When birds sleep they normally sleep on one leg, why? surely two legs would be far more stable than two and does not this sole leg recieve severe pains and cramp during the night.  thanks xx

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.