Flamingos balance on one leg almost without using any muscle

Biomechanics researchers studying flamingo cadavers and live birds found the one-legged stance is a passive balancing act, not a feat of muscular endurance — the joints lock into a stable position under gravity alone. That's likely why flamingos can sleep standing on one leg for hours, and why they were found to be more stable on one leg than on two. The pose may also cut heat loss by tucking one leg against the warm body.

— Young-Hui Chang and Lena H. Ting, Mechanical evidence that flamingos can support their body on one leg with little active muscular force — Biology Letters, vol. 11 (2015)