Bird Lungs and Mammal Lungs

"From a structure–function standpoint, the bird lung is superior." — John West, Rebecca Watson and Zhenxing Fu, European Respiratory Journal, 2007¹

"In mammals, one adaptation is lung collapse at shallow depths, which limits absorption of nitrogen. Blood N2 levels remain below the threshold for decompression sickness. No such adaptive model is known for birds." — Gerald Kooyman and Paul Ponganis, 1998²

Summary

The parent article, Arguments from Bad Design, sets a test: name the task, name the cost, and compare a feasible alternative. The bird-lung argument passes the first steps better than most bad-design claims. It names a measurable task, loading oxygen into blood, and its alternative is not a drawing but a lung that about 9,000 living bird species actually use.¹

The claim is made by physiologists, not only by popular writers. John West and two colleagues concluded in 2007 that "from a structure–function standpoint, the bird lung is superior."¹ New Scientist the same year called bird lungs "far superior to ours."³
On gas exchange the claim is correct. Air flows one way through a rigid bird lung, pumped by air sacs. Its crosscurrent layout lets blood leave the lung with more oxygen than the exhaled air holds, which a mammal lung cannot do.¹ The bird blood-gas barrier averages 0.19 µm against 0.47 µm in mammals.⁴
Altitude is where it shows. Sparrows flew at a simulated 6,100 m that left mice comatose,⁵ and bar-headed geese flew in a wind tunnel on air equal to about 9,000 m, helped also by their haemoglobin, larger lungs and a lowered metabolic rate.⁶⁵⁶
At sea level the mammal lung is not the limit. Ewald Weibel's group matched every step of the mammalian oxygen pathway to peak demand and found the lung was the one step with spare capacity, "up to a factor of 2."⁷ Bats fly on a tidal lung with a higher morphometric diffusing capacity than birds.⁸ The cost appears only at the extremes: elite runners lose arterial oxygen and racehorses bleed into their lungs.⁹⁴
Diving is the one measured mammalian advantage. Seal and sea lion lungs collapse at depth, which limits nitrogen uptake and saves oxygen for the ascent.¹⁰¹¹ The rigid air capillaries of penguins need some other protection, not yet identified.¹²
The usual design defence, that an animal also has to talk, cough and pant, mostly fails. Birds sing on exhaled air at the same pressures as human speech, parrots mimic speech, and ostriches pant for eight hours.¹³¹⁴¹⁵ Coughing differs between the classes, but nobody has measured the difference as an advantage.¹⁶
One-way airflow is not unique to birds. It was measured in alligators in 2010 and in two lizards in 2014, none with bird-style air sacs, and not for aerobic capacity.¹⁷¹⁸ That kills the creationist premise that "all other vertebrates" breathe in and out through the same passage.¹⁹
Creationists concede the performance claim. They call the bird lung "an excellent engineering design"²⁰ and answer that the human lung is "rather impressive" for its own tasks.²¹

Verdict: "better at oxygen extraction" is established. "Better designed" does not follow, because the mammal lung's measured job at sea level is not limited by extraction and it has one demonstrated advantage the bird lung lacks. The design reply is weaker than its advocates usually make it: most proposed trade-offs are unmeasured or false, the fittest mammals do hit the lung's limit, and the class-by-class distribution of the two lungs fits common descent without extra assumptions.

Part 1: The claim

The scientific form

In January 2007 John West, a respiratory physiologist at the University of California, San Diego, published "The human lung: did evolution get it wrong?" with Rebecca Watson and Zhenxing Fu. Their abstract lists the costs of tidal breathing: "The result is that mammals have a reduced alveolar and arterial oxygen tension, a potential for uneven ventilation, and relatively large terminal air spaces."¹ Large air spaces leave the capillaries poorly supported, so the blood-gas barrier must be thicker. The paper ends by asking "why did evolution take what appears to be such a flawed path in the mammal?"¹ West's 2009 review closes more bluntly: "Nature has produced a more efficient respiratory system in birds than in mammals."⁴

West does not argue about design. He calls the evolutionary question "probably not a useful one."¹

The popular form

New Scientist printed "Evolution's greatest mistakes" by Claire Ainsworth and Michael Le Page on 11 August 2007. Its first entry, "Bad ventilation," states the verdict: "Bird lungs are far superior to ours and, the more they are studied, the longer the list of their advantages becomes. We mammals might have been even more successful had we inherited or evolved similar lungs."³ It adds a causal chain that West did not: alveoli must be large, so their walls are thick and delicate, "leading to a condition called emphysema."³

Two details in the popular version are wrong. West's own paper only says "It is tempting to believe" that repeated stretching of alveoli may contribute to emphysema.¹ The National Heart, Lung, and Blood Institute names smoking as "the main risk factor" for the disease.²² And the "griffon vulture" that New Scientist dates to 1975³ was a Rüppell's griffon that struck an airliner at 37,000 ft over Abidjan on 29 November 1973, according to Roxie Laybourne, who identified its feathers.²³ Neither slip touches the core claim.

The strongest form of the argument

As an argument against design, the claim runs like this. A better lung exists in living animals. A designer able to build it gave mammals a worse one, including mammals that fly and climb. So the mammalian lung is either careless design or no design. Unlike most bad-design arguments, it compares two lungs that both work, not a real organ with an imagined one.

Part 2: How each lung works

The mammal lung is a pool

A mammal breathes in and out through the same tubes, which end in blind sacs, the alveoli. In West's figures, "a small tidal volume of ∼0.5 liter in humans is delivered into a large resting volume of the lung of perhaps 3.5 liters."⁴ Fresh air mixes into stale air, so the gas in the alveoli holds less oxygen than the air breathed in: "in the human lung, PA,O2 is typically ∼13.3 kPa (100 mmHg) in contrast to the oxygen pressure of the inspired gas, which is ∼20 kPa (150 mmHg)."¹ Blood leaving an alveolus can at best match that alveolar gas.¹ Because fresh air must reach the far end by mixing, "the alveoli have a diameter of ∼300 μm, whereas the air capillaries that surround the blood capillaries in the bird lung have a diameter ∼5% of this."¹

The mammalian diaphragm divides chest from abdomen, and in humans at least the start of each breath out comes from passive recoil. Birds have no diaphragm, and both breathing in and breathing out are driven by muscle.¹³

The bird lung is a through-flow exchanger

John Maina, who has published measurements of bird lungs since the 1980s, summarises it: the lungs "serve as gas exchangers and air sacs that operate as ventilators. Utterly rigid, the avian lungs are deeply fixed to the ribs and the vertebrae."²⁴ Air sacs push air one way through narrow tubes, the parabronchi, and blood crosses the airflow at right angles.²⁴ This crosscurrent layout "potentially allows the oxygen pressure of the blood leaving the lung to exceed that of the expired gas. This is impossible in the mammalian lung."¹ The layout was proved by experiment: Johannes Piiper and Peter Scheid reversed the airflow through duck lungs and found no change in blood oxygen, which a countercurrent system could not do.²⁴

The barrier is thinner, and the reason matters

West compiled harmonic-mean barrier thickness for 34 bird and 37 mammal species: "0.47 and 0.19 μm, respectively," a factor of 2.5. A violet-eared hummingbird measures 0.099 µm and a shrew 0.34 µm.⁴ Birds also carry "about a 15% larger surface area" of barrier than mammals of the same mass.⁴ The advantage is not universal among birds. The ostrich (0.56 µm) and the Humboldt penguin (0.53 µm) have thicker barriers than the mammalian average.⁴

The reason is the important part. Bird capillaries can be thin because the tiny air capillaries around them brace them, and "the support is available because the terminal air spaces in the avian lung are extremely small due to the flow-through nature of ventilation."⁴ The thin barrier is not a separate improvement that could be fitted to a tidal lung. It comes with the whole avian package: a rigid lung, air sacs, and a body cavity divided around them. A fair comparison must price the package, not the barrier alone.

On gas exchange, then, the critics are right. Creationist writers agree: the Institute for Creation Research says birds have a system "much more efficient" than an oxygen mask, "thanks to their Creator."²⁵

Part 3: Altitude, endurance and diving

Altitude: a real bird advantage

Vance Tucker's 1968 experiment is still the cleanest comparison. As a 2015 review summarises it, "even lowland sparrows can fly in a wind tunnel while breathing air that simulated the Po2 at 6,100 m, which was sufficiently hypoxic to render domestic mice comatose." The same review reports that hypoxia-tolerant birds survive an oxygen pressure of about 2.7 kPa, against about 4.7 kPa for the most tolerant mammals, mole rats.⁵

Bar-headed geese cross the Himalaya mostly at 5,000 to 6,000 m, and one tracked bird reached 7,290 m.⁵ In 2019 Jessica Meir and colleagues hand-reared geese and flew them in a wind tunnel on low-oxygen gas. Three birds flew at 7% oxygen, "equivalent to an altitude of roughly 9,000 m," though only for 60 to 468 seconds.⁶ At that oxygen level one resting goose's arterial oxygen pressure was 36.7 mmHg.⁶ Acclimatised climbers sampled at rest at 8,400 m on Everest averaged 24.6 mmHg.²⁶²⁷

The lung is only one reason. Scott's review lists breathing control, tissue capillaries, a haemoglobin with higher oxygen affinity (P50 of 4.0 kPa against 5.3 kPa in the lowland greylag), and "∼25% larger lungs than lowland waterfowl."⁵ Meir's team concluded that hypoxic flight "is largely achieved via the reduction in metabolic rate."⁶ The altitude record belongs to the whole bird.

Sea level: the mammal lung has spare capacity

Ewald Weibel, C. Richard Taylor and Hans Hoppeler tested whether each step of the oxygen pathway in mammals, from lung to mitochondria, is built just large enough for peak demand. Blood, heart, muscle capillaries and mitochondria matched. The lung did not: it "maintains a considerable excess diffusing capacity, up to a factor of 2 in sedentary species," which "also allows sedentary animals to maintain their aerobic capacity in a hypoxic environment."⁷⁷ In most mammals at sea level, better oxygen extraction in the lung would not raise peak aerobic output, because the lung is not what sets it.

Bats are the test case the critics must face. They achieve powered flight on a tidal lung. Maina and Anthony King measured five bat species in 1984 and found "a higher DLO2 in the bat than either in the birds or in the terrestrial mammals," achieved with larger lungs and more surface.⁸ Maina's conclusion in 2000: "Bats improved a fundamentally mammalian lung to procure the large amounts of oxygen needed for flight. The lung/air sac system of birds is not therefore a prescriptive morphology for flight."²⁸

The honest limit: the most athletic mammals do reach the lung's edge. Jerome Dempsey's group found that 12 of 16 elite runners lost 10 mmHg or more of arterial oxygen pressure during hard running at sea level, and suggested a diffusion limit in the lung.⁹ Thoroughbred racehorses, bred for centuries for aerobic output, show stress failure of lung capillaries: "essentially all of these horses in training are bleeding into their lungs."⁴ The mammal lung is adequate for ordinary life and costly at the extreme.

Diving: the one measured mammalian advantage

A diving mammal takes air down with it, and under pressure its nitrogen dissolves into the blood, risking the bends on ascent. In 1985 K. J. Falke and colleagues drew arterial blood from free-diving Weddell seals and found nitrogen tensions that peaked at 40 to 80 m, "indicating that the seal's lung collapses by 25 to 50 meters." Their conclusion: "alveolar collapse and redistribution of blood nitrogen allow the seal to avoid nitrogen narcosis and decompression sickness."¹⁰ In 2012 Birgitte McDonald and Paul Ponganis logged a California sea lion whose lungs collapsed at about 225 m. The collapse also "preserved a pulmonary oxygen reservoir that supplemented blood oxygen during ascent."¹¹

Soft alveoli can empty into stiffened airways. A rigid bird lung cannot. Kooyman and Ponganis: "No such adaptive model is known for birds."² A 2015 CT study of penguins found their air sacs too small to shield "the non-collapsing, rigid air capillaries" on the deepest dives, and concluded that some other volume reduction "must occur," by a route still unknown.¹² The deepest recorded bird dive is an emperor penguin's 564 m;²⁹ the deepest mammal dive is a Cuvier's beaked whale's 2,992 m.³⁰ Depth also depends on body size and oxygen stores, so the gap does not measure the lung alone. The collapse mechanism, though, is measured, and it is a job the tidal design does that the through-flow design cannot do in the same way.

Part 4: Talking, coughing and panting

The design reply usually lists other jobs a mammal lung must do. Each is a claim to test.

Talking. The through-flow lung does not prevent vocal skill. Songbirds sing on the exhaled airstream, and "during phonation" their air-sac pressure "ranges from 10–60 cm H2O ... This range is similar to that found during human speech and singing."¹³ Canaries take mini-breaths between syllables up to 33 times a second, and some species sing trills lasting "30 seconds to a minute."¹³ Parrots shape sound with the tongue, which "may thus in part explain the well-known ability of parrots to mimic human speech."¹⁴ Speech is not a demonstrated reason for a tidal lung.

Coughing. Here the classes do differ. The mammalian trachea has C-shaped rings with a soft back wall, "considered an important feature of the clearance mechanism of the mammalian cough," while "the bird's tracheal rings are complete." Brown and colleagues add: "Although birds do perform a coughlike action, we do not know if such behavior represents an effective mechanism for clearing debris."¹⁶ West's paper cuts the other way: in a mammal a blocked airway shuts down the gas exchange beyond it, while in a bird inhaled material "will end up in the nonvascular air sacs," so "Presumably the bird avoids these problems."¹ Maina's 2023 review rejects the older idea that birds have weak lung defences, crediting them with "an exceptionally efficient mucociliary escalator."³¹ Birds are more sensitive to carbon monoxide, which is why miners carried canaries, but Brown and colleagues write that "we do not know the physiologic basis" for the difference.¹⁶ Clearing is a proposed trade-off, not a measured one.

Panting. Birds pant. Knut Schmidt-Nielsen's team kept ostriches below 40 °C for eight hours at 50 °C by respiratory evaporation, with breathing rising from 6 to 12 breaths a minute to 40 to 60. Ostriches avoided alkalosis, though "all other birds examined (nine species) exhibit marked alkalosis after heat stress."¹⁵¹⁵ Panting is not a reason for a tidal lung.

Growth and size. Human lungs keep adding alveoli through childhood; between ages 7 and 21 their number rises about 1.94-fold.³² A bird lung, by contrast, "cannot be inflated out of a collapsed state," and air begins to enter it "three to four days before hatching," as Michael Denton reports from the anatomist Hans-Rainer Duncker.¹⁹ A through-flow lung might therefore struggle in an animal born from a fluid-filled womb, but that is untested. The tidal lung spans barrier areas from about 0.02 m² in a shrew to over 1,000 m² in a horse,⁴ and whales are larger still, but no measurement shows that a bird lung could not scale as far.

Part 5: Alligators, lizards and the history of the bird lung

Until 2010 one-way airflow was thought to be a bird invention for flight. Colleen Farmer and Kent Sanders then showed "that air flows unidirectionally through parabronchi in the lungs of the American alligator, an amphibious ectotherm without air sacs."¹⁷ In 2014 Emma Schachner and colleagues found it in the savannah monitor lizard, outside the crocodile-bird group entirely,³³ and Robert Cieri's team found it in the green iguana, which has no parabronchi. Their conclusion: "unidirectional flow is not an adaptation for expanding aerobic capacity," and "we know of no clear evidence for increased efficiency of gas exchange in nonavian reptiles over that seen in mammals."¹⁸¹⁸

This has three consequences.

For the "later improvement" framing. On the standard evolutionary account, one-way flow is not a recent upgrade that mammals missed. Schachner's team puts its origin, if it evolved once, "in a small ectothermic tetrapod during the Palaeozoic era at least a hundred million years before the origin of birds."³³
For the creationist "no intermediate" argument. Denton wrote in 1986 that "In all other vertebrates" air moves "in and out through the same passage," and that the bird lung could not work until parabronchi and air sacs were "both highly developed."¹⁹¹⁹ Alligators and iguanas show one-way flow working without air sacs. The strong form of the objection is dead. A weaker form survives: the full bird system, a rigid lung sealed off by a horizontal septum, is still not shown in any fossil, and Schachner and Andrew Moore argued in 2025 that "a fully avian lung should not be reconstructed in non-avian ornithodirans."³⁴
For both models. The discovery surprised evolutionary biologists too: one-way flow had "long been considered a unique and specialized trait" of birds.³³ But the two lungs now sort by lineage. One-way flow has been measured in birds, crocodilians and two lizards, and in no mammal. Common descent accommodates that sorting without further assumptions. A design model has to say why the tidal lung went to every mammal, including flying bats, and the through-flow lung to birds and some reptiles. Brian Thomas of the Institute for Creation Research answered that "birds and alligators look as though they were separately and uniquely engineered."³⁵ That is compatible with the data, but it does not predict the pattern.

Part 6: What creationists have said

The creationist literature treats the bird lung as a problem for evolution and as evidence of design, and it does not dispute the performance claim.

Michael Denton, 1986: "Just how such an utterly different respiratory system could have evolved gradually from the standard vertebrate design is fantastically difficult to envisage."¹⁹ In a 1999 interview he called it "almost inconceivable."³⁶
Answers in Genesis, 1999, on the bat point: "Actually, bats do very well with the “bellows” type of lung."³⁶
Jonathan Sarfati, Creation Ministries International, 2009: the bird system is "an excellent engineering design," while "Reptilian lungs operate like bellows ... The stale air is then breathed out the same way it came in."²⁰²⁰ The page was updated on 31 August 2024 and still does not mention the alligator, monitor or iguana results.
Creation Ministries International, replying to New Scientist in 2007, argued that "some bats can fly with comparable speed and superior manoeuvrability to birds, yet accomplish this using the bellow lung system," and compared the complaint to calling a moped deficient beside a jet fighter.²¹²¹

Priced against the sources, the replies are mixed. The bat point is right and is their strongest, but CMI footnoted it to a 2005 paper whose abstract compares one bat with two rodents, not with birds.³⁷ The support is Maina and King 1984.⁸ Answers in Genesis and Sarfati describe the bird lung as countercurrent, blood flowing "in the opposite direction" to air.³⁶²⁰ The flow-reversal test shows the main exchange is crosscurrent.²⁴ CMI's claim that pterosaurs could fly "without" the avian system²¹ is unsettled: pterosaur bones carry air-sac traces, and Farmer suggests one-way flow may date back to them.¹⁷³⁴ The moped analogy concedes the gap, and rightly says a gap is not a defect unless the slower machine fails at its own job.

Objections and responses

"Better oxygen extraction simply is better design. Oxygen is what a lung is for." Only if extraction limits what the animal does. Weibel's data show it does not for most mammals at sea level, where the lung is the one oversized step.⁷ Where it does limit, at extreme altitude and in elite athletes, the critics have a point.
"The design side invents trade-offs after the fact." Often, yes. Talking and panting fail, and coughing and live birth are unmeasured. An honest design reply rests on diving and on the sea-level spare capacity, not on a list of guesses.
"Bats show the design argument's problem: why not give them the better lung?" Bats reach flight's oxygen demand with a larger tidal lung and more barrier surface.⁸ No study shows a bat limited by its lung. But that shows the bat lung is adequate. It does not explain why bats got this lung rather than the other.
"Emphysema proves the mammal lung is fragile." West offered that link only as "tempting to believe,"¹ and the main measured cause is smoking.²²
"Racehorses bleeding shows the mammal lung is second-rate." It shows that a thin, unsupported barrier fails under the pressures reached by an animal bred for hundreds of years for aerobic performance. That is a real cost of the design at the extreme, and it should be conceded.
A creationist objection: "One-way flow in reptiles is separate engineering, not ancestry." Separate engineering fits the data but says nothing about which animals should have which lung. Common descent did not predict one-way flow in reptiles, but once it was found, it explains why the trait follows the diapsid lineage and skips the mammals.

Issues and unanswered questions

Weaknesses of the design reply. The avian lung is better at its central job, and the fittest mammals, human and horse, hit the limits of theirs. The one measured mammalian advantage, diving, serves few mammals. The two lungs sort by lineage, which common descent explains more simply than a design model does.
Unmeasured costs. No study has measured whether a through-flow lung would impair human speech breathing, clearing of the airway, or birth from a fluid-filled womb. These remain proposals, and this article treats them so.
Not obtained. Tucker 1968 (Journal of Experimental Biology 48:55) is cited through Scott et al. 2015. The full texts of Canals et al. 2005, Maina 2000, Farmer and Sanders 2010 and Falke et al. 1985 were not opened; their abstracts were. Duncker's account of bird-lung aeration is reported through Denton. Nathan Lents's Human Errors (2018) could not be searched, so whether it makes the bird-lung argument is unchecked.
Penguins. How penguins protect rigid air capillaries on deep dives is unknown,¹² and the answer may narrow the diving comparison.

Verdict

The bird lung is the better gas exchanger. The barrier is thinner, the exchange is crosscurrent, and it wins in thin air. That much of the bad-design claim is measured and should be granted.

"Second-rate" does not follow. The mammalian lung carries spare capacity at sea level, supports powered flight in bats, and collapses safely under pressure in a way a rigid lung cannot. Those are measured. The popular defence that an animal must also talk, cough and pant is mostly wrong: birds sing, mimic speech and pant.

What remains is a trade-off argument with one solid measured item, diving, and one solid negative finding, that the mammal lung does not limit most mammals at sea level. That is enough to deny that the mammal lung is a design failure. It is not enough to show that it is the best possible lung for a mammal, and the lineage pattern of the two designs remains easier for common descent to explain.

Sources

1.
West, John B. et al. “The human lung: did evolution get it wrong?” European Respiratory Journal 29(1):11–17. 2007. Full-text HTML read through a browser session on 2026-09-27; the HTML carries no page numbers, so claims are located by section heading. West is a respiratory physiologist at the University of California, San Diego.
1. 1a.
  Section Abstract: “From a structure–function standpoint, the bird lung is superior.” ↩ ↩
2. 1b.
  Section Abstract: “A major difference is that the ventilation of the gas-exchanging tissue has a flow-through pattern in the bird but is reciprocating in the mammal. The result is that mammals have a reduced alveolar and arterial oxygen tension, a potential for uneven ventilation, and relatively large terminal air spaces.” ↩
3. 1c.
  Section The evolutionary success of birds: why did mammals take a different route?: “So why did evolution take what appears to be such a flawed path in the mammal? The question is probably not a useful one.” ↩ ↩
4. 1d.
  Section Is repetitive distortion of alveolar tissue a factor in its destruction?: “It is tempting to believe that the distortion that occurs in the delicate gas-exchanging parenchyma with every breath may be a factor in these conditions.” ↩ ↩
5. 1e.
  Section Flow-through versus reciprocating ventilation: “For instance, in the human lung, PA,O2 is typically ∼13.3 kPa (100 mmHg) in contrast to the oxygen pressure of the inspired gas, which is ∼20 kPa (150 mmHg).” ↩
6. 1f.
  Section The reciprocating pattern necessitates large terminal air units: “For example, in the human lung, the alveoli have a diameter of ∼300 μm, whereas the air capillaries that surround the blood capillaries in the bird lung have a diameter ∼5% of this (fig. 4), although this is variable depending on the species.” ↩
7. 1g.
  Section Rigid versus deformable gas-exchanging tissue: “The net result of this “cross-current” gas exchange, as it is called, is that it potentially allows the oxygen pressure of the blood leaving the lung to exceed that of the expired gas. This is impossible in the mammalian lung, where the best result that can be achieved is that the end-capillary blood reaches the oxygen tension of the alveolar gas which is subsequently exhaled.” ↩ ↩ ↩
8. 1h.
  Section The evolutionary success of birds: why did mammals take a different route?: “the number of extant species (9,000 in birds versus ∼4,200 in mammals)”. ↩
9. 1i.
  Section Occlusion of airways by aspiration or secretions: “This scenario of substantial impairment of gas exchange following blockage of an airway is an inevitable result of combining gas exchange and ventilatory functions in the same lung parenchyma. Presumably the bird avoids these problems to a large extent, as inhaled material will end up in the nonvascular air sacs, with little resulting impairment of gas exchange.” ↩
2.
Kooyman, Gerald L., and Paul J. Ponganis. “The physiological basis of diving to depth: birds and mammals.” Annual Review of Physiology 60:19–32. 1998. Abstract read through Europe PMC.
1. 2a.
  Section Abstract: “In mammals, one adaptation is lung collapse at shallow depths, which limits absorption of nitrogen. Blood N2 levels remain below the threshold for decompression sickness. No such adaptive model is known for birds.” ↩ ↩
3.
Ainsworth, Claire, and Michael Le Page. “Evolution's greatest mistakes.” New Scientist 195(2616):36–39. Aug 11, 2007. Read from a text reprint without page breaks, linked in Eric's research notes. The "Bad ventilation" section opens the article, so it is on page 36, the page Creation Ministries International also cites. Mirror: Answersinscience.org
1. 3a.
  Section Bad ventilation: “Verdict: Bird lungs are far superior to ours and, the more they are studied, the longer the list of their advantages becomes. We mammals might have been even more successful had we inherited or evolved similar lungs.” ↩ ↩
2. 3b.
  Section Bad ventilation: “Our air passages end in air sacs, or alveoli, which unlike the tubes of bird lungs have to be relatively large to ensure adequate air flow. This means less surface area for gas exchange. Large alveoli also need thick walls for support, but these reduce gas exchange. As a result, humans have delicate alveolar walls that are prone to become damaged, leading to a condition called emphysema.” ↩
3. 3c.
  Section Bad ventilation: “But that achievement pales in comparison with the feat of the griffon vulture that set the record for the highest recorded bird flight in 1975 when it was sucked into the engine of a plane flying at 11,264 metres.” ↩
4.
West, John B. “Comparative physiology of the pulmonary blood-gas barrier: the unique avian solution.” American Journal of Physiology: Regulatory, Integrative and Comparative Physiology 297(6):R1625–R1634. 2009. Read from the PMC HTML; claims located by section heading. The thickness figures are Maina and West's compilation of 34 bird and 37 mammal species. Mirror: PMC
1. 4a.
  Section Thinness of the blood-gas barrier: “The values of the mean harmonic thickness are 0.47 and 0.19 μm, respectively, which means that the mean thickness in these two groups differs by a factor of 2.5. Even in members of the two classes that have small body sizes, there are large differences. For example, the thickness in the violet-eared humming bird is 0.099 μm, whereas that in the shrew is 0.34 μm, a 3.4-fold difference (21).” ↩ ↩
2. 4b.
  Section Thinness of the blood-gas barrier: “within the birds, the ostrich has a mean thickness of 0.56 μm, and the value in the Humboldt penguin is 0.53 μm.” ↩
3. 4c.
  Section Surface area of the blood-gas barrier: “Birds have about a 15% larger surface area of the blood-gas barrier compared with mammals of the same body mass despite the fact that the volume of the gas-exchanging tissue in the bird parabronchi is much smaller than that in the mammal (19).” ↩
4. 4d.
  Section Surface area of the blood-gas barrier: “For example, the surface area varies from ∼0.02 to over 1,000 m2 between the smallest shrew and the horse.” ↩
5. 4e.
  Section Abstract: “This external support comes from epithelial plates that are part of the air capillaries, and the support is available because the terminal air spaces in the avian lung are extremely small due to the flow-through nature of ventilation in contrast to the reciprocating pattern in mammals.” ↩
6. 4f.
  Section Reason for the small terminal air-spaces in the avian lung: “In the mammal, a small tidal volume of ∼0.5 liter in humans is delivered into a large resting volume of the lung of perhaps 3.5 liters.” ↩
7. 4g.
  Section Stress failure of the blood-gas barrier: “The most striking example is so-called exercise-induced pulmonary hemorrhage in thoroughbred racehorses. Tracheal washings from these animals contain hemosiderin-laden macrophages, indicating that essentially all of these horses in training are bleeding into their lungs (41).” ↩ ↩
8. 4h.
  Section Perspectives and Significance: “Nature has produced a more efficient respiratory system in birds than in mammals.” ↩
5.
Scott, Graham R. et al. “How bar-headed geese fly over the Himalayas.” Physiology 30(2):107–115. 2015. Read from the PMC HTML. Its reference 50 is Tucker, V. A., Respiratory physiology of house sparrows in relation to high-altitude flight, Journal of Experimental Biology 48:55–66, 1968, which was not obtained. Mirror: PMC
1. 5a.
  Section Avian Physiology as High-Altitude Exaptations: “Early work showed that even lowland sparrows can fly in a wind tunnel while breathing air that simulated the Po2 at 6,100 m, which was sufficiently hypoxic to render domestic mice comatose (50). Birds are generally more tolerant of hypoxia than mammals, and bar-headed geese and some other hypoxia-tolerant birds can survive lower Po2 (lowest survivable Po2, ∼2.7 kPa) than the most hypoxia-tolerant euthermic mammals (e.g., mole rats, ∼4.7 kPa) (49).” ↩ ↩
2. 5b.
  Section The Challenges of High-Altitude Flight: “Most birds reach altitudes of 5,000–6,000 m during the migration, where the Po2 is roughly half of that at sea level, and they occasionally fly even higher (e.g., one recorded bird reached 7,290 m) (16, 24, 47).” ↩
3. 5c.
  Section The Specialized Physiology of Bar-Headed Geese: “Bar-headed geese also have ∼25% larger lungs than lowland waterfowl of comparable body mass (45)”. “The hemoglobin of bar-headed geese has a higher affinity for O2 (whole-blood P50 of 4.0 kPa at pH 7.4 and CO2 tension of ∼5 kPa) than that of closely related lowland geese (5.3 kPa in greylag goose under the same conditions) (32)”. ↩ ↩
6.
Meir, Jessica U. et al. “Reduced metabolism supports hypoxic flight in the high-flying bar-headed goose (Anser indicus).” eLife 8:e44986. 2019. CC-BY. Read from the PMC HTML. Mirror: PMC
1. 6a.
  Section Discussion: “Three bar-headed geese also flew in severely hypoxic conditions [equivalent to an altitude of roughly 9,000 m (0.07 FiO2)], at least for the short duration of flights in this study.” Table 1: Flight length in severe hypoxia 139 ± 29 seconds, range 60–468, from 3 birds. ↩ ↩ ↩
2. 6b.
  Section Results: “In the one bird for which we have data at all O2 levels, arterial PO2 fell to 56.5 ± 5.4 and 36.7 ± 0.54 mmHg preflight for FiO2=0.105 and FiO2=0.07, respectively.” ↩
3. 6c.
  Section Abstract: “We conclude that flight in hypoxia is largely achieved via the reduction in metabolic rate compared to normoxia. Arterial Po2 was maintained throughout flights.” ↩ ↩
7.
Weibel, Ewald R. et al. “The concept of symmorphosis: a testable hypothesis of structure-function relationship.” Proceedings of the National Academy of Sciences 88(22):10357–10361. 1991. Read from the PMC scan; pages anchored by the running heads. The scan prints "02" for O2, restored here. Mirror: PMC
1. 7a.
  Page 10357: “The study of allometric and adaptive variation leads to the conclusion that the hypothesis of symmorphosis is acceptable for all internal compartments of the respiratory system (blood, heart, muscle capillaries, and mitochondria), whereas it must be refuted for the lung that forms the interface to the environment.” ↩
2. 7b.
  Page 10360: “It rather appeared that the lung maintains a considerable excess diffusing capacity, up to a factor of 2 in sedentary species, that can be partly exploited in athletic species to achieve higher uptake rates; this also allows sedentary animals to maintain their aerobic capacity in a hypoxic environment (31).” ↩ ↩ ↩
8.
Maina, John N., and Anthony S. King. “Correlations between structure and function in the design of the bat lung: a morphometric study.” Journal of Experimental Biology 111:43–61. 1984. Abstract read through Europe PMC. Five bat species, 5 to 173 g. DLO2 is the lung's morphometric diffusing capacity for oxygen.
1. 8a.
  Section Abstract: “The bats had a remarkably higher VL than either birds or terrestrial mammals.” “In the bats the tau ht was thicker than in the birds but thinner than that of the terrestrial mammals. These pulmonary structural adaptations culminated in a higher DLO2 in the bat than either in the birds or in the terrestrial mammals.” ↩ ↩ ↩ ↩
9.
Dempsey, Jerome A. et al. “Exercise-induced arterial hypoxaemia in healthy human subjects at sea level.” Journal of Physiology 355:161–175. 1984. Abstract read through Europe PMC. Sixteen highly trained runners, maximal oxygen uptake 72 ml/kg/min.
1. 9a.
  Section Abstract: “Four runners maintained arterial PO2 (Pa, O2) within 10 mmHg of resting values, another four showed 10-15 mmHg reductions in Pa, O2, and the remaining eight showed reductions of 21-35 mmHg, i.e. in all cases to a Pa, O2 of less than 75 mmHg and to less than 60 mmHg in two cases.” “By exclusion, we suggest that hypoxaemia may be attributed to a diffusion limitation secondary to very short red cell transit times in at least a portion of the pulmonary circulation.” ↩ ↩
10.
Falke, K. J. et al. “Seal lungs collapse during free diving: evidence from arterial nitrogen tensions.” Science 229(4713):556–558. 1985. Abstract read through Europe PMC; full text not obtained. Weddell seals, 47 arterial samples, dives to 230 m.
1. 10a.
  Section Abstract: “Peak arterial blood nitrogen tensions of between 2000 and 2500 millimeters of mercury were recorded at depths of 40 to 80 meters during descent, indicating that the seal's lung collapses by 25 to 50 meters.” “In a single dive, alveolar collapse and redistribution of blood nitrogen allow the seal to avoid nitrogen narcosis and decompression sickness.” ↩ ↩
11.
McDonald, Birgitte I., and Paul J. Ponganis. “Lung collapse in the diving sea lion: hold the nitrogen and save the oxygen.” Biology Letters 8(6):1047–1049. 2012. Read from the PMC HTML, which drops the PO2 symbol; it is restored in brackets. Mirror: PMC
1. 11a.
  Section Abstract: “Lung collapse is considered the primary mechanism that limits nitrogen absorption and decreases the risk of decompression sickness in deep-diving marine mammals.” “(iii) lung collapse at depth preserved a pulmonary oxygen reservoir that supplemented blood oxygen during ascent so that mean end-of-dive arterial [PO2] was 74 ± 17 mmHg (greater than 85% haemoglobin saturation).” Depth of collapse 225 ± 25 m in 48 dives to 306 ± 35 m. ↩ ↩
12.
Ponganis, Paul J. et al. “Penguin lungs and air sacs: implications for baroprotection, oxygen stores and buoyancy.” Journal of Experimental Biology 218(5):720–730. 2015. Abstract read through Europe PMC. CT scans of live Adélie, king and emperor penguins.
1. 12a.
  Section Abstract: “the presumed maximum air sac volumes resulted in air sac volume to air capillary/parabronchial/tracheobronchial volume ratios that were not large enough to prevent barotrauma to the non-collapsing, rigid air capillaries during the deepest dives of all three species, and during many routine dives of king and emperor penguins. We conclude that volume reduction of airways and lung air spaces, via compression, constriction or blood engorgement, must occur to provide pulmonary baroprotection at depth.” ↩ ↩
2. 12b.
  Section Abstract: “We emphasize that the maximum pre-dive respiratory air volume is still unknown in penguins.” ↩
13.
Riede, Tobias, and Franz Goller. “Peripheral mechanisms for vocal production in birds: differences and similarities to human speech and singing.” Brain and Language 115(1):69–80. 2010. Read from the PMC author manuscript. Mirror: PMC
1. 13a.
  Section b) The avian and human lung system: “Unlike in humans, the avian chest and abdominal cavity are not separated into two separated compartments by a diaphragmatic muscle (e.g., McLelland, 1989b). The two-compartment morphology in humans affects temporal patterns of breathing, speech and singing”. “Whereas at least the initial pressurization of the lung for expiration originates in the passive recoil of the diaphragm, the abdominal muscle sheet can contribute actively to the generation of expiratory pressure.” “In birds, the pressure conditions of inspiration and expiration are always actively generated by muscles.” ↩
2. 13b.
  Section c) Respiratory patterns and vocal temporal patterns: “Song is typically generated using an expiratory airstream, although occasional inspiratory phonation has been documented”. Section d) Respiratory contribution to airflow control: “During quiet respiration subsyringeal expiratory pressure in birds is approximately 1 cm H2O and during phonation ranges from 10–60 cm H2O (reviewed by Brackenbury, 1989). This range is similar to that found during human speech and singing”. ↩ ↩
3. 13c.
  Section c) Respiratory patterns and vocal temporal patterns: “These mini-breaths replenish the air used for sound production during the preceding or following syllables (Hartley & Suthers, 1989; Goller & Daley, 2001), enabling birds to sing very long songs. For example, the song of some species consists of trills that last 30 seconds to a minute (e.g., Brewer's sparrow, Spizella breweri; canary, Serinus canaria).” “The highest mini-breath rate confirmed by air sac pressure recordings is 33 Hz in Waterslager canaries (Hartley & Suthers, 1989)”. ↩
14.
Beckers, Gabriël J. L. et al. “Vocal-tract filtering by lingual articulation in a parrot.” Current Biology 14(17):1592–1597. 2004. Abstract read through Europe PMC. Five monk parakeets.
1. 14a.
  Section Abstract: “We suggest that lingual articulation may thus in part explain the well-known ability of parrots to mimic human speech, and, even more intriguingly, may also underlie a speech-like formant system in natural parrot vocalizations.” ↩ ↩
15.
Schmidt-Nielsen, Knut et al. “Temperature regulation and respiration in the ostrich.” The Condor 71(4):341–352. 1969. The archive PDF has a garbled text layer, so page 351 was read from a 300 dpi image with optical character recognition; page anchored by its running head.
1. 15a.
  Page 351: “The Ostrich can maintain its body temperature below 40°C during 8 hr at ambient temperatures as high as 50°C. This is achieved by increased evaporation from the respiratory tract (panting).” “Respiratory rates rose from 6-12 cycle/min at rest to about 40-60 cycle/min during heat load.” ↩ ↩
2. 15b.
  Page 351: “The complete absence of alkalosis in Ostriches after panting for up to 8 hr suggests that during panting the lungs are bypassed by much of the ventilation volume”. “In contrast, all other birds examined (nine species) exhibit marked alkalosis after heat stress (Calder and Schmidt-Nielsen 1966, 1968).” ↩
16.
Brown, Robert E. et al. “The avian respiratory system: a unique model for studies of respiratory toxicosis and for monitoring air quality.” Environmental Health Perspectives 105(2):188–200. 1997. Public domain (US federal journal). Read from the PMC scan; pages anchored by the journal's page footers. The scan's "dearance" is read as "clearance". Mirror: PMC
1. 16a.
  Page 189: “the bird's tracheal rings are complete”. “In the mammalian trachea, the collapsible membrane spanning the gap between the ends of the incomplete tracheal rings is considered an important feature of the clearance mechanism of the mammalian cough (8). Although birds do perform a coughlike action, we do not know if such behavior represents an effective mechanism for clearing debris from their longer tracheas.” ↩ ↩
2. 16b.
  Page 197: “The pathophysiologic effects of carbon monoxide on a range of animal taxa have been well documented and indicate the following relative sensitivities (84-89): canaries (most sensitive), sparrows, pigeons, chickens, mice, guinea pigs, rabbits, and dogs (least sensitive).” “Yet, despite all the studies of the toxicity of CO in a variety of animals, we do not know the physiologic basis for the different sensitivities among the different kinds of animals.” ↩
17.
Farmer, C. G., and Kent Sanders. “Unidirectional airflow in the lungs of alligators.” Science 327(5963):338–340. 2010. Abstract read through Europe PMC; full text not obtained.
1. 17a.
  Section Abstract: “Here, we show that air flows unidirectionally through parabronchi in the lungs of the American alligator, an amphibious ectotherm without air sacs, which suggests that this pattern dates back to the basal archosaurs of the Triassic and may have been present in their nondinosaur descendants (phytosaurs, aetosaurs, rauisuchians, crocodylomorphs, and pterosaurs) as well as in dinosaurs.” ↩ ↩ ↩
18.
Cieri, Robert L. et al. “New insight into the evolution of the vertebrate respiratory system and the discovery of unidirectional airflow in iguana lungs.” Proceedings of the National Academy of Sciences 111(48):17218–17223. 2014. Read from the PMC text in the source library. Mirror: PMC
1. 18a.
  Section Discussion: “These results indicate that unidirectional flow does not require either a bronchus that has grown deep into the lung as a mesobronchium or intercameral perforations.” “The results of this study also suggest that unidirectional flow is not an adaptation for expanding aerobic capacity and did not arise coincident with vigorous sustained locomotion or with endothermy.” ↩ ↩
2. 18b.
  Section Discussion: “However, we know of no clear evidence for increased efficiency of gas exchange in nonavian reptiles over that seen in mammals, for example, ventilation-perfusion matching during exercise in the savannah monitor is very similar to that of many mammals (28).” “However, convergent evolution has not been ruled out”. ↩
19.
Denton, Michael. Evolution: A Theory in Crisis. Adler & Adler, Bethesda, MD. 1986. First published in London by Burnett in 1985. Denton is a molecular biologist and was not a creationist. Read with archive.org search-inside; the book cannot be borrowed. Page numbers are leaf minus 4, checked at leaf 181 (page 177) and leaf 310 (page 306). Mirror: Archive.org
1. 19a.
  Page 210: “In all other vertebrates the air is drawn into the lungs through a system of branching tubes which finally terminate in tiny air sacs, or alveoli, so that during respiration the air is moved in and out through the same passage.” ↩ ↩
2. 19b.
  Pages 211–212: “Just how such an utterly different respiratory system could have evolved gradually from the standard vertebrate design is fantastically difficult to envisage, especially bearing in mind that the maintenance of respiratory function is absolutely vital to the life of an organism to the extent that the slightest malfunction leads to death within minutes.” ↩
3. 19c.
  Page 212: “so the avian lung cannot function as an organ of respiration until the parabronchi system which permeates it and the air sac system which guarantees the parabronchi their air supply are both highly developed and able to function together in a perfectly integrated manner.” ↩
4. 19d.
  Page 212: “the avian lung cannot be inflated out of a collapsed state as happens in all other vertebrates after birth. In birds, aeration of the lungs must occur gradually and starts three to four days before hatching”. Denton is reporting Hans-Rainer Duncker, whose primary account was not read. ↩
20.
Sarfati, Jonathan. “Bird breathing anatomy breaks dino-to-bird dogma.” Creation Ministries International. Jun 16, 2009. Retrieved Aug 3, 2026. Updated 31 August 2024. Read from the copy in the source library.
1. 20a.
  Section Avian lung design: “This system keeps air flowing in one direction through special tubes (parabronchi, singular parabronchus) in the lung, and blood moves through the lung’s blood vessels in the opposite direction for efficient oxygen uptake,2 an excellent engineering design.” ↩ ↩ ↩
2. 20b.
  Section Avian lung design: “Reptilian lungs operate like bellows (like our own lungs, although the reptile lung structure is different). The stale air is then breathed out the same way it came in.” ↩
21.
Creation Ministries International. “The good, the bad and the evolutionary.” creation.com. Aug 21, 2007. Retrieved Sep 27, 2026. Updated 29 August 2007. No author is named on the page. Linked in Eric's research notes.
1. 21a.
  Section All out of puff: “For instance, some bats can fly with comparable speed and superior manoeuvrability to birds, yet accomplish this using the bellow lung system. In some small bats, they may in fact have a higher respiratory efficiency than birds of flight that are the same weight.2”. ↩
2. 21b.
  Section All out of puff: “If anything, this is an argument against evolution. If bats and pterosaurs could fly without them, then why would the avian lung system need to be evolved in the first place?” ↩
3. 21c.
  Section All out of puff: “As they themselves admit, the human lungs are rather impressive machines already for their specified tasks.” “That’s like comparing the engines of a jet fighter and a moped and saying the moped is deficient because it doesn’t go as fast as the jet.” ↩ ↩
22.
National Heart, Lung, and Blood Institute. “COPD: Causes and Risk Factors.” NIH. Oct 4, 2024. Retrieved Sep 27, 2026. Public domain. Emphysema is one form of COPD.
1. 22a.
  Section Causes and Risk Factors of COPD: “Smoking: This is the main risk factor for COPD. Three out of four people who have COPD smoke or used to smoke.” ↩ ↩
23.
Laybourne, Roxie C. “Collision between a vulture and an aircraft at an altitude of 37,000 feet.” Wilson Bulletin 86(4):461–462. Dec 1974. Laybourne identified the feathers against comparative material in the US National Museum of Natural History. Read from the archive PDF; the scan prints "Riippell's", restored here. 37,000 ft is 11,278 m.
1. 23a.
  Page 461: “On 29 November 1973, a Rüppell's Griffon (Gyps rueppellii) collided with a commercial aircraft at 37,000 ft over Abijan, Ivory Coast, western Africa. The altitude is that recorded by the pilot shortly after the impact, which damaged one of the aircraft's engines and caused it to be shut down.” ↩
24.
Maina, John N. “Structure and function of the avian respiratory system.” Philosophical Transactions of the Royal Society B 380(1920):20230435. 2025. Read from the PMC HTML. Mirror: PMC
1. 24a.
  Section Abstract: “Anatomically, the avian respiratory system is separated into lungs that serve as gas exchangers and air sacs that operate as ventilators. Utterly rigid, the avian lungs are deeply fixed to the ribs and the vertebrae.” ↩
2. 24b.
  Section 3(b) Crosscurrent gas exchange system: “The rather perpendicular (orthogonal) disposition between the directions of air flow in the parabronchial lumen and that of venous blood into the exchange tissue forms the crosscurrent gas exchange system.” “In the lungs of the domestic duck (Anas platyrhynchos domesticus), Scheid and Piiper [71] showed experimentally the existence of a crosscurrent gas exchange system: under steady state breathing, reversing the direction of air flow in the lung did not produce an effect on the partial pressure of O2 (PO2) in air as well as that in the arterial and mixed venous blood.” ↩ ↩ ↩
25.
Johnson, James J. S. “High-Altitude Flying Is for the Birds.” Institute for Creation Research. Feb 29, 2016. Retrieved Sep 27, 2026.
1. 25a.
  “What they have—thanks to their Creator—is much more efficient than Antis’ custom-made oxygen mask!” Antis was a German shepherd that flew bombing missions with a modified oxygen mask. ↩
26.
Grocott, Michael P. W. et al. “Arterial blood gases and oxygen content in climbers on Mount Everest.” New England Journal of Medicine 360(2):140–149. 2009. Abstract read through Europe PMC; full text not opened.
1. 26a.
  Section Abstract: “In four samples taken at 8400 m (27,559 ft)--at which altitude the barometric pressure was 272 mm Hg (36.3 kPa)--the mean PaO(2) in subjects breathing ambient air was 24.6 mm Hg (3.28 kPa), with a range of 19.1 to 29.5 mm Hg (2.55 to 3.93 kPa).” ↩ ↩
27.
The two numbers show scale, not a controlled comparison. Meir and colleagues equate 7% oxygen at normal pressure with about 9,000 m, a little higher than Grocott's 8,400 m, where the barometric pressure was 272 mmHg.⁶²⁶ The goose value comes from one hand-reared bird that was not acclimatised, breathing thin gas at sea-level pressure. The climbers had spent weeks acclimatising and breathed thin air at low pressure. Breathing effort and carbon dioxide levels were not matched. ↩
28.
Maina, John N. “What it takes to fly: the structural and functional respiratory refinements in birds and bats.” Journal of Experimental Biology 203(20):3045–3064. 2000. Abstract read through Europe PMC; the full text is behind a paywall.
1. 28a.
  Section Abstract: “Bats improved a fundamentally mammalian lung to procure the large amounts of oxygen needed for flight. The lung/air sac system of birds is not therefore a prescriptive morphology for flight: the essence of its design can be found in the evolution of the reptilian lung, the immediate progenitor stock from which birds arose.” ↩
29.
Wienecke, Barbara et al. “Extreme dives by free-ranging emperor penguins.” Polar Biology 30(2):133–142. 2007. Abstract read from the publisher's page. Published online 19 July 2006.
1. 29a.
  Section Abstract: “The two most extreme dives (564 m depth, 21.8 min duration) were separate dives and were performed by different individual penguins.” ↩
30.
Schorr, Gregory S. et al. “First long-term behavioral records from Cuvier's beaked whales (Ziphius cavirostris) reveal record-breaking dives.” PLoS ONE 9(3):e92633. 2014. CC-BY. Mirror: PMC
1. 30a.
  Section Abstract: “The effort resulted in 3732 hr of dive data with associated regional movements – the first dataset of its kind for any beaked whale – and included dives to 2992 m depth and lasting 137.5 min, both new mammalian dive records.” ↩
31.
Maina, John N. “A critical assessment of the cellular defences of the avian respiratory system: are birds in general and poultry in particular relatively more susceptible to pulmonary infections/afflictions?” Biological Reviews 98(6):2152–2187. 2023. Abstract read through Europe PMC.
1. 31a.
  Section Abstract: “Without empirical evidence, early observations led to the supposition that birds in general, and poultry in particular, have weak innate and adaptive pulmonary defences and are therefore highly susceptible to injury by pathogens. Recent findings have, however, shown that birds possess notably efficient pulmonary defences”. “(iii) an exceptionally efficient mucociliary escalator system that efficiently removes trapped foreign agents”. ↩
32.
Narayanan, Manjith et al. “Alveolarization continues during childhood and adolescence: new evidence from helium-3 magnetic resonance.” American Journal of Respiratory and Critical Care Medicine 185(2):186–191. 2012. Abstract read through Europe PMC. 109 healthy subjects aged 7 to 21. Mirror: PMC
1. 32a.
  Section Abstract: “The number of alveoli is estimated to increase 1.94-fold (95% CI, 1.64-2.30) across the age range studied.” ↩
33.
Schachner, Emma R. et al. “Unidirectional pulmonary airflow patterns in the savannah monitor lizard.” Nature 506:367–370. 2014. Abstract read through Europe PMC.
1. 33a.
  Section Abstract: “The unidirectional airflow patterns in the lungs of birds have long been considered a unique and specialized trait associated with the oxygen demands of flying, their endothermic metabolism and unusual pulmonary architecture.” ↩
2. 33b.
  Section Abstract: “The presence of unidirectional flow in the lungs of V. exanthematicus thus gives rise to two possible evolutionary scenarios: either unidirectional airflow evolved independently in archosaurs and monitor lizards, or these flow patterns are homologous in archosaurs and V. exanthematicus, having evolved only once in ancestral diapsids”. “If unidirectional airflow is plesiomorphic for Diapsida, this respiratory character can be reconstructed for extinct diapsids, and evolved in a small ectothermic tetrapod during the Palaeozoic era at least a hundred million years before the origin of birds.” ↩ ↩
34.
Schachner, Emma R., and Andrew J. Moore. “Unidirectional airflow, air sacs or the horizontal septum: what does it take to make a bird lung?” Philosophical Transactions of the Royal Society B 380(1920):20230418. 2025. CC-BY. Read from the PMC HTML. Ornithodirans are pterosaurs plus dinosaurs. Mirror: PMC
1. 34a.
  Section Abstract: “Specifically, we argue that without evidence for the horizontal septum, a fully avian lung should not be reconstructed in non-avian ornithodirans, despite the presence of post-cranial skeletal pneumaticity.” ↩ ↩
35.
Thomas, Brian. “Alligator Lung Design Prompts Evolutionary Rewrite.” Institute for Creation Research. Jan 28, 2010. Retrieved Sep 27, 2026.
1. 35a.
  “The unidirectional flow aspect of an alligator’s lung design appears fully integrated into its particular overall breathing apparatus, which is in turn well integrated into the alligator’s mode of living. In short, birds and alligators look as though they were separately and uniquely engineered.” ↩
36.
Answers in Genesis. “Blown Away by Design: Michael Denton and Birds’ Lungs.” Creation 21(4):14–15. Sep 1999. Retrieved Sep 27, 2026. No author is named. The Denton quotations come from a video interview sold by Access Research Network.
1. 36a.
  “I think it doesn’t require a great deal of profound knowledge of biology to see that for an organ which is so central to the physiology of any higher organism, its drastic modification in that way by a series of small events is almost inconceivable.” ↩
2. 36b.
  “Actually, bats do very well with the “bellows” type of lung. This makes the selectionist argument for the origin of birds” lungs (i.e. that they “needed” them) even more difficult to sustain.” ↩
3. 36c.
  Section The amazing bird lung: “This is known as counter-current exchange. Such very efficient lungs help birds to handle the energy demands of flight, especially at high altitudes.” ↩
37.
Canals, Mauricio et al. “Functional and structural optimization of the respiratory system of the bat Tadarida brasiliensis (Chiroptera, Molossidae): does airway geometry matter?” Journal of Experimental Biology 208(20):3987–3995. 2005. Abstract read through Europe PMC; the full text is behind a paywall and was not obtained, so whether its body compares bats with birds is unchecked. This is the paper Creation Ministries International cites for its bat claim.
1. 37a.
  Section Abstract: “We studied structure and function of the respiratory system in the bat Tadarida brasiliensis and compared it with those of two species of rodents, Abrothrix andinus and A. olivaceus.” ↩