Why do birds stand on one leg?

Why do birds stand on one leg? Many birds - short/tall, small/large - often roost in a one-legged position, while keeping the other leg tucked up into their body.

Here I got some spoonbills in Mai Po marshes to model this for me.

Birds have adaptations to manage heat loss, like feathers etc. The arteries that transport blood into the legs lie in contact with the veins that return blood to the bird’s heart. The warm arteries heat the cooler veins. Because the veins also cool the arteries, the bird’s feet are closer to environmental temperature and thus don’t lose as much heat as they would if they were at body temperature. sooooo……a bird with its foot tucked up reduces by half the amount of heat lost through its unfeathered limbs.

And here a Pied Avocet in the early morning light, also i Mai Po, keeping warm, in, er 25 degrees. hmmmmm

You can also see that some birds, for extra warmth, tuck their beaks under their shoulder feathers, (not under their wings). You can also see other birds who do not stand on leg fluff out their feathers to try and cover their legs.

How do they balance? No idea….also, i am going on what bird researchers have written, so I take this at face value. BUT...I have seen this behaviour in Mai Po in 30degree temperature....so, im not sure how it helps then....maybe keeps them cool? dammit, ill go off and find an expert and get back to y'all.