The Unique Respiratory System of Birds
Birds cannot survive without breathing, and their naturally high metabolic rate increases their oxygen demand. Some deep-diving birds can go over 20 minutes without breathing, while a Hummingbird may take a staggering 250 breaths per minute!
Avians have a highly efficient respiratory system, entirely different from our own. While its goal remains to introduce oxygen and remove carbon dioxide from the bloodstream, the path of airflow and the elements that make up this complex system are unique to birds and a few other surviving animals.
Studying the avian respiratory system is critical to understanding bird health and behavior for veterinarians and anyone interested in our feathered friends. This fact-filled guide will introduce key details on the unique avian respiratory system.
Anatomy of the Avian Respiratory System
Trachea and Bronchi
Air circulating through the avian respiratory system flows through the trachea (windpipe) that runs from the throat to the lungs. This structure is reinforced with rings of cartilage that prevent it from collapsing under the negative pressure caused during inspiration.
The trachea forms an inverted ‘Y’ junction where it branches into smaller tubes called bronchi, which later branch further into smaller passages like ventrobronchi, dorsobronchi, and parabronchi. Each bronchus enters a lung and passes right through it and into air sacs located below and behind the lungs.
Bird Lungs
The avian respiratory system includes two symmetrical lungs, although they look and function very differently from our own. They are over 25% smaller than mammal lungs relative to body mass and have a spongy consistency. While our lungs can blow up like balloons, avian lungs have a more or less fixed volume.
Air Sacs
Most birds have a system of nine air sacs that surround the lungs and connect to hollow (pneumatic) bones. These air sacs are expanded and compressed by muscular action, causing changes in air pressure that push and pull air through the respiratory system.
Air sacs are not vascular enough to contribute to oxygen absorption, although they perform an essential function in keeping birds cool. Evaporation within the respiratory system removes heat and lowers a bird’s core temperature, which is especially important during and after the physical exertion of flight.
Birds can actively manage their body temperature by using certain breathing techniques, so you’ll often see them panting on hot days.
Diagram of the Respiratory System of Birds
The Mechanics of Bird Respiration
One-Way Airflow
Birds have a one-way (non-tidal) airflow through the respiratory system. This avoids the problem of exhaled air mixing with inhaled air and ensures a constant supply of oxygen. The pathway of air is relatively long, so birds require two inhales and exhales for each cycle.
Gas Exchange Process
While mammals exchange oxygen and carbon dioxide in the alveoli of the lungs, birds have a system of extremely thin-walled air capillaries and blood capillaries where gases diffuse in and out of the bloodstream. The avian system is superior since the air capillaries are interconnected and allow through-flow, unlike the blind-ending structures of the mammalian lung.
Respiratory Cycle
The avian respiratory cycle is a little more complex than our own. It is powered by muscles that shift the position of the ribs and sternum, creating pressure changes that increase the volume of the air sacs to push and pull air through the various structures of the respiratory system. During flight, expansion and contraction of the air sacs caused by the upstroke and downstroke can also assist in ventilation.
The (simplified) four-stage cycle:
- On the first inspiration, air is drawn through the lungs and into posterior air sacs.
- On the first expiration, the air in the posterior air sacs is pushed into the lungs for gas exchanges.
- On the second inspiration, the now deoxygenated air is drawn into the anterior air sacs.
- On the final expiration, the air in the anterior air sacs is expelled through the mouth or nares.
Song Sparrow breathing on a cold winter morning
Respiratory System Efficiency and Flight
High Metabolic Demand
Flying is a highly effective means of locomotion that allows birds to forage over large distances or even migrate between opposite ends of the world and back each year.
Even with lightweight feathers, powerful muscles, and a frame perfectly adapted for the air, flight is very costly, using roughly twice as much energy as mammalian running. The high energy and oxygen costs of flight, coupled with a bird’s naturally high metabolic rate and body temperature, make an efficient respiratory system essential for survival.
Oxygen Utilization
The high breathing rate, efficient one-way airflow through the lungs, and the extensive contact area between blood and air capillaries all work together to maximize oxygen diffusion. These adaptations help birds get up to 25% more oxygen from the air, increasing their endurance and ability to survive at high altitudes.
Adaptations and Evolution
Evolutionary Development
While the complex and efficient unidirectional respiratory system has clear benefits for flight, it did not evolve in birds. The presence of soft tissues is difficult to confirm from fossils, although evidence suggests that extinct archosaurs and non-avian dinosaurs, such as pterosaurs, sauropods, and theropods, had a system of pneumatic bones and air sacs long before avians evolved.
In fact, the unidirectional airflow system that evolved in archosaurs still occurs in crocodilians and some large lizard species today. However, specializations like rigid lungs and a cross-current blood-exchange pattern are thought to be unique to birds, which have a much higher oxygen demand. The fossil record indicates that this system has occurred in avians for over 120 million years.
Special Adaptations
The avian respiratory system is generally considered the most advanced of any vertebrate animal. Some species have even evolved special adaptations to further advance their performance, depending on their habitat and foraging habits.
High-altitude birds
Birds that live, soar, or migrate at high altitudes risk losing consciousness due to low oxygen levels in the air. Hemoglobin in blood is responsible for transporting oxygen around the body, and high-flying birds, like Vultures may have two or more types of this vital protein, allowing them to absorb more oxygen in thin air.
Nare coverings
Many birds, including Crows and Woodpeckers, have bristle-like feathers that cover their nares (nostrils). These are thought to prevent foreign bodies from entering the respiratory system. Diving birds like Gannets and Pelicans have evolved coverings over their nares to prevent water from entering the nostrils when diving.
Common Cranes can fly at heights of over 10,000 feet. Hemoglobin in blood allows birds to absorb more oxygen in thin air.
Implications for Avian Health
Common Diseases
Birds suffer from various respiratory diseases caused by bacterial, fungal, and viral infections. Some of these pathogens are highly infectious, causing massive losses in poultry farms where birds are housed in close quarters.
Avian influenza is a well-known avian virus that can be particularly devastating to poultry, although it also affects wild birds of prey and waterfowl.
Aspergillosis is another respiratory disease caused by a fungal infection that affects birds after contact with moldy food sources. This infection occasionally occurs in wildfowl that eat old grain.
Mycoplasma bacteria cause conjunctivitis and respiratory system infection in wild birds like House Finches and Purple Finches. This infection is spread through contact with infected surfaces like bird feeders, which is why excellent hygiene is so important when feeding wild birds.
Environmental Sensitivities
Pathogens aren’t the only cause of disease in the avian respiratory system. Harmful chemical pollutants in the form of gases and particulate matter can be devastating to wild and captive birds.
The following air pollutants are known to affect birds:
- Benzene
- Sulfur dioxide
- Ozone
- Carbon monoxide
- Nitrogen oxides
Mycoplasma bacteria can be harmful to birds like Purple Finches, who can get conjunctivitis and respiratory system infection
Summary
The avian respiratory system is uniquely adapted to deliver oxygen, remove carbon dioxide, and cool the body, all of which are vital for the survival of birds.
A naturally high metabolic rate, coupled with the rigors of flight, has shaped a complicated system of air sacs and inflexible lungs, further refined for the specialized foraging techniques and extreme habitats of various species.
Whether it’s paying extra attention to hygiene at your bird feeders, supporting local conservation projects, or minimizing your use of fossil fuels and pollutants, there’s always something we can do to help our feathered friends and their remarkable and sensitive respiratory systems.