Wild Neighbors: At Least One Leg to Stand On

By Joe Eaton
Wednesday November 16, 2011 - 02:30:00 PM
Greater white-fronted goose: why doesn't it fall over?
Alan Vernon (Wikimedia Commons)
Greater white-fronted goose: why doesn't it fall over?

Someone once asked the science fiction writer Barry Longyear where his ideas came from. “Schenectady,” he replied. I think he eventually published a book called It Came From Schenectady. The column-writing process is similar. Ron and I recently had an article on the autumnal florescence of garden, AKA pumpkin, spiders in another publication. It drew a fair amount of reader response, one of which could be paraphrased: “If you know so much about spiders, how do birds manage to stand on one leg?” 

I found an answer for him and responded. And then I thought: “This stuff is too good to waste on one email.” Hence today’s venture into biomechanics. 

If you’ve spent any amount of time around birds, you’ll have observed the one-legged stance. Large waders like flamingos and storks are notorious for it, but waterfowl (see goose photo), shorebirds, and songbirds do it too. There are also, just to confuse things, a few genuinely one-legged birds around: usually seabirds and waterbirds. Spend a lot of time floating on the surface of the water with appendages dangling and something may take a bite. 

At least one travel guide to Hawaii lists the one-legged owl among the islands’ extinct avifauna. This appears to be a garbled version of the long-legged owl, or alternatively the stilt-owl. There are no congenitally one-legged bird species. 

Why do they do it? It’s all about thermoregulation, minimizing heat loss. There’s a part of the avian circulatory system called the rete mirabile that allows a counter-current heat exchange. The arteries that transport warm blood from the heart to the legs are in contact with the veins that return cooler blood to the heart. The arteries transfer some of their heat to the veins. Standing on one leg halves the amount of heat lost through the featherless legs. (But do birds with feathered legs, like the rough-legged hawk or the great horned owl, stand on one leg?) According to one study, flamingoes assume the one-legged posture more often when standing in water than when standing on dry land, and in cooler as opposed to warmer temperatures. 

As for the how, I am indebted to the web site of Professor Doctor Reinhold Necker, formerly of the University of the Ruhr in Bochum, Germany, for his detailed explanation of why these birds don’t topple over ( Necker says it’s long been speculated that long-legged birds like flamingoes, at least, had some kind of snapping mechanism that locked the intertarsal joint of the extended leg in place. However, such a mechanism has only been demonstrated in the ostrich, which does not stand on one leg. Other long-legged birds have a structure in the trochanter, where the femur attaches to the hip, that prevents the body from tipping over forward and stabilizes it against rotation toward the non-supported side. 

Bear in mind that a bird’s leg is not quite like a human’s, although the basic structures are similar. Birds walk on their toes; what appears to be a reversed knee is actually their ankle joint. The true knee—the thick end of the drumstick—is concealed by feathers. When standing, a bird’s knees are flexed and the knee joint is near their center of gravity. The femur is nearly horizontal, held in place by the ligaments of the hip joint. 

Necker also writes that birds have balance regulators in their inner ears, as we do. They also have an auxiliary equilibrium sensor in the lumbosacral region of their vertebral column that acts directly on the motor system of the legs. 

So it seems to be a combination of bone structure and sensory apparatus that does the trick. None of this, of course, applies to the various human cultures that have adopted a one-legged posture, like those cattle-herders in South Sudan who used to be a National Geographic staple.