“When I take my kids to the zoo, the first thing you see is the flamingo exhibit,” says Chang, who’s based at Georgia Tech. “The first question people ask is: How do they stand on one leg?” So he teamed up with Ting, an expert in postural control at Emory University, to find out.

(Georgia Tech / Rob Felt)

First, they went to Zoo Atlanta with a force-plate—a fancy 3-D bathroom scale that measures the forces a foot exerts in all directions. “I’m used to studying things that walk or run, where you have to chase them across the force-plate or have them chase you,” says Chang. “This was more boring. We just had to sit around and wait for the flamingos to fall asleep.” Some of the youngsters did eventually cooperate, and one even fell asleep while standing on the plate. And as it lost consciousness, Chang and Ting saw that it became more stable. Its body swayed less, and its center of gravity moved by mere millimeters.

The birds are so steady that no one at the zoo could remember an instance of a flamingo falling over. “We really wanted to do an experiment where we just walked over and gave them a little prod,” says Chang. “But the zoo wouldn’t let us.”

In lieu of that, the duo decided they needed a detailed look at the birds’ legs. They put a call out to local zoos, and within a day, Birmingham Zoo said that they had recently euthanized two flamingos because of poor health. They had been frozen, so Chang drove over with a cooler. Back in his lab, he and Ting defrosted and dissected the birds. And that’s when he lifted one.

To understand why that worked, it will help to clarify some terms, because bird legs are deeply confusing. Many people think that birds have backward-bending knees, but the joint that they think of as the “knee” is actually the ankle. The real knee is much higher; it’s obscured by the feathers on the bird’s belly and bends in the same way ours do. (Look at the image below, in which I’ve marked the top parts of the flamingo’s right leg.) Birds are perpetually crouching, holding their thigh bones almost horizontally against their bodies. The two long bones that you see extending downwards are, respectively, the shin and a fusion of several foot bones.

(Jean Paul Pelissier)

When a flamingo shifts onto one leg, two things happen. First, the leg inclines so that the foot moves from being directly under the hip to being directly under the center of the body. Second, the center of mass moves to just in front of the flamingo’s (hidden) knee, so its body weight naturally pulls the hip and knee forward. These two changes, combined with gravity’s pull and the shape of the leg bones, keeps all the joints in place.

When Chang held the bird up by the shin, he engaged the same mechanics. Even when he tilted the bird at a 45-degree angle, the leg didn’t buckle. “You definitely feel a very large difference if you just get it in the right posture,” he says. “If you press down on the front of the bird, it’s very solid.” And this only works on one leg. In a two-legged stance, both legs are perfectly vertical, and the joints become more unstable. So, counter-intuitively, flamingos are actually more stable on one leg than on two.