Some avian synapomorphies (a characteristic unique to birds) include feathers, rigid respiratory anatomy, and unidirectional flow of air through the respiratory system.
By the way, the term warm-blood is waaaay outdated. Animals that regulate body temperature by generating internal heat are endotherms. Animals that regulate body temperature vi external heat sources are ectotherms. These two terms address thermoregulation from a functional/mechanistic viewpoint.
Endotherms ("warm-blooded") does not imply that body temperature is held constant. That would be homeothermy. Therefore, humans would be endothermic homeotherms because we regulate body temperature by generating internal heat and we have a stable body temperature. All birds and mammals are endotherms but not necessarily homeotherms. Some small birds and mammals are endothermic heterotherms. Their body temperature varies across different regions of the body and/or across time (regional and temporal heterotherms) BUT they still regulate their body temperature by generating internal heat. Penguins are a good example. They are birds and therefore endotherms, but they are regional heterotherms. They shunt blood away from their limbs in order to conserve heat and keep their cold. Having cold limbs helps prevent them from getting stuck to the ice🙂
Reptiles, fish, and amphibians are ectotherms. The majority are also heterotherms (more correctly called poikilotherms). There are some ectothermic endotherms though. A good example is deep sea teleost fish. The water temperature is pretty much constant at the depths that these fish live in and therefore so is their body temperature.
Animal physiology rocks.
