Why do most water birds tend to stand on one leg?

Question

Is there a functional reason for this?

Answer 1

One theory is that birds lose a lot of their body heat through their feet, so they tuck one leg under to minimise the heat loss.

Some ornithologists have suggested that flamingos may sleep on one leg, essentially turning half of the brain off to rest, and balancing on the leg which is connected to the part of the brain that is awake. Certainly tucking one leg under the body would help to conserve body heat, because it would minimize the amount of surface area exposed to the air.