Why birds used to sleep on his one leg ?

Why birds used to sleep on his one leg ?

Reason 1: Some birds stand on one leg when they sleep while some don't.

As for the birds that do, like cranes and flamingos, apparently, no one really knows why they do it.

So there are many theories.

One theory is for thermoregulation (in this case to keep warm). Keeping one leg tucked in close to the body would surely warm that leg up. Then the bird will switch legs and this time tuck in the leg that it was initially standing on.

Birds alternate legs and they don't stand on the same one all night so the stresses are spread evenly. They also have special tendons that allow them to lock their knees which reduces the amount of stress.

Reason 2: Birds have cycles of NREM and REM sleep, but with some differences. One of the most striking differences is that both NREM and REM sleep episodes are quite short in birds; their NREM sleep episodes average only about 2 1/2 minutes, and REM sleep episodes only 9 seconds. Also, most birds do not lose muscle tone during REM sleep as consistently as mammals do, which is understandable, since many birds sleep while standing or perching. Some birds, such as pheasants and partridges, sleep on the ground. Water fowl can sleep while swimming. Some parrots sleep hanging down. It is suspected that some birds sleep aloft during long transoceanic flights.