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Why don’t birds’ legs freeze in extreme cold weather?

Hi, my name is Dr. Glenn Tattersall from Brock University, and I’m answering the question “Why don't birds' legs freeze in cold weather?”

There are actually two components to this question. The first could be asked: “Why don’t birds lose a lot of heat from their legs?” and following that comes the answer to the question “Why don’t birds legs freeze in cold weather?”

Most birds have the ability to alter blood flow to their legs through the use of a counter-current system of blood vessels. When blood flows down a main arterial vessel in each leg, the venous blood that returns from the webbing/toes flows past the arteries but in the opposite direction (thus, the notion of the flow being counter-current). In this manner, arterial blood is cooled on the way down to the toes and venous blood is warmed on the way back up to the heart through the veins. This type of blood flow arrangement saves heat and actually helps the birds keep a fairly efficient insulation against the cold. There’s not a lot of active tissue in a bird foot. The muscles are mostly located in the thigh region, and so that leaves the mostly hollow bone, skin and some connective tissue. So, the bird feet do not require a high amount of blood flow. Nevertheless, this kind of blood flow pattern is argued to provide birds and mammals with the ideal solution: blood still flows and provides the tissues with nutrients and oxygen, while not dumping precious body heat to the environment. The tissues themselves can handle being cold, provided they do not actually reach temperatures below freezing.

Now, the system described above won’t necessarily help a bird when the air or ground temperature goes really cold, like below freezing. At least in the birds that have been studied (geese and giant fulmars, an Antarctic petrel), legs also have shunt blood vessels that can by-pass the counter-current system I described above. Analogous to a by-pass surgery on your heart, these shunt vessels provide the bird with an alternative pathway for blood to take on the way down to the toes and feet. Essentially, the bird can flood the toes with warm internal body heat to keep them from freezing. I suspect in the wild when this is happening, the birds also lie down on their feet to further insulate them, but they do not always do this. I have seen many Canada geese in the wild hanging around all winter long standing up, clearly having to make use of these cardiovascular adaptations. I should add that there are also some unpublished observations by colleagues of mine suggesting that some geese may have antifreeze proteins on the skin of their feet as well. Perhaps in some species there is potential for actual skin freezing to be a problem. Clearly not all bird species may exhibit these responses. Many birds simply fly south to get away from the cold. Our Canada geese, however, did not get this memo, and so will have to make use of their altered blood flow to keep their toes from freezing.

Canada Geese
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