A 2011 paper published in Trends in Cell Biology echoed a comment "enucleation [of mammalian red blood cells] has critical physiological and evolutionary significance in that it allows an elevation of hemoglobin levels in the blood". Increased hemoglobin levels in the blood, in turn, permitted increased blood oxygen solubility and, thereby, metabolic rate, which may have paved the way for endothermy, which is characterized by high metabolic rate, to evolve in mammals. However, if all of this is true, why do you suppose that enucleation of red blood cells did not evolve in birds, which are also endothermic?