Birds have a respiratory system that is uniquely adapted for flight. Their lungs are particularly sensitive compared to other animals for several important reasons:
Small and compact
Bird lungs need to be small and lightweight so that birds can fly. Their lungs only make up around 5-7% of their total body volume, compared to around 12% in mammals. This means each lung is smaller with less surface area for gas exchange. As a result, they have less reserve capacity and are more sensitive to any disturbances or irritants.
Rigid structure
A bird’s lungs are compact and rigid in structure. They do not readily inflate and deflate like mammalian lungs. Air flows in a single direction through the parabronchi (the site of gas exchange) during both inhalation and exhalation. This rigid system is more vulnerable to over-inflation, collapse or occlusion that could disrupt oxygen supply.
Thin blood-gas barrier
The blood-gas barrier where oxygen and carbon dioxide are exchanged is extremely thin in birds. In mammals it is typically 0.5 microns thick, but in passerine birds it can be as thin as 0.2 microns. This allows for very rapid gas exchange to meet their high metabolic demands. However, it also means the barrier is particularly fragile and easily damaged by inflammation or environmental pollutants.
Fewer air capillaries
While bird lungs are highly efficient, the number of air capillaries is reduced compared to many mammals. For example, humans have around 480 million alveoli while barn owls have only around 14 million parabronchi. With fewer sites for gas exchange, loss of function in even a small number of units can have a big effect on blood oxygen levels.
Smaller functional reserve
The proportion of lung tissue that is actually involved in gas exchange (as opposed to structural support) is much higher in birds. However, their functional reserve is smaller, meaning they have less excess capacity in their system. When demand increases they have little ability to improve gas exchange. So any faults or inefficiencies quickly impact oxygen supply.
Vulnerable air sacs
Birds have extensive air sacs that penetrate their hollow bones and body cavities. These help maintain airflow through the rigid lung tissue. However, air sacs are extremely thin and delicate, making them prone to damage, inflammation or infection. As they are integral to the respiratory system, any compromise can severely affect breathing.
Generally lower oxygen levels
The normal partial pressure of arterial oxygen is lower in birds compared to equivalent sized mammals. For example, a typical bird may have a PaO2 around 9.3 kPa compared to around 13 kPa in mammals. This lower baseline oxygenation means their arterial saturation is more vulnerable to even small desaturation events.
Oxygen saturation:
The table below compares oxygen saturation levels between birds and mammals:
Animal | Normal arterial oxygen saturation |
---|---|
Bird | 87-95% |
Mammal (human) | 95-100% |
As shown, birds typically have lower normal saturation compared to mammals. So a small drop that may not affect a mammal could significantly impact a bird.
Highly sensitive to toxins
Birds are extremely sensitive to environmental toxins and pollutants. Their highly efficient respiratory system and thin blood-gas barrier readily absorbs particles and chemicals. These can cause severe inflammation, air sac and lung damage. Birds have been called the “canary in the coal mine” as their response can serve as an early warning of environmental hazards.
Limited breathing mechanisms
During exercise, mammals can increase their breathing rate and tidal volume many fold. Birds are constrained by their rigid lung anatomy. They can only increase ventilation by raising breathing frequency, up to around 120 breaths per minute in small birds. They have very limited capacity to improve oxygen supply when demand increases during flight.
Susceptible to oxygen damage
The intense metabolic activity required for flight produces reactive oxygen species that can damage delicate lung tissues. Birds are thought to be particularly susceptible to these oxygen free radicals. Their huge gas exchange surface and lack of an antioxidant enzyme (glutathione peroxidase) leaves lungs vulnerable to injury.
Metabolic rate during flight:
Animal | Metabolic rate (multiples of basal) |
---|---|
Pigeon in flight | 15x |
Human during marathon | 10-15x |
Birds have metabolic rates during flight far exceeding those reached by even elite human athletes. This extreme demand generates reactive oxygen species that are damaging to avian lungs.
Limited collateral ventilation
In mammalian lungs, collateral ventilation from neighboring alveoli can help maintain oxygenation if some airways are blocked. However, between bird parabronchi there is minimal interconnections. Total blockage of just a few units can markedly impact overall gas exchange.
Minimal lung defenses
Mammalian lungs have various protective mechanisms to keep them sterile, clear debris and fight infection. These include coughing, mucociliary clearance, alveolar macrophages and secretory antibodies. Birds lack many of these defenses, leaving their lungs vulnerable to pathogens and particles.
Highly vascular
Birds have extensive pulmonary blood vessel networks surrounding their parabronchial gas exchange units. This helps provide oxygen quickly to the tissues. However, swelling or minor bleeding into the lungs can readily lead to pulmonary congestion that disrupts oxygen transfer.
Difficult to assess function
Assessing avian respiratory function and diagnosing lung disease is extremely challenging in birds. Observational methods used in mammals such as chest auscultation have very limited application. Very few diagnostic tests have been developed specifically for investigation of avian pulmonary disorders.
Frequent high altitude flight
Many bird species, especially migrants, regularly fly at extremely high altitudes of over 9000 feet. This exposes their lungs to hypobaric hypoxia, increasing their susceptibility to altitude-related pulmonary damage and edema. Sudden descents can also over-inflate delicate lung tissues.
Conclusion
In summary, birds have exquisitely sensitive respiratory systems adapted for their tremendous oxygen needs during flight. Their rigid, compact lungs with immense gas exchange surfaces are highly efficient but vulnerable to many forms of damage. They lack protective mechanisms and reserves seen in mammalian lungs. For birds, maintaining oxygen supply is literally a matter of life and death. Even minor disturbances can rapidly turn fatal. This explains why their lungs are so delicate and unstable compared to other species.