What aromorphoses allowed birds to spread widely in the airborne habitat?

1) structural features and functions associated with flight: hollow bones, the transformation of the forelimbs into wings, the rapid digestion of food, etc.
2) features that provide a high level of metabolism and warm-bloodedness: 4-chamber heart, a special structure of the respiratory system, the consumption of a large amount of food, etc.
3) the development of the central nervous system, complex behavior (flights, care for offspring, etc.).

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