The study is based on the results of the integrated measurement of the energy expenditure at rest and common activity in birds belonging to various systematic groups. Homeothermy has formed in birds and mammals independently and in different geological ages. However, in both groups it originated as a side effect of selection for aerobic metabolism improvement that provided a higher level of activity. Advantages of having high and stable body temperature, which were inevitably related with metabolism intensification, led to development of thermoregulatory adaptations such as fur and feathers. This made it possible to retain the metabolically generated heat and reduce heat absorption in hot environments. Emergence of homeothermy with aerobic supply of motion activity, possibilities to regulate the level of metabolism and thermal conductance, has opened a lot of opportunities for homoeothermic animals. Achieving such a level of energy utilization allowed them to maintain activity for a longer time, while its sensory support led to complication and diversification of birds’ behavioral repertoire (as well as that of mammals) facilitating the conquest of almost entire part of the biosphere that was suitable for living. This process was favored by the development of nurturing and passing on the information, collected throughout the life, to new generations. Formation of high levels of aerobic metabolism in birds and mammals was proceeding in parallel among different groups of reptilian ancestors. The level of homeothermy, at which aerobic metabolism was able to maintain prolonged activity, developed in birds and mammals in different ways: they had got dissimilar partitioning of venous and arterial networks, erythrocytes with or without a cell nucleus, different lungs design—but, at that, similar minimum metabolic power and rather close body temperatures which corresponded well to the environmental conditions on the Earth. Natural selection allowed animals with high energetic metabolism to increase their diversity and abundance, but only when homoeothermic animals could satisfy their demands for food resources, that had risen manifold. That happened in the middle of Cretaceous, in time with the appearance of angiosperms and expansion of related fauna of invertebrates.