Subject: Had warm-bloodedness evolve independently in birds and mammals?

Current endotherms are birds and mammals, and I know there is a 
theory that some dinosaurs where, too. I know that Mammals 
evolved from reptiles, and that thay already existed before the 
dinasaurs, and that birds evolved much later from a family of 
dinosaurs. If both mammals and birds are endotherms, and 
assuming that both mammals and dinosaurs evolved from exotherm 
reptiles, did this trait evolve twice, independently? If not, it 
must mean that all dinasaours where endotherms, doesn't it? Does 
this trait operate in the same way in birds as it is in mammals?

Re: Had warm-bloodedness evolve independently in birds and mammals?