Flamingos often stand on one leg because their anatomy makes the posture surprisingly efficient.
One leg can be mechanically stable
A human balancing on one leg constantly uses muscles to stay upright.
Flamingos are built differently.
Their joints and body weight can line up so that one-legged standing requires very little active muscular effort.
It may save energy
If the skeleton can support the body with little muscle activity, the posture becomes a useful resting position.
Flamingos can remain that way for long periods and may even sleep standing on one leg.
What about heat conservation?
Flamingos spend a great deal of time standing in water.
Long bare legs can lose heat.
Pulling one leg up against the body reduces the amount of leg exposed to cold water and air.
That may be especially useful in cooler conditions.
Do they always use the same leg?
No.
Flamingos switch.
The posture is normal, not a sign that one leg is injured or tired.
Why does the leg anatomy look so strange?
The joint that looks like a backward knee is actually closer to the ankle.
The true knee is higher and partly hidden by feathers.
Bird anatomy is not arranged like a human leg on stilts.
The same lesson applies to horses sleeping while standing: an animal’s comfortable posture can look bizarre when judged through human anatomy.
What looks like a balancing act to us may be the flamingo version of taking a load off.
And that’s the short explanation.
Related explanations: Why can horses sleep standing up?, Why do penguins huddle together?, and Why do birds fly in a V formation?