Evolutionary biologists have long been intrigued by the evolution of endothermy, particularly how birds and mammals evolved endothermy independently of each other. One of the key differences between the two taxa is the presence of nucleated erythrocytes along with cellular organelles in birds, and enucleated erythrocytes without any cellular organelles in mammals. It is widely assumed that mammals evolved enucleated erythrocytes as a means to pack more haemoglobin into erythrocytes to cope with the increased aerobic metabolic demand of endothermy. However, the idea of enucleated erythrocytes having more haemoglobin per cell volume has not been tested using a phylogenetic framework. Additionally, birds evolved endothermy independently of mammals and presumably have similar selection pressure for endothermy, yet they possess not just nucleus, but also other cell organelles. To investigate whether enucleated erythrocytes have more haemoglobin per cell volume, we compared mean cell haemoglobin concentration (MCHC), along with other blood parameters of birds and mammals using a both conventional statistical approach and phylogenetic framework. Preliminary results suggest that MCHC did not differ between birds and mammals, suggesting that perhaps mammals did not extrude cell organelles from their erythrocytes to maximize intracellular haemoglobin content, and/or birds evolved a different way to maintain high MCHC. At the meeting, we will discuss some of the ecological and evolutionary forces that allow birds to retain nucleus and other cell organelles in their erythrocytes, as well as propose ways to test some of the hypotheses regarding the roles of different cells organelles in avian erythrocytes.