What progressive features of the structure of birds ensured their flourishing and the ability to fly?

1) the development of the brain led to the building of nests and caring for the offspring. The development of the cerebellum provided good spatial coordination;
2) the emergence of a four-chambered heart with complete division of blood into arterial and venous;
3) the occurrence of warm-bloodedness and an increase in the level of metabolism;
4) fusion of some parts of the skeleton, the presence of cavities in it and the appearance of wings;
5) a general decrease in body weight (due to the facilitation of the reproductive organs, the characteristics of the excretory system).

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