When looking at a vertebrate phylogeny tree, both reptiles and birds separate from other species under the branch reptilia. From there, they part under different classes: reptilians (non-avian) and aves. Being that they came from the same ancestors, they have some similarities such as being vertebrates and having scales instead of skin. As both species continued to evolve, there became differences between the two. Aves (birds) evolved to have feathers on top of their scales making the lighter making flight possible. The main difference between the two is that reptiles are exothermic, meaning that they regulate body heat through the outside environment (the sun). Where as, Birds (Aves) are endothermic and can regulate their own body temperature.