Warm blooded animals can maintain constant body temperature, irrespective of the surrounding environment, i.e., their body temperature is fixed. Warm blooded body is the common characteristic feature in birds and mammals. This characteristic was first evolved in higher reptiles; birds and mammals acquire this feature from higher reptiles during evolution.
Pigmented skin is the adaptive feature in mammals, and is not present in birds. Pneumatic bones are found only in birds as their flight adaptation. These reduce body weight for flight. Viviparity is shown by mammals and not by 'birds as they are oviparous, i.e., lay eggs.