There are several theories as to why birds, especially large wading birds such as herons and flamingoes, stand on one leg, or “unipedal resting” as scientist like to refer to the trait. The seemingly most convincing and best-proved theory is that birds stand on one leg to conserve body heat. It is shown that birds stand on one leg more often when wading, which again points to the thermoregulation hypothesis because water draws away more body heat.
Standing on one leg is not necessarily a sleeping and resting habit. I have filmed Great Blue Herons and Snowy Egrets meticulously preening while standing on one leg. The characteristic is not limited to large wading birds; species with shorter legs, including ducks and swans, also stand on one leg. Another popular theory suggests that wading birds stand on one leg to look less suspicious to aquatic prey.
To read more about avian sensory physiology, visit the website of Professor Dr. Reinhold Necker. Additional images courtesy Professor Necker’s website.