Respiration

Breathing is one of the most critical bodily functions for animals.  There are very few species that can go a long time without oxygen and chickens aren’t one of them.

In this section, we will learn more about how the respiratory system plays a key role in helping chickens (i) absorb the oxygen they need to power their cells, (ii)release carbon dioxide and other waste products to maintain their internal chemical balance, (iii) regulate their body temperature by releasing excess heat, and (iv) make chirps or squawking noises to help communicate with others around them.

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Respiratory Anatomy

Chickens normally take about 30 breaths per minute.  This may increase to about 35 breaths per minute when they are active during the day and drop as low as 15 breaths per minute when they are resting at night.  In hot weather, they may start to pant (defined as greater than 150 breaths per minute) to help them get rid of excess heat through the evaporation of water from the lining of the respiratory tract.  This is really important for chickens because they don’t have sweat glands like we do to let them use evaporative cooling from their skin.

There are 8 main components to the chicken respiratory tract that allow chickens to pull fresh air into the body, circulate it through the lung tissue to allow gas exchange and then push the used air back out:

Most of the time chickens will breathe air in through their nostrils.  They may start to pant through their mouth or open-mouth breathe if they are getting too warm, if the nostrils are blocked, or if they are not getting enough oxygen from gas exchange.

The larynx is the structure towards the back of the mouth that prevents food, water, and other objects from entering the lungs. When the birds swallows, this triggers the glottis (flap at the opening of the airway) to close.  When the bird has finished swallowing, it opens again to allow air flow.

The trachea is the main tube that connects the outside with the inside.  It’s made up of tough cartilaginous rings to stop it from collapsing from the suction pressure pulling air into the lungs.  Think of how a balloon collapses when you try to suck air out. 

The syrinx sits just inside the body cavity and is the chicken equivalent of a voice box.  By using muscles that pull on the valve in the syrinx to modify its shape, chickens can create a whole variety of noises. Whether or not chickens crow depends on the level of male hormones (testosterone) and female hormones (estrogen).  Roosters crow because they have high testosterone and low estrogen.  Some hens may start to crow if their ovaries become diseases and stop producing estrogen.

Air sacs are balloon-like structures that act like bellows to pull air into body and push it out again.  They don’t move themselves, but rather respond to changes in pressures as the chest wall expands and contracts. The air sacs also connect into the pneumatic bones (bones in the skeleton that are hollow and air-filled to keep them light including the skull, humerus, clavicle, keel, pelvis, and spine).

Bronchi are the larger tubes that allow air to pass from the trachea, into the air sacs, into the lungs, and then back out again.

Parabronchi are continuous tubes inside the lungs that allow air to flow through in one direction.

Chicken lungs are small, stiff, and attached firmly to the rib cage. Unlike us, the lungs don’t expand or contract very much so they rely on the air sacs to regulate the flow of air. The lung tissue has a lot of small blood vessels that allow gas exchange to occur. The effective exchange surface per volume unit is about ten times larger than our lungs.

Respiratory Cycle

The chicken respiratory system is designed to be very efficient with air always flowing in a one-way direction rather than the tidal system we have where fresh air goes in and waste air comes back through the exact same set of pipes.

To make this happen, each breath a chicken takes is divided into two cycles, each with an inspiration and expiration phase:

Cycle 1
Inspiration
Cycle 1 - Inspiration

During this phase, the ribs and sternum (also called the keel or breast bone) expand creating negative pressure that pulls air into the caudal air sacs from the outside world.

Cycle 1
Exhalation
Cycle 1 - Exhalation

Next, the ribs and sternum contract to push air up through the bronchi and parabronchi inside the lungs.

Cycle 2
Inspiration
Cycle 2 - Inspiration

The ribs and sternum expand again creating negative pressure that pulls air from the lungs into the cranial air sacs.

Cycle 2
Exhalation
Cycle 2 - Exhalation

The ribs and sternum contract again pushing air to the outside world through the trachea.

Since chickens don’t have a complete diaphragm, their main way of drawing air into their lungs is by expanding their rib cage and sternum. 

That’s why when you hold a chicken, it’s really important to have one hand supporting them underneath their body behind the sternum and one hand lightly holding the wings against their body from above their rib cage.  Squeezing a chicken tightly against you will stress them out because they cannot breathe.