Snowy egret

A snowy egret high in a tree balances on one leg. Aparently the reason some long-legged birds do this is called 'rete mirabile' and has to do wth heat exchange. However having read up about it I am none the wiser, so sadly I'm unable to give a simple clarification here. If this photo had been taken in a cold climate it might have made more sense but as these birds spend much of their time in cold water and this photo was taken in warm Spain I am left a little confused. Unless the opposite is true and they are protecting their legs from the heat.