|Immature Red-tailed hawk standing on one foot|
Seeing the one-legged hawk on our recent Nature Ramble tweaked my curiosity.I knew that wading birds, ducks and shorebirds often stood on one leg and I recalled a reason from the coursework in my distant past: heat conservation. Birds that frequent bodies of water, especially those that stand in it, lose a lot of body heat through their feet. Some have an arrangement of blood vessels in their skinny legs that helps to reduce heat loss from the body. The major artery in the leg that supplies blood to the foot runs next to the vein that carries blood from the foot back to the body. This side-by-side arrangement allows heat to flow from the artery to the vein, warming the venous blood as it returns to the body. Of course the arterial blood gets cooled on its way to the feet. But by allowing the feet to cool, heat is retained in the bird's core, reducing the amount of energy that is needed to maintain body temperature.
Birds, like mammals, are warm-blooded, only more so. Their normal body temperature is higher than that of mammals. A survey including birds of all sizes found that their average resting body temperature was 102°F and their body temperature during activity is even higher. The maximum recorded resting body temperature was 105.4°F. A fever that high would be enough to make most humans delirious. So birds are even more warm-blooded than we are.
But back to the feet. Despite the counter-current flow of venous and arterial blood in the legs, birds still lose heat through their feet, especially when standing in water below their body temperature. (And almost every body of water is below their body temperature.) So it would make sense to reduce that heat loss by standing on one leg and tucking the other one into the fluffy, insulating feathers to let it warm up. Perhaps our immature Red-tailed hawk is doing some foot-warming itself, even though it's not standing in water. The tree branch it is standing on is probably cool to the touch from losing heat over night. The Cornell Laboratory of Ornithology has a web cam on a Red-tailed hawk and has a similar opinion; click on this link and scroll down to Question no. 54. Other websites have noticed the same one-legged behavior in other kinds of hawks. So maybe our young hawk friend is just getting cold feet.
Prinzinger, R., Pressmar, A., Schleucher, E., 1991. Body temperature in birds. Comparative Biochemistry and Physiology Part A: Physiology 99, 499–506. doi:10.1016/0300-9629(91)90122-S