Birds, like mammals, can regulate their body temperatures within close limits. therefore birds and mammals are called

I believe they are called endothermic. 
Endotherms also called warm blooded animals are those that maintain a constant body temperature independent of the environment. They primarily includes, birds and animals, but some fish are also endothermic. On the other hand ectothermic animals or cold blooded animals are those whose regulation of body temperature depends on the external sources such as sunlight. 

That would be a working theory.