I have seen it done a few different ways, the microwave unfortunately and in the oven with some stock underneath to reheat it. but the best method i have found is to roast the birds and then half them and store them in a cryovac. then you just put them in a Sous Vide bath or hot pot of water. if you use this method you then ususally have to put the bird in the salamander to crisp the skin.