How is the adaptability of the structure of the bird’s skeleton to flight expressed?

First of all, fitness is expressed in signs that reduce the weight of the bird. Among these signs are:

Tubular bones filled with air sacs
Decrease in the number of bones
Lack of bladder, teeth, rectum.

In addition to reduced weight, fitness for flight is expressed in other structural features:

Streamlined body shape,
Powerful wing muscles capable of long flight.

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