Many observers have noted that flamingos and other wading birds often stand on only one leg while tucking the other leg up under the body. Flamingos exhibit this behavior both in the wild and in captivity, suggesting that it is entirely natural, rather than the result of stress due to confinement. A number of theories have been put forward to explain this phenomenon, but ultimately no one is really sure about why flamingos stand on one leg.
One of the more widely accepted theories is that flamingos stand on one leg to conserve body heat and energy. 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. Alternating legs may also allow the flamingo to rest and promote circulation through both legs.
Many scientists have agreed with the circulation theory. The long legs of flamingos require extra work from the heart to circulate blood fully throughout the body. Especially when flamingos are standing in cool water, the heart is forced to circulate more blood to keep both legs warm. By tucking one leg closer to the body, flamingos may reduce the load on their hearts.
Standing on one leg may also help to camouflage a flamingo, as the single leg resembles the reeds and grasses flamingos stand in. Given that flamingos eat plant matter, crustaceans, and mollusks, it seems unlikely that they need to develop camouflage to conceal them from prey animals. When one considers that the bodies of flamingos are also an outstanding and distinctive pink color in the wild, it also seems like they may not be entirely successful at masquerading as reeds.
It has also actually been suggested by some ornithologists that flamingos may stand on one leg so that ducks only run into them half the time. While this theory does have merit, it dismisses other bird species at risk of duck collisions which share habitat with flamingos such as spoonbills, skimmers, and geese. Since flamingos tend to congregate in large flocks, it is also probable that ducks have learned to avoid flamingos on their own.
As we can see, numerous theories have been posited for this peculiar trademark flamingo behavior, although a concrete explanation may never be reached. Like most mysteries in nature, the truth is probably a combination of several theories, although it is most likely related to energy conservation and resting behavior. Long term study of flamingos and other wading birds both in the wild and in captivity has not resulted in a concrete answer to the question, but perhaps it might some day.