For flamingos, standing on one leg might actually be easier than standing on two. A report published Wednesday in The Royal Society's Biology Letters revealed that flamingos might literally be built to stand on one leg, which would explain how they're able to do so while they sleep without toppling over.
When the researchers' first idea for testing flamingos' balance — walking over to the birds in a zoo and giving them "a little prod" — was rejected, they turned to studying flamingo cadavers. To their surprise, they found that while a dead flamingo can't stand on two legs, it can still balance on one. When the researchers began studying live flamingos — by watching them until the birds dozed off — they found out flamingos' balance became better as they fell asleep.
That led researchers to realize that flamingos' fantastic balance might have something to do with their anatomy, specifically a built-in "stay mechanism." The Washington Post explained the phenomenon:
The bird's skeleton appears to be the key. As with humans, flamingos have two main joints on their leg. The one you can see, that bends backward, is not the knee. That's actually the bird’s ankle. Its knee, meanwhile, is hidden in the bird's features at the fatter part of its body.
When the flamingo is ready to nod off, it lifts one leg and instinctively moves its body so its single foot isn't under its hip. Instead, it's centered directly under the carriage of bird. Meanwhile, pulling the other leg up forces the knee to bend, which the flamingo rests on. All the joints essentially snap into place.
[...] As the flamingo remains nearly perfectly still while sleeping, gravity does the rest, keeping the bird in place. [The Washington Post]
Though the researchers might be one step closer to figuring out how flamingos stand on one leg, the question of why they do so remains a mystery. Flamingo studier Matt Anderson, of St. Joseph's University, pointed out to The Atlantic that if standing on one leg saved flamingos so much energy, "one would expect flamingos to employ the one-legged resting stance constantly."