All the features of the internal structure of birds associated with flight.

The structural features of the bird’s body that are associated with flight include:
– changing the forelimbs into wings;
– Acquisition of a streamlined body shape;
– the presence of air bags;
– the presence of a feather cover;
– the acquisition of a skeleton with light bones;
– loss of bladder and teeth;
– the presence of a keel;
– warm-bloodedness;
– the presence of double breathing;
– good development of the nervous system.
All these features appeared in birds during a long evolutionary process, which now help them feel comfortable in the air.