I know, by observing people and from my own experience (you can try it yourself while standing, but not too consciously and in a loose way) that they, while standing, tend to do this on one leg. Just observe yourself and other people. Many birds do this as well. Like the stork:

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The reason for this is to reduce heat loss:

Why do some birds stand around on one leg? A. One widespread theory among bird experts involves heat conservation. Herons, hawks, geese, ducks, and gulls — birds both short-legged and long-legged — are on the Audubon Society list of those that tuck up one leg under their feathers and balance on the other.

The question remains: why do people too tend to do this? We don't pull up our entire leg though. Many times I've seen people standing on one leg, their other leg crossed over the other.

Leaning on one leg Leaning more on one leg while standing, sometimes referred to as “hanging on one hip”, can feel comfortable, especially if you've been standing for a while. Instead of using your buttocks and core muscles to keep you upright, you place excessive pressure on one side of your lower back and hip. Mar 26, 2016

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    $\begingroup$ Again, editing rebuttals to close reasons into your question is not likely to be received well. If you are unclear why a question is closed, you can ask on Meta. If you edit a question to solve the problems with it, people can vote to reopen it, but this will only work if you can solve the problems. Some questions cannot be salvaged because they do not fit the site. Adding prior research to your questions might improve them as well and encourage reopening or upvotes. $\endgroup$ – Bryan Krause Sep 15 '20 at 22:09
  • $\begingroup$ Observing myself, I don't, generally. So I think you really need some support for the claim. $\endgroup$ – jamesqf Sep 16 '20 at 5:05
  • $\begingroup$ birds that stand on one leg are rare and they do so for very specific purposes. $\endgroup$ – John Sep 16 '20 at 12:16
  • $\begingroup$ @John Our chicken does it very often! Like the well-known example of storks. $\endgroup$ – Deschele Schilder Sep 16 '20 at 13:13
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    $\begingroup$ Asking why some birds stand on one leg would be a good question I suggest asking that instead. $\endgroup$ – John Sep 16 '20 at 15:16

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