What More than Wings?

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Have you ever wondered what tools birds use to fly besides wings and feathers? The answer lies within a bird’s respiration system and skeleton.

An easy way to understand avian respiration is to compare it to our own. When humans breathe the flow of air makes a U shape, flowing into the lungs then directly out. When we breathe there is a bit of air left behind, as our lungs do not contract completely when we exhale (i). When birds breathe airflow is in more of a circular pattern so no air is left behind – making avian respiration more efficient than human respiration.

Birds have a pair of lungs located in the center of the body cavity (i). Think of a bird like a hot air balloon with a small motor in the middle (the lungs), but a big sac of air that really helps generate lift and stay in the air (air sacs). Birds don’t have one big air sac; they actually have 7-9 air sacs located throughout the body depending on the species Figure 1 (i).


Figure 1. Avian respiratory anatomy

The air sacs inflate like balloons when air passes through them via the lungs or another air sac (i). Air moves through the bird by way of pressure changes in the air sacs passing through the lungs on the way in and out of the bird. This creates a circular flow of air(i). Generally speaking, the respiratory cycle has four steps after entering the trachea: anterior air sacs to lungs to posterior air sacs and then out.

Birds have a few nifty adaptations in their skeletons to help with breathing and flying. One adaptation is the uncinate processes located on the rib cage in Figure 2 (ii). This process serves to strengthen the rib cage during flight as well as during respiration (ii). Interestingly Codd’s work found that the process is not present in the famous fossil Archaeopteryx or in a modern bird known as a screamer (ii). Codd’s study of the uncinate process helps explains how birds achieve respiration successfully since their lungs are anchored to their ribs.


Figure 2. Uncinate processes highlighted in red

Another adaptation is having pneumatized or hollow bones. Having hollow bones helps reduce the weight, but the bones are not just hollow like a tube. They are more similar to an airplane wing; which is hollow but has a network of supporting bars inside. Having bones that are hollow but containing supports allows for maximum efficiency of the skeleton by reducing weight without compromising strength (iii). Also, skeletons of birds have been found to be stronger and stiffer than mammal skeletons in relation to weight (iii).

Paired together air sacs, uncinate processes, and pneumatized bones are a few of the key adaptations that birds use for flight beyond just their wings and feathers.

If you want to check out a video explaining the respiration system try this link:

Brittany Culp, Hospital Intern, 2016

(i) Foster, Dr., and Smith, Dr. “Respiratory System of Birds: Anatomy and Function.” Peteducation.com. N.p., n.d. Web. 16 Nov. 2016.
(ii) Codd, Jonathan R. “Uncinate Processes in Birds: Morphology, Physiology and Function.” Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 2010 (2009): 303-08. Web. 28 Apr. 2015.
(iii) Dumont, Elizabeth R. “Bone Density and the Lightweight Skeletons of Birds.” Proceedings of the Royal Society B: Biological Sciences 277.1691 (2010): 2193–2198. PMC. Web. 17 Nov. 2016.

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