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Avian Biology 101

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Well it has been over a year since I first proposed to write about avian anatomy and physiology here on this board. Now that I am in my third year of study to be a veterinarian, I believe I have enough knowledge to write articles on birds from a medical and biological point of view.

I will try to keep this going as long as I can - I am aiming to write at least an article a week but this will vary. Articles may be long or short and will cover a broad spectrum of avian biology.

Feel free to discuss the topics and ask me questions – I will do my best to answer. I hope this is in the right section as it is about birds in general but I will refer specifically to budgies wherever I can.


So let’s begin – from the inside out.


The Avian Skeleton

Birds have special requirements for their skeletons. They need a structure that will provide support for their powerful wing muscles and withstand the stresses of flight, yet will not weigh them down in the air. A characteristic feature of the avian skeleton is the presence of pneumatic bones, particularly in bird species that fly extensively. These are bones that are semi-hollow with criss-crossing struts and filled by air sacs – so the respiratory system is directly connected to the skeletal system. Examples of pneumatic bones are the humerus, femur and pelvis. Beaks are also characteristic of birds, and these structures are much lighter than the hefty mammalian jaw with teeth. These features help lighten the bird while retaining significant strength and size of the bones. However, it is not without consequence - pneumatic bones are more brittle, so they fracture more easily. Due to its association with air sacs, sometimes pockets of air will form under the skin around a fracture site (emphysema).


Another difference in the bird skeleton to the mammalian skeleton is that there are ossifications (fusions) between many bones. In the spine they are grouped into notarium, synsacrum and pygostyle. The notarium is formed from the first 3-5 thoracic vertebrae, and helps provide rigidity in the back when a bird is flapping its wings. The synsacrum is formed from the lumbar and pelvic vertebrae, and helps absorb shock when landing. The pygostyle are the fused and flattened caudal vertebrae that support the tail. It is important to note that the vertebrae between notarium and synsacrum are not fused, presenting a point of weakness in the spine. This area is usually the site of trauma when a small bird is accidentally stepped on, leading to compression of the spinal cord and paralysis of the legs.


*Diagram of the bird skeleton: http://www.birdwatching-bliss.com/images/bird_skeleton.jpg

*Diagram of the fused bones: http://people.eku.edu/ritchisong/RITCHISO//birdskeleton5.gif


Flying birds have a very large carina (keel) of the sternum (breast bone), which is the site of attachment for their powerful pectoral (breast) muscles. The pectoral girdle has three components, akin to the reptiles (mammals have two). These are clavicle (collarbone), scapula (shoulder blade) and coracoid. The clavicles meet to form the furcula (wishbone) and helps withstand pressure when the bird flaps its wings. Each rib has a projection called an uncinate process which overlaps the rib behind it, to prevent the ribcage from collapsing when the bird is flapping.


The pelvic girdle is composed of synsacrum and fused bones of the pelvis. Like mammals, the pelvis is made of ischium, ilium and pubis. In birds the pubis part of the pelvis is open rather than forming a ring, assisting passage of a large egg. There is also considerable fusion in the bird’s forelimb and hindlimbs, forming carpometacarpus, tibiotarsus and tarsometatarsus (breaking down these names will give you the unfused bones in a mammal – carpus, metacarpus, tibia, tarsus, metatarsus). These fusions provide strength for feather attachments and wing/leg strength for flying, landing and takeoff.


*Bird skeleton demonstrating the fused bones: http://academic.emporia.edu/sievertl/verstruc/birdbody2.JPG

*Cat skeleton for comparison: http://www.csd.net/~abyman/images/catskllg.jpg


In conclusion, birds have a highly adapted skeletal system for the stresses of flight. From head to tail they are designed for being in the air, takeoff and landing, tasks which involve incredible forces on the body. The avian skeleton is truly amazing in its high specialisation.


Further reading

Excellent site for diagrams, especially of the fused bones: http://people.eku.edu/ritchisong/RITCHISO//skeleton.html

Good introductory site for each section: http://fsc.fernbank.edu/Birding/skeleton.htm

Veterinary information on avian orthopaedics: http://www.exoticpetvet.net/avian/orthopedic.html

Edited by Elly

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Respiratory system


The respiratory system of birds happens to be my favourite topic in avian anatomy. There are similarities between a bird’s breathing system and a mammal’s, and also some very striking differences. What follows will be a short series about avian respiratory anatomy.


The respiratory system functions to deliver air from the external environment, move it into the body, and then allow the vital molecules of oxygen to be diffused into blood capillaries within the lungs (and then travel in the blood to power the body). At the same time, the waste product carbon dioxide diffuses out of blood to be exported out of the body. The respiratory system is made up of upper (oral cavity, nasal cavity, pharynx, larynx) and lower (trachea, bronchi, secondary bronchi, parabronchi, lungs, air capillaries, air sacs) tracts. The mechanism of breathing in a bird is also vastly different when compared to a mammal.


Part 1: The upper respiratory system and trachea

In the roof of a bird’s mouth there is a slit, the medial choana, allowing direct connection between the nasal cavity and oral cavity. This slit is open while the bird is breathing, and closes when it swallows to prevent the movement of food into the nasal cavity. Birds do not have a soft palate.


The opening to the trachea (windpipe), the glottis, lies in the centre on the floor of the mouth, just behind the base of the tongue. Unlike mammals, birds do not have vocal cords in their larynx, nor do they have an epiglottis. It is important to avoid pointing straight down at the centre of the mouth when introducing a needle for crop feeding, as it may enter the larynx into the trachea and lungs. The correct method is to roll the ball of the needle over the tongue (and hence over the glottis), pointing it more horizontally until it gently contacts the back of the mouth, the entrance to the oesophagus. The glottis is also much smaller than the oesophagus making it more difficult to enter.

In mammals, the trachea is supported by U shaped cartilage rings set apart at fixed distances. In a bird however, the cartilage forms complete rings which overlap, covering the entire trachea. This makes it feel very firm and can be felt on the right side of a bird’s neck. If a crop needle enters the trachea, the ball cannot be felt from the skin, but if it has correctly entered the oesophagus the ball can be seen/felt across the soft wall. In some water birds, the trachea actually coils down into the breast bone, forming loops before entering the thorax.


Birds have an extra feature in their trachea. At the point where the trachea divides into the two bronchi, the walls have membranes that produce sound when vibrated by moving air. The tension of the membranes and hence the pitch of sound can be changed by contracting muscles outside the trachea. This apparatus is called the syrinx. This difference in vocalisation organs (syrinx versus larynx) is the reason why birds can form very complex noises that our own vocal cords cannot replicate, and likewise they cannot produce some of the sounds that we can. Songbirds can operate the two sides of the syrinx independently – allowing it to sing a duet with itself!

Further reading


Stay tuned for the next article, which is about the lower respiratory tract.

Edited by Chrysocome

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Part 2: The lower respiratory system

Following the respiratory system down, the trachea divides into two bronchi. Each bronchus further divides into secondary bronchi, which enter the lungs or air sacs as parabronchi. The parabronchi further divide into air capillaries, which form loops through lung tissue. Air capillaries are the site of gas exchange between oxygen and carbon dioxide, and therefore lie in close association with blood capillaries. (Mammals have alveoli instead).

In mammals, the lungs shrink and expand with each breath, supported by the diaphragm. In a bird the lungs are rigid and of fixed volume, and they do not have a diaphragm. This makes the thoracic cavity and the abdominal cavity one continuous space (the coelomic cavity). The lungs do not wrap around the heart as they do in mammals, instead they are located near the dorsal (spinal) area.


So how does a bird breathe without changing the volume of its lungs? The answer is that they possess air sacs, located throughout their body cavity near many organs. The air sacs expand and contract to move air through the lungs. No gas exchange (O2 for CO2) occurs in the air sacs. Besides acting as bellows, they also lighten the body and extend into the bones to help with flight. Thus, the air sacs closely link the respiratory system, thoracic organs, abdominal organs and musculoskeletal system, which may be of consequence in systemic disease, and makes respiratory infection particularly dangerous.


The positions of the air sacs are: two cervical (not present in some species), one interclavicular, two cranial thoracic, two caudal thoracic, two abdominal.

The movement of air through the sacs is dependent on the position of the sternum (and ribs), controlled by chest muscles. A bird must always be allowed to expand its chest. If the chest is restricted such as when being held incorrectly, the air sacs cannot fill, air cannot be moved through the lungs, and the bird will suffocate.


Another important function of the respiratory system is to do with thermoregulation. Unlike mammals, birds have no sweat glands to cool down. Instead, they rely on increasing their body surface area (see later) and changing how they breathe. They take in more cool air by breathing faster. The movement of air causes evaporation of the moisture in the lining of the respiratory system, taking heat with it. Thus if a bird cannot breathe fast or deep enough (such as being held too tight) it not only has the potential to suffocate but it can overheat as well.


A summary of the major differences between birds and mammals:

Avian | Mammalian

Vocalize with syrinx | Vocalise with larynx

Complete tracheal rings | U-shaped tracheal rings

No diaphragm | Diaphragm

Air sacs | No air sacs

Rigid lung | Expanding lung

One-way air flow | Two-way air flow*

Gas exchange associated with inspiration and expiration | Gas exchange associated with inspiration*


Further reading


*Next time - all about how this system actually works

Edited by Chrysocome

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Part 3: The mechanics of breathing

Note: This topic of article has some difficult concepts but I have tried to keep it easy to read so you can appreciate how the system works. However, feel free to ask questions here as I am always ready to discuss this topic. Also, in this article I have used words like ‘cranial’ and ‘caudal’. Assume for now that cranial means the front end (towards the head). Caudal refers to the back end (towards the tail).


As we saw in the previous article, birds have a complex respiratory system in close association with abdominal/thoracic organs and the bones. In birds, the act of breathing relies on the lungs, air sacs, chest/abdominal muscles and sternum (keel/breast bone). Birds have a unique mechanism of breathing that is very efficient, and flow occurs in one direction (as opposed to the ‘in, then out of lungs’ type of breathing in mammals). In mammals, fresh air reaches the lungs only on inhalation; exhalation pushes the air out of the body. In birds, fresh air arrives at the lungs when they breathe in and out.





Volume and pressure are intimately related (a general law of physics). Imagine we have a fixed amount of air (and therefore a fixed pressure) inside a rubber ball with a hole in it. If we squeeze the ball (decrease its volume), the pressure rises inside so air goes out of the ball. When we let go, the elasticity in the ball makes it change volume and air will go back in. In general, we can think of the air sacs (and mammalian lungs) as a rubber ball with a hole in it. The sternum and chest muscles act as the squeezing/releasing force.


For the diagrams below, the lungs are represented by ‘paleopulmonic parabronchi’ at the top of the picture. The first two air sacs on the left I will collectively call ‘cranial air sacs’; the two on the right are the ‘caudal air sacs’.


Inhalation (filling air sacs)

  • The sternum is lowered and the ribs moved outwards. This allows the air sacs to expand.
  • Fresh air moves from outside, down the trachea, and to the caudal air sacs. (1)
  • Stale air from the lungs is pushed to the cranial air sacs. (3)





Exhalation (emptying air sacs):

  • Sternum is raised, abdominal muscles contract. Air is forced out of the air sacs.
  • Fresh air from the caudal air sacs enters the lungs. (2)
  • Stale air from the cranial air sacs exits the body. (4)




It takes two inhalations to complete a cycle in the respiratory system (follow the numbers above). To summarise, air moves from outside, down trachea to caudal air sacs, across lungs, to cranial air sacs, and out of the body. In both expiration and inspiration, fresh air is moving across the lungs. This makes the birds highly efficient breathers and it is why they are so sensitive to smaller insults than mammals are – for example birds are much more sensitive to the fumes from Teflon cookware because they absorb it into their bodies when both breathing in and out.


Because of the coelomic position of the air sacs, a bird’s tail may pulse (tail bobbing) with each breath if it is having trouble breathing. This may occur if it has just been stressed or exercising, or due to any illness making it hard for the air sacs to change size (this can be anywhere in the body). The tail muscles assist with the sternum and ribs to help push air out of its air sacs.


Respiratory system: Key points

  • Birds rely on the movement of their chest to breathe, so never restrict a bird’s chest
  • They also cool down via evaporation of warm moisture through breathing – not being able to inhale deep enough will lead to overheating
  • Highly efficient breathing means that they are more susceptible to smaller insults than mammals (Teflon fumes, infection)
  • The respiratory system is physically close to the thoracic organs, abdominal organs and bones. Infections of the respiratory system can quickly become widespread, and disease can move between any of these areas. So you may see signs in one body system even if the real problem is in another.

Further reading:


Next up: Anatomical directions. This will help you understand general anatomical descriptions in all animals (and maybe what I’ve said previously will make more sense!)

Edited by Chrysocome

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That is excellent stuff Chryso. Good revision for me :) Very good and you have written your articles so well.


I'll add some stuff that really interests me:


Many birds are also able to move their maxilla (upper jaw/beak) separately from their cranium, this also occurs in some fish, whereas in mammals this joint is fused.


The temporomadibular joint is also quite different to mammals. Mammals have a very unique jaw, which is one of the many things that define them. A mammalian jaw connects to the temporal bone with the ramus of the mandible travelling up under the zygomatic arch (the bone which forms the lower part of the eye socket in most animals and forms our cheek bone), this forms a complex joint. In birds, reptiles, amphibians and fish this joint is a simple hinge joint.



Dog skull


Secretary bird skull


On your first article: you forgot to mention that the sacrum is also fused to the pelvic and lumbar bones to form the synsacrum. But that is me just nit picking.

Good work

Edited by Sailorwolf

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