If you’ve spent any time observing birds, you might have noticed that some birds like to stand or rest on one leg, often for extended periods of time. Why do birds assume this unipedal posture? As humans, we would find it quite hard to balance on one leg, which probably makes us that much more curious about this phenomenon.
George A. Clark Jr. wrote a paper about unipedal posture in 1973 . He observed that this behaviour was most notable in families such as flamingoes and ducks, while other birds such as loons rarely do this (of course, loons are also not usually observed on land).
He proposed a couple of hypotheses for this behaviour. First, from his own observations, some birds such as the House Sparrow seemed to stand on one leg on cold days. Because heat can be lost through the legs and feet, one possibility is that birds shift to a unipedal posture to prevent heat loss.
Another hypotheses is that birds stand on one leg to give the other a rest. Clark Jr. did not give any data for these hypotheses. Also, it should be noted as with every behaviour and trait, that different reasons may exist for different species, and the behaviour evolved multiple times over the eons.
Because flamingos are one of the most noticeable birds to the general public, their unipedal behaviour has been more heavily scrutinized. Matthew J. Anderson and Sarah A. Williams  looked at this question for seventeen captive Caribbean Flamingos, and came to some fascinating conclusions.
They first examined another hypothesis put forward by some researchers that standing on one leg allow flamingos to make their escape from predators more quickly. Presumably the greater resting of the leg would make it more capable in movement.
To do this, they measured how long it took for the birds to start moving after resting, and measured the differences between those standing on one leg and those standing on two legs. They found the result to be statistically significant that the birds standing on one leg actually took longer to start up than birds standing on two legs. The authors were quick to point out that captive flamingos can act differently than wild flamingos, so this is far from the final word on the matter, but it does seem to go against the quick-escape hypothesis.
They also found a significant negative correlation between temperature and standing on one leg, as well as a greater likelihood of standing on one leg while in the water compared to land. Therefore, there may be something to the heat conservation hypothesis.
Another hypothesis is the stability hypothesis: that standing on one leg increases the stability of the flamingos. This might be quite strange to humans, but may be more likely in birds. However, the authors found no correlation between wind speed and percentage of birds standing on one leg. However, I would posit that wind speed might not be the best independent variable to choose in this case. Perhaps a larger variance in wind speed over time would be a better variable to use, as if the wind is blowing at a constant speed (and direction), it would be easier to obtain stability compared to having the wind blowing at various rapidly changing speeds and directions.
In fact, Young-Hui Chang and Lena H. Ting did a more in-depth study  on the stability of Chilean Flamingos both on videos and using a dead flamingo. They paid more careful attention to the physiology of the flamingo. They found that a more stable joint configuration could actually be obtained standing on one leg as opposed to two legs. From this conclusion, they posit that it is much more likely that the main benefit of standing on one leg is actually to reduce energy expenditure rather than reduce heat loss. This perhaps is the most likely explanation for Chilean Flamingos.
Timothy D. Harker and F. Roger Harker found another piece of the puzzle in a pilot study on New Zealand Birds . He found something different than Anderson and Williams for captive flamingos: that birds tended to stand on one leg at higher temperatures. The authors also observed that birds that rest on one leg also do so for resting and sleeping reasons. This also seems supported by the study of Chang and Ting, who determined that Chilean flamingos can rest more stably on one leg as opposed to two. If birds are more likely to rest at warmer temperatures, then this would explain the positive correlation with temperature. Harker and Harker also observed that the variance was quite large in his observations, and he explains that the behaviour of the birds tended to change rapidly if they were disturbed as any birdwatcher knows.
In concluding, we can see that the function of resting on one leg is a complex phenomenon! At least for the flamingo and possibly for other birds, resting on one leg may aid in stability. Still, in other situations and other species, preventing heat loss may play a greater role. The question certainly could use more exploration, such as comparative analyses across different species using many explanatory variables.
- Clark, G. A. Unipedal postures in birds. Bird-Banding, 1973, 44, 22-26
- Anderson, M. J. & Williams, S. A. Why do flamingos stand on one leg? Zoo Biology, 2010, 29, 365-374
- Chang, Y.-H. & Ting, L. H. Mechanical evidence that flamingos can support their body on one leg with little active muscular force Biology letters, The Royal Society, 2017, 13, 20160948
- Harker, T. D. & Harker, F. R. Why do birds stand on one leg?–A pilot study of exotic and native New Zealand birds Notornis, 2010, 57, 173-177