Because of the energy required for flying,birds have a very high demand for oxygen. Even thugh chickens dont fly much, they still maintain these characteristics.
When a bird inhales, half the air passes through bronchi to the posterior (back) air sacs, and the other half goes to the posterior (front) sacs and the lungs. That in the posterior sacs is emptied into the lungs, rather than alveoli, gas exchange takes place between the blood capillaries and the air ‘capillaries’ known as parabronchi. Compared to mamalian gas exchange, that that takes place in birds is much more efficient because the partial pressure is the same as that of the enviroment.
Their lungs are quite different to mammal lungs, because, rather than being a single ‘bag’ for air, they have air sacs spread around their organs, which keep the pressure of air in the lungs constant.
They have no diaphragm, and therefore air flows into the lungs and is pushed out by muscular contraction.
Lcoated at the bottom of the trachea (windpipe) is the syrinx, the birds voice box, and as they exhale they can crow, cluck etc…