at Birdfacts, and several links to other sites with bird information at Birdlink
Hope this answers your question!
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Tim Susman "For the animal shall not be measured by man." -- Henry Beston
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