STEIN: Unlike our feet, birds’ feet are little more than bone, sinew and scale, with very few nerves. But it takes more than a lack of nerves to keep their feet from freezing. A miraculous adaptation called rete mirabile is responsible. This fine, net-like pattern of arteries that carry warm blood from the bird’s heart is interwoven with the veins carrying cold blood from the feet and legs. This interweaving warms the cold blood in these veins, before it reaches the bird’s heart. This system keeps the bird’s legs and feet warm, even without leggings and slippers.