Chickens have a specialised system for getting oxygen into their bodies and removing carbon dioxide. In doing so they employ an efficient combination of two methods to obtain oxygen, allowing them to indulge in highly energetic activities such as flying and running. As you might expect, air enters through the nostrils, and passes through the pharynx and trachea. Chickens have long necks because they need to preen those hard to reach places and peck into every nook and cranny, and as a result they have a long trachea which allows a relatively large amount of air to enter in one breath. This function results in a low breathing rate of roughly 30 – 35 breaths per minute, and cockerels roughly 18 – 20 per minute.
The trachea is protected by the larynx which makes sure that food does not enter the airway when the chicken is swallowing. The trachea branches into smaller bronchi, which enter the lungs and then branch into secondary bronchi and hundreds of small loops called parabronchi. This results in a large surface area within a relatively small space and allows for more efficient transfer of oxygen.
The lungs are attached to the ribs and only expand a little when air is breathed in. The lungs do not expand like a balloon unlike human lungs, because the rib cage in chickens has to be far more rigid than in mammals to provide a strong anchoring point for the large flight-enabling muscles.
Chickens also use their bones to help them breathe. In addition to the trachea and lung system, the avian secondary bronchi also lead into separate air sacs, and from there into the pneumatic bones which contain a light honeycomb structure of air-filled cavities. These bones provide a secondary route by which air exchange can occur. The air sacs which connect the lungs to the pneumatic bones have very thin walls and act like bellows to pull air into the relatively rigid lungs. Pneumatic bones also make birds lighter, allowing them to fly.