Why do birds stand or sleep on one leg?

There are two main reasons why a bird might sleep on one leg. First, it will reduce the amount of heat loss as the foot helps insulate the underside. The second reason listed by experts is to give one of the legs a break from standing. Of course, this doubles the weight on the downed leg. It is worth noting that birds who will sleep on one leg will switch the downed leg

https://www.audubon.org/news/why-do-birds-rest-one-leg