Wings and feathers first and foremost, but also by the fusion and elimination of some bones and the "pneumatization" (hollowing) of the remaining ones.
The respiratory system of birds is also adapted to the demands of flight. A bird's respiratory system is proportionately larger and much more efficient than ours -- as might be expected, since flight is a more demanding activity than walking or running. An average bird devotes about one-fifth of its body volume to its respiratory system, an average mammal only about one-twentieth
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