Do Chickens Breathe Through Their Butts?

Bird lungs fill 20% of their bodies, ours, 5%

I see that somebody asked that question in a forum online. It met with dismissive and incredulous ridicule. But I KNOW why that curious person asked, because, while knowing a bit more about avian respiration from comparative anatomy days, I wondered why I was seeing the rhythmic puckering of our hens’ rumps. I assumed it was not digestive but probably somehow related to their breathing, and I think I’m right.

Chickens (and all birds) have a respiratory process that is quite un-mammalian and not surprisingly related to their high metabolic rate and the physics and blood chemistry associated with flight. Their lungs are not like ours, lacking the tiny permeable alveoli where gas exchange takes place.

Bottom line (and then you can read more fascinating details about bird breathing) is that what I think I’m seeing is the filling and emptying of the abdominal air sacs that fill the posterior gut near the feathery puckering poop-port. So, dear forum member, no, they don’t breathe through their butt, but their breathing accounts for why you ask your question, which is not a stupid as it sounds, after all.

from…Respiratory Cycle of a Bird Birds do not have a diaphragm; instead, air is moved in and out of the respiratory system through pressure changes in the air sacs. Muscles in the chest cause the sternum to be pushed outward. This creates a negative pressure in the air sacs, causing air to enter the respiratory system.

Expiration is not passive, but requires certain muscles to contract to increase the pressure on the air sacs and push the air out.  Because the sternum must move during respiration, it is essential that it is allowed to move freely when a bird is being restrained. Holding a bird “too tight” can easily cause the bird to suffocate!

Because birds have air sacs that reach into the bones, and have no diaphragm, respiratory infections can spread to the abdominal cavity and bones.

Bird lungs do not expand or contract like the lungs of mammals. In mammalian lungs, the exchange of oxygen and carbon dioxide occurs in microscopic sacs in the lungs,  called alveoli.  In the avian lung, the gas exchange occurs in the walls of microscopic tubules called ‘air capillaries.’

From…Enchanted Learning Birds breathe using a unique system in which air follows a one-way route through the respiratory system. This system is unlike our lungs, in which the air backtracks where it came from. Their system of respiration (breathing) is very efficient – much more efficient than our system.

Birds have two relatively small lungs (where gas exchange occurs), but the lungs are augmented by bellows-like air sacs (where no gas exchange occurs). These air sacs keep the lungs perpetually inflated (even when the bird is exhaling). Our lungs alternately fill and empty out. The bird’s respiratory system takes up 20% of a birds’s volume (our respiratory system takes up only 5% of our volume).

In the bird’s respiratory system, air first flows through air sacs (located even inside their hollow bones) that direct fresh, oxygenated air into the tube-like lungs (parabronchi, where gas exchange occurs) both when the bird inhales and when it exhales.

This system increases birds’ respiratory efficiency and gives them enough oxygen for flight.

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Fred First holds masters degrees in Vertebrate Zoology and physical therapy, and has been a biology teacher and physical therapist by profession. He moved to southwest Virginia in 1975 and to Floyd County in 1997. He maintains a daily photo-blog, broadcasts essays on the Roanoke NPR station, and contributes regular columns for the Floyd Press and Roanoke's Star Sentinel. His two non-fiction books, Slow Road Home and his recent What We Hold in Our Hands, celebrate the riches that we possess in our families and communities, our natural bounty, social capital and Appalachian cultures old and new. He has served on the Jacksonville Center Board of Directors and is newly active in the Sustain Floyd organization. He lives in northeastern Floyd County on the headwaters of the Roanoke River.

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  1. Well, now … that is certainly an interesting bit of knowledge. I never gave the matter any thought, but then, I don’t have chickens, either. Thanks for sharing, Fred.

  2. Once we get passed the fact that these birds are made of meat, and sort the edible from the in-edible we discern many mysteries.
    I have sat in the cold and fog, dipping plucking and gutting birds and meditated on the contents, to no avail; trying to sort out what the “extra stuff” is for our kids. And to know a reason for all the puffery associated with yard birds… Aint science wonderful?
    Thanks for the lesson, next time this is going to be really good.

  3. I can report with 100% certainty that this is something I would never, ever have questioned or even wondered about!