On some of the coldest, windiest days this past winter, we noticed many birds tucked into the trees, balancing on one leg while the other leg and foot was tucked up so close to the breast that it was almost impossible to see. It looked like the bird was missing a leg.
How can a bird balance like this? A bird can easily balance on one leg for several reasons. In addition to the balance sensors in the inner ears, quite like us, a bird possesses an additional balance sensor in the pelvis. So instead of teetering while perched on one leg, a bird can compensate with this extra balance sensor located in the lower body.
The structural anatomy of a bird also makes it easier for this one leg up, one leg down pose. A bird’s knee joint is actually located within the bird’s body, and the portion of the bird’s leg outside the body that you see is the ankle joint with a long bone and toes attached.
Now that we understand some of the physical anatomy and physiology, let’s return to the original question: Why do birds stand on one leg? According to birdnote.org, it is the same reason you put your hands in your pockets when it is cold!
Rete mirabile is the adaptation birds’ legs have to minimize heat loss. Arteries carry warm blood from the heart to the extremities, while veins return cool blood back to the heart to be warmed and filled with oxygen. In a bird’s legs, the arteries and veins in the legs lie very close together. The arteries can help warm the veins and the veins help warm the arteries keeping a bird’s legs closer to the environmental temperature. By standing on one leg, a bird can halve the amount of heat it loses by tucking the unfeathered, unprotected ankle, foot, toes close to its warm body.
Recent studies on flamingos, a bird that does not experience cold weather, but does spend much of its life on one leg, exposed this single leg posture as a different way flamingos conserve energy. Standing on a single leg changes the center of gravity so it is directly over the leg. The bone structure essentially locks the leg into position. The end result is precious little muscular energy expended by the flamingo on maintaining balance. This physical ability may transfer to other yet unstudied species.
Check out some of our feathered friends in their energy-efficient balanced positions!