The size and surface area of a beak seem to be a compromise between heat loss and foraging requirements. Likewise, leg and toe lengths are compromises between locomotory needs and heat loss. The old joke I lay on beginning birdwatchers begins “Why do some birds stand on one leg?” The answer: “Because if they lifted it up they would fall down.” After the groans subside I explain that to reduce heat loss in cold temperatures, the birds tuck one leg up into the feathers of their abdomen while resting, or both feet while sitting down or floating on the water.

         Ducks and a number of other birds have what is called a counter-current heat exchange system between the arteries and veins of their legs. Blood in the arteries comes from the heart and is warmer than the blood returning from the extremities. The arteries and veins are intertwined in a structure in the lower leg called the “rete tibiotarsale, (net of the tibiotarsus).” which may consist of  as few as three arteries and five to seven veins, as in owls, or up to 60 arteries and 40 veins in flamingos. Birds that do not have this net have instead a pair of veins running closely on either side of an artery. In some species of gulls and guillemots, the arteries and veins are closer to each other in the birds of more northern areas than those in more southerly locations. In both cases, the warmer arteries pass on some of their heat to the colder veins, returning some warmth to the heart but still providing enough heat to the legs and feet to keep them from being frostbitten. A duck standing on ice will lose body heat, but only five percent of that loss will come from its feet. Blood also serves to supply oxygen and nutrients to the feet and legs, but there is not much muscle there so little blood is needed. The muscles that operate the feet and legs are mainly concentrated in the upper leg and utilize long tendons for mobility. At lower temperatures that threaten the feet with frostbite, blood flow is increased to the lower limbs by special valves that open in the arteries. By the same mechanism, birds can lose body heat in a hot environment by shunting more blood to their legs and feet.