(2) The characteristics of bird bones are suitable for flying life. Some of the pigeons' bones are very thin, and some are healed together. Most of the longer bones are hollow and filled with gas. This kind of skeleton can not only reduce the body weight, but also strengthen the firmness, which is suitable for flying life.

(3) There are developed air sacs in birds, which are distributed between viscera, and some air sacs protrude into the bone cavity, which can reduce the friction between viscera. Birds' lungs are places for gas exchange, and airbags themselves have no gas exchange function. The function of airbag is to store air, so that the inhaled air can pass through the lungs twice, ensure the lungs to fully exchange gas, assist the lungs to complete double breathing, and provide sufficient oxygen for flight.

So the answer is: (1), streamlining; Resist.

(2)、④; sternum

(3), double breathing; Oxygen; Airbags.