12. Evolution of Birds

Black is White - Up is Down - Birds are Dinosaurs

Hear ye, hear ye, its official, read all about it: Birds are dinosaurs. Birds are dinosaurs. Birds are dinosaurs. Birds are dinosaurs. Right or wrong, true or false, big or small, it does not matter to the authorities who propagate this message, they know that if they repeat their claim often enough then people will believe them. That is how propaganda works. But is this how science works?

And with that the ‘science’ was settled, the investigation is over, the case is closed. We should always be leery whenever authorities move quickly to close a case instead of seeking answers to questions. Almost always it means that they are hiding something and usually it is something that exposes their misconduct.

Birds are dinosaurs? On the surface this claim seems preposterous; the definition for birds and the definition for dinosaurs are not remotely close. But as we dig deeper into the evidence, and clarify our definitions of birds and dinosaurs we will discover that … yea, the claim that birds are dinosaurs really is preposterous. Furthermore, we will discover that the ‘birds are dinosaurs’ claim did not come out of nowhere; paleontologists have a nefarious agenda in play and they have plenty to hide in their cover-up.

It’s time to ask questions.

I would rather have questions that cannot be answered, than answers that cannot be questioned.
Richard Feynman

People Are Fish

Birds are dinosaurs in the same way that people are fish. This is because of the classification rule that says if one classification of species evolves from another classification of species then technically the new classification is still a part of the original classification. Hence, since birds supposedly evolved from dinosaurs then technically birds are dinosaurs. Furthermore, since we can also state that dinosaurs are reptiles, we must likewise conclude that birds are reptiles. Do you see where this is going? Birds are dinosaurs, birds are reptiles, birds are amphibians, and finally birds are fish. And by the same logic we can declare that people are fish!

It is highly questionable if birds even evolved from dinosaurs, and even if this claim were technically true, placing birds and dinosaurs in the same classification nullifies the whole point of why we classify species. The reason we classify species is to expedite the process of understanding each individual species. Once a classification is assigned, we instantly gain significant knowledge about the species, and from that point on, we only need to describe the few remaining features that make the species unique among the other members of its classification.

By claiming that birds are dinosaurs, these paleontologists are engaging in a misleading game of semantics that does not contribute to the advancement of science.

When did science become mind games?

No, Birds Are Not Dinosaurs

Are we all going nuts, or are these leading authorities gaslighting the public when they tell us that birds are dinosaurs? It is gaslighting, and it does not require a paleontology degree to recognize that these 'Birds Are Dinosaurs' (BAD) paleontologists are misleading the public. A person just needs to look over the feathered dinosaur fossils to identify the numerous bird features before realizing that these are not dinosaurs; they are birds.

Paleontologists have made numerous questionable claims regarding the evolution of birds and their supposed relationship to dinosaurs. Oddly enough, even though numerous species of birds had already evolved tens of millions of years earlier during the late Jurassic period, most paleontologists claim that birds evolved from early Cretaceous theropod dinosaurs. From these early Cretaceous theropod dinosaurs emerged theropod dinosaurs that had beaks and looked like something ostriches, with long flexible tails. They did not have wings, and they certainly did not fly. This lineage of so-called feathered dinosaurs - that may or may not have had feathers - died out with the K-T mass extinction at the end of the Cretaceous period. Hence, this lineage of dinosaurs has no connection to the birds that evolved during the Mesozoic era, and since they died out, they cannot have any connection to modern birds either. Nevertheless, paleontologists use this lineage of dinosaurs to make the misleading claim that birds are dinosaurs.

The main arguments clarifying why birds could not have evolved from Saurischian theropod dinosaurs are as follows:

  1. Does Not Follow I - Expert paleobiologist Alan Feduccia has spent decades pointing out the evidence refuting the claim that birds evolved from dinosaurs. His published works cover everything from revealing the numerous conflicts in paleontologists’ evolutionary history of birds (phylogenetic systematics), their claimed improbable origins for flight, to the renaming of Jurassic and Cretaceous birds as feathered dinosaurs. His direct embryological research with ostriches shows that while both theropod dinosaurs and birds have a reduction of the digits from five down to three digits, they are not the same digits. While theropod dinosaurs have digits one, two, and three, birds have digits two, three, and four. This mismatch means that one group could not have evolved from the other.

  2. Picture of Ornithischia and Saurischia hips, Models of T-Rex and Triceratops The 'Birds are Dinosaurs' paleontologists claim that birds evolved from the Saurischian dinosaurs shown on the right. However, the hip structure of birds more closely resembles the Ornithischian dinosaurs shown on the left. Birds needed a more open space to accommodate their large respiratory system, so the pubis is pointed backwards. On the other hand, theropods could have used the robust pubis bone that ends with a flattened boot, as this would have given them the ability to slide on their hips much like an alligator, providing an effective means of staying low and unseen while sneaking up on their prey.

    Archaeopteryx In 1861, only a few years after Darwin published On the Origin of Species, workers at a German limestone quarry made one of the most important fossil discoveries of all time: the 145 million-year-old Archaeopteryx. Notice the feathered wings and the long, stiff tail, indicating that this was a flying bird.
  3. Does Not Follow II - The hips of theropod dinosaurs are distinguished by having a robust pubis bone that points downward and forward. This theropod dinosaur hip bears little resemblance to the hips of birds whose pubis bone is not nearly as robust and points to the rear. Zoologist Devon Quick and vertebrate paleobiologist John Ruben of Oregon State University found that the position of the thigh bone — the rear-pointing pubis — and muscles in birds are critical to their ability to have adequate lung capacity for sustained long-distance flight.
  4. Bad Timing - Birds could not have evolved from early Cretaceous period theropod dinosaurs, as there are several species of birds that existed more than forty million years earlier in the late Jurassic period.
  5. Failing Evolution Theory 101 - Claiming that a vertebrate can form wings by jumping numerous times until it sprouts wings is an excellent example of how evolution does NOT work! For a species to evolve a new feature, there must be some benefit throughout the evolution of the new feature, and there is no survival benefit that comes from jumping up and down on the ground that would encourage wings to form. This is in contrast to taking ever longer glides from one tree to another to either escape predators or to simply move more efficiently about a forest. The vertebrate’s ability to achieve this survival task improves with every bit of wing growth. Hence, every class of flying vertebrates has evolved by first becoming gliders before evolving into true flyers.

While the first three points highlight problems indicating that the claim 'birds evolved from early Cretaceous dinosaurs' is all but certain to be wrong, it is the last point that is upsetting simply because the Theory of Evolution is so crucial to the acceptance of science in our society. What should the public think about the Theory of Evolution when these 'scientists' so casually dismiss it because it does not fit their agenda? Creationists — anti-science religious people who also have their own agenda — have seized upon these paleontologists' claim that dinosaurs could 'evolve wings by jumping up and down' by asking the question 'What good is a half-formed wing?' By promoting a scientific claim that conflicts with the Theory of Evolution, these paleontologists have opened the door to attacks on the Theory of Evolution and science in general.

What is a Bird?

Bird features Chart of features that distinguishes birds from crocodiles: their closes living relatives.
University of Maryland Department of Geology

There is an expression “if it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.” A similar statement could be said about birds in general.

Most biologists would say that birds are warm-blooded, egg laying vertebrates that have feathers, a beak, two wings and two feet. They may also add that birds have a high metabolism, a four chamber heart, a unique respiratory system, keen vision, and light hollow bones. Last and certainly not least, biologists would state that most birds can fly, although there are some birds that have lost their ability to fly.

It is so easy to identify modern birds that most children can do this; however, identifying who the first birds were is not so easy. Species may evolve considerably over time, such that ancestral species will not have nearly all the features of present-day species. In this chain of evolving forms, how far back in time do we go before declaring that earlier ancestral species are unrecognizable as being birds? We need agreement on what the minimum critical features are that a species must have before we consider it a member of the bird community.

The task of determining a dividing line for declaring when a new species or group of species has come into existence is not as difficult as it may first seem. This is because evolution does not move forward at a constant pace; rather, its movement is more accurately described as fits and starts, known as Punctuated Equilibrium. It is common for species to remain unchanged for many millions of years until there is a breakthrough creating a new species. The breakthrough occurs when individuals cross a specific barrier that allows them to exist in a new environment. In the process of crossing this barrier, individuals quickly evolve to be better suited to their new environment. On a generation-to-generation basis, the evolution of a new species is so subtle that it is impossible to notice. Yet, from the perspective of geological time, it represents a dramatic jump; while looking at a timeline on the scale of millions of years we blink and suddenly we see a new species.

Flying Confuciusornis This is a drawing of Confuciusornis sanctus that flew during the early Cretaceous period. Does this look like a dinosaur to you?

For the feathered arboreal archosaurs of the Mesozoic era their big evolution moment came when they evolved from tree climbers to gliders to flyers. This is something that had already been done by the pterosaurs millions of years earlier so it is not like the feathered archosaurs did not have competition in the air. To compete in this new frontier of flying these earliest birds had to be better than the pterosaurs, and they were. It is because birds have feathers that they were able to compete with the already established pterosaurs.

While the evolution of the long light and strong flight feathers proved to be very advantageous for smooth aerodynamic flying, it was actually the insulating ability of simple body feathers that first set the birds apart from the pterosaurs. By providing superior insulation the body feathers set the stage for a whole host of evolutionary developments that gave birds superiority as hot blooded high powered fliers. The first birds are distinguished by having feathers indicating that they were warm blooded vertebrates along with whatever evidence there is indicating that these vertebrates were flying. Thus, if a fossil of a vertebrate shows both evidence of feathers and evidence that vertebrate was flying then that vertebrate was a bird.

Birds (Aves) are advanced feathered archosaurs that fly, or at least had ancestors that flew.

What is a bird? What is a dinosaur? Why are definitions important? Sometimes mistakes that lead to disagreements occur when people are sloppy about how they define terms, while at other times there are those who intentionally engage in misconstruing terms for their own benefit. Paleontologists cannot explain the existence of large dinosaurs, flying pterosaurs, AND the exceptionally large flying birds of the Mesozoic era, and so they are trying to convince the pubic that the large flying birds were non-flying feathered dinosaurs. Clear, rational, working definitions for terms such as birds and dinosaurs allow science to move forward by putting an end to these shenanigans.

Fossil showing the feathered wing of Confuciusornis Feathered wing of Confuciusornis

During the Cretaceous period, there were numerous birds that were substantially larger than modern flying birds. Apparently, when these species were first being discovered, paleontologists had difficulty accepting the evidence that they were flying birds, so they mislabeled them as dinosaurs. However, the evidence continued to come in, indicating that these 'dinosaurs' were actually large flying birds. Most problematic for the paleontologists was the evidence showing that either these large birds or their smaller close relatives had feathered wings and asymmetric feathers. So, the paleontologists yielded to the evidence showing that these vertebrates had feathers but continued to deny the fact that these birds could fly. In every description of these large flying birds, they finish their description by saying that despite the 'feathered dinosaur' having feathered wings, it is their opinion that it could not have flown. Let us take a closer look at the evidence indicating that many of the large vertebrates paleontologists are referring to as 'feathered dinosaurs' were actually flying birds.

Wings: Fossils of feathers, and likewise feathered wings, are not as likely to be preserved as well as bones, and yet there are still several fossils showing the feathered wings of the early Cretaceous 'feathered dinosaurs'. Either these wings are seen directly on these exceptionally large 'feathered dinosaurs', or at least on some of the smaller members within their species' family, such that no one contests the fact that all of the members of Troodontids, Dromaeosaurs, and Avialae had wings. Even though the wings on these birds were relatively small by today’s standards the size of their wings is appropriate for the much thicker Mesozoic atmosphere.

Paleontologists want to dismiss the idea that these were flying birds and so they introduced the slippery slope argument that these feathered vertebrates could have evolved their wings for sexual display or whatever. While nature is full of surprises the Theory of Evolution does not favor improbable explanations. Of the current 70,000 known species of vertebrates 70,000 known species of vertebrates there are approximately twelve thousand winged species that fly: 10,500 birds and about 1,500 bats. There is not a single species that evolved wings for some purpose other than flying or at least gliding. This does not mean that it would be impossible for a species to evolve wings for some purpose other than flying, but it does say that it is so astronomically improbable of happening that we should not waste our time considering the possibility. Hence, if we discover a species with wings then we should conclude that it or its ancestors were using its wings for flying or at the very least gliding.

Drawing showing why feathers are asymmetric
Drawing identifying parts of a asymmetric

Asymmetric Feathers: The flight feathers of birds are asymmetric because the lifting pressure on the leading vane of the feather is much stronger than the lifting pressure on the trailing vane. The same principle applies to an airplane wing: there is at least as much lift on the forward one-fourth to one-third of the wing as there is on the remaining trailing portion of the wing. Whether it is a flight feather or the wing of an airplane, the main support tube needs to be placed one-fourth to one-third of the way back from the leading edge to counteract the lifting force. It is only the wing feathers of flying birds that experience these unbalanced lifting forces, so it is only these feathers that are asymmetric.

Image of several feathers Wing feathers are more likely to be asymmetrical than tail feathers.
Image by Beverly Buckley.

It is common practice in paleontology to assume that whatever features found on one member of a family or clade are shared among all the members of the family. Long asymmetric feathers, used by modern birds, are also found on the wings of Archaeopteryx and Confuciusornis, members of the Avialae family. Similarly, long asymmetric flight feathers are found on Microraptor of the Dromaeosaurid family and Troodontid of the Troodontidae family. It is collectively accepted that asymmetric flight feathers existed on all the members of the Troodontids, Dromaeosaurs, and Avialae families, thus favoring the belief that all of the members of these families were flying vertebrates.

Stiff Tail: The stiff tail of these fossilized vertebrates is the clincher in distinguishing whether the vertebrate was a flyer or grounded. Similar to the rudder and elevator on the tail of an airplane, the tail of a bird guides the direction of its flight. For this to work effectively, the bird’s tail needs to be stiff everywhere other than at its base. As the bird flies, the stiff tail changes the direction of its flight by deflecting the air that flows past the flying bird. By deflecting the air, a force is applied back on the tail, which turns the bird in the desired direction. In contrast, a flexible tail is not as effective in guiding a bird, as it yields or bends in response to the force applied by the wind.

Anchiornis Huxleyi: Late Jurassic feathered dinosaurs or ancestral bird Fossil shows Anchiornis Huxleyi of the Late Jurassic (160 mya). Being older than Archaeopteryx and having features such as a fused tail and feathers on all of its limbs means that it may actually deserve the title of being the ‘first bird’.

While modern birds and a few Jurassic and early Cretaceous birds used long stiff tail feathers as their rudder, most of the Jurassic and early Cretaceous birds had their tail vertebrae fused together to create a long, feather-covered rod that served as their rudder. Because fossilized bones are more likely to be preserved than feather imprints, the straight fused tail is an easily recognizable feature that can be used to identify vertebrates as birds. By using this identifier, we find that most, if not all, of the 'feathered dinosaurs' of the early Cretaceous period are actually flying birds. This fact is not surprising, as these vertebrates are all members of family groups that have already been identified as having at least one member showing well-defined feathered wings and asymmetric feathers.

Besides being far more likely to be preserved, the fused bony tail is just as effective as a feathered wing in distinguishing between flyers and non-flyers. While a stiff tail is extremely useful to a bird while flying, it can be a hindrance to any vertebrate on the ground. Most ground-dwelling vertebrates live in small dens and need to be able to turn around without being poked or having their tail bent or damaged. Similarly, when birds are not flying, they often hold their stiff tail feathers up at nearly a 45-degree angle to avoid this problem, whether on the ground or squatting on their nest. A stiff tail can be such an annoyance that if a bird evolves into a flightless species, it is more likely to lose its stiff tail before its shortened wings. Hence, the presence of a stiff tail is a strong indicator that the vertebrate is a flyer.

The reason many vertebrates have a muscular, flexible tail is that it can be an effective means of propelling an animal through a thick fluid. However, while an oscillating tail can provide strong thrust when the animal is starting to move or moving slowly in a dense fluid, the ability of a flexible tail to provide positive thrust diminishes as the animal attempts to move at higher speeds through the fluid. For this reason, we see most animals using their tail to push themselves through water, but we do not see animals using a flexible tail to push themselves while flying through the air.

alligator on a dock An alligator’s flexible tail is it primary means of propelling itself through the water.
Public domain image by Photo by Shelly Collins on Unsplash
snake swimming in the water A snake is effectively a head attached to a long flexible tail, a flexible tail that it uses to propel itself through the water or over the land.
sparrow standing on a post When on the ground or near other objects, a stiff tail is more likely to be damaged than a flexible tail. This may be the reason why many birds hold their tail up high when they are on the ground.

Birds need to be moving fairly fast for their wings to generate enough lift for flying. Unlike an airplane's propeller, an oscillating flexible tail cannot move fast enough to be effective in pushing the flyer forward. Hence, birds do not have a flexible tail; instead, they have a stiff tail that they use only to determine the direction of their flight. Birds flap their wings to produce the forward thrust needed for flying.

'Feathered Dinosaur' named Zhenyuanlong Fossil of Zhenyuanlong suni of the Cretaceous period (125 mya). Zhenyuanlong was a 6-foot-long Velociraptor. Notice the fused vertebra tail along with the brownish tint silhouette in the shape of wings thus indicating that Velociraptor was a flying bird.

During the Mesozoic era, the atmosphere was much thicker than it is today, which influenced the anatomy of the animals of that time. Firstly, the dense atmosphere enabled even reptiles to fly. Similarly, birds also evolved, and they too were capable of flight, but because of the thickness of the Mesozoic atmosphere, these birds did not need their wings to be nearly as large as either the pterosaurs or modern birds. However, Mesozoic birds still required a stiff tail to direct their flight since they were flying at high enough speeds that a flexible tail would have been useless. This was not the case for dinosaurs, as they did not move nearly as fast as birds. At the dinosaurs’ typically slower speeds, a muscular flexible tail provided propulsion through the thick Mesozoic atmosphere. Whereas a modern alligator can only use its tail while in water and its legs as it travels over land, most dinosaurs used their flexible tails and rear legs simultaneously to propel themselves forward through the thick air and across the land.

Birds are advanced feathered archosaurs that fly, or at least had ancestors that flew. Dinosaurs, on the other hand, are an extinct group of unusually large archosaurian reptiles that had their hind legs extending directly beneath the body. These two groups share a common ancestry of archosaurian reptiles, so it is not surprising that they share many common features: both lay eggs, scales are typically found on dinosaurs, and there are also scales on the legs of birds. Additionally, members of each group can have either a mouth full of teeth or a beak. Hence, it would not be shocking if paleontologists were to find a dinosaur that had feathers. However, if that 'feathered dinosaur' also flew, then it would not be a dinosaur; it would be a bird.

The Giant Flying Birds of the Cretaceous Period

troodontid Troodontid J. tengi was a three foot (0.9 m) tall bird of the early Cretaceous period. Troodontids are known for having a large brain, binocular vision, and asymmetrical flight feathers.

Birds had evolved by at least the mid to late Jurassic period, with some paleontologists claiming that birds may have existed as early as the late Triassic period. Thus, birds coexisted with dinosaurs and pterosaurs for most of the Mesozoic era. The first birds of the Jurassic period were small and unusual compared to modern birds; they had teeth instead of a beak, claws on their wings, a long stiff tail, and many had flight feathers on their rear legs. At the extinction event that ended the Jurassic period, many of these early bird species went extinct, but they were soon replaced by similar birds that emerged in the early Cretaceous period. The most notable difference about Cretaceous period birds is that many, if not most, were much larger. Like the dinosaurs and pterosaurs, many Cretaceous period birds were incredibly large compared to modern species.

Avialea chart

Originally, many of the species that paleontologists now classify as feathered dinosaurs were placed in 'waste basket' categories, indicating uncertainty about their identity and evolutionary relationships. However, it is now possible to reclassify nearly all of these specimens as either birds or dinosaurs, while also determining their proper place in taxonomy.

The paleontological community's insistence on depicting birds as evolving from theropod dinosaurs, coupled with their reluctance to acknowledge the existence of exceptionally large birds from the Cretaceous period, has led to significant confusion regarding the classification of various groups of birds and dinosaurs. Consequently, the phylogenetic trees depicting the evolution of birds often vary widely and can sometimes be nonsensical. For instance, it is not uncommon for these trees to depict a new family of birds evolving from another group of birds that do not appear in the fossil record until tens of millions of years later. Similarly, they tend to overlook late Jurassic birds such as the iconic Archaeopteryx, which lived between 160 and 145 million years ago.

To identify the earliest birds, we need to return to the most fundamental definition: if it has feathers and it flies, then it is a bird. This means that the lineage of birds begins in the mid to late Jurassic period, encompassing iconic species like Archaeopteryx, as well as more recently discovered ones such as Anchiornis huxleyi and Confuciusornis sanctus.

While many of these species became extinct at the end of the Jurassic period, a few families of birds managed to survive and thrive into the early Cretaceous period. These included Troodontids, Dromaeosaurs, and Avialae.

The fused tail is the most obvious indicator that a late Jurassic or early Cretaceous vertebrate is a flying bird. By being flexible only at the base, the fused tail acted like a rudder, guiding the direction of these birds as they flew through the air. Further confirmation of their flying nature comes from observing that the pubis bone of the hip is pointed backwards to accommodate the bird’s large respiratory system. Additionally, many, if not most, of these specimens show actual evidence of feathers, often in the form of outstretched wings. Please examine the fossil images to confirm for yourself that Troodontids, Dromaeosaurs, and Avialae were indeed true flying birds.

Several Predatory Birds of the Cretaceous Period The birds of the Dromaeosaurs family flew during the Cretaceous period, and many of them were considerably larger than modern flying birds. Despite their varying sizes, they shared striking similarities in shape and features: teeth instead of a beak, relatively small wings, deadly sharp talons, and a stiff tail. While paleontologists may initially envision only the smallest of these birds as capable of flight, calculations demonstrate that in a thick atmosphere, all of these birds were highly proficient fliers. They were formidable predators.
(Wikipedia creative commons file first uploaded by Fred Wierum)
fighting dinosaurs
This fossil is know as the ‘Fighting Dinosaurs’ even though it is actually the entangled remains of a dinosaur (Protoceratops) and a bird (Velociraptor). Similar to the killing strategy of present day eagles and hawks, the Velociraptor was attempting to make its kill by ripping its talons through the large arteries in the neck of its prey. However, in this dangerous world of frequent aero attacks, the Protoceratops had evolved a shield that made it difficult for the Velociraptor to get to the Protoceratops’ neck. This time the deadly encounter ended in a draw. Notice the stiff tail of the Velociraptor, indicating that it was a flying bird.
Dakotaraptor Dakotaraptor is one of the larger members of the Dromaeosaurs family of birds.
Drawing by Fred the Dinosaur Man.

Using the flight equations derived in chapter three we can calculate the power ratios of these feathered vertebrates based on best estimates of their weight, wingspan, and other factors important to flight. Using the Earth’s current sea level atmospheric density (1.2 kg/m3) the calculations show that it not be possible for these feathered vertebrates to fly in our present atmosphere. While on the other hand when the equations are reworked using the much higher density value of the thick Cretaceous atmosphere (660 kg/m3) the calculation show that these feathered birds of the Cretaceous period were very capable flyers.

Here are the equations used for making the flight calculations followed by a table giving the data input estimates along with the calculated values.

N = Fg (1 - ρF / ρS)

v_min =  [(2 W^2) / (3 A C b^2 ρ^2)]^1/4

P_T-min =  4/3 [ W^2 / (b^2 ρ v_min)]

Pav = 1.5[(69 W3/N2) W2]1/3

Flyer Weight
Front Area
Drag Coefficient
(Front Area)
for least Power
Minimum Power
for Flight
Available Power
Power Ratio
Thin Atm
3000 0.25 0.25 3.5 48 17 1.0 0.06
Thick Atm
1020 0.25 0.25 3.5 2.1 0.10 1.0 10
In order to fly, a potential flying object needs a power ratio greater than one. The first time through the calculations are done based on the incorrect assumption that on Cretaceous atmosphere was similar to the present atmosphere. Using the current sea level atmospheric density of 1.2 kg/m3, the resulting value for the power ratio is only 0.06 thus indicating that it is falling far short of having enough power to fly in the present atmosphere. However when the numbers are crunched again using the much higher density value of the Cretaceous period the power ratio comes out to 10 thus indicating that this bird was an extremely capable flyer in the much thicker atmosphere.
Leopard Shark
Unlike other fish, sharks do not have an air-filled swim bladder to help ‘float’ their body, so sharks must use their pectoral fins as their wings to give them lift. The shark’s ‘wings’ are relatively small because small wings are all that are required when ‘flying’ in a dense fluid.

To better understand why these large birds had relatively small wings, let’s consider a present-day example of a vertebrate 'flying' in a dense fluid. A shark’s body is denser than the surrounding water, so if it did not swim, it would slowly sink to the bottom of the ocean. However, sharks do not sink because they have pectoral fins on their sides that act as effective wings, giving them lift as they propel themselves forward with their tail.

Think of a shark in the ocean as being similar to an airplane or bird in the atmosphere: all of these 'fliers' propel themselves forward so that fluid flows over their wings, producing the lift required to maintain their altitude. However, unlike airplanes and modern birds that fly in the thin atmosphere, sharks need only relatively small wings because they are flying in the much denser ocean water. This inverse relationship between the size of wings and the density of the fluid likewise applies to the birds that flew in the dense Cretaceous atmosphere. The extremely dense Cretaceous atmosphere was the reason why most of the birds of the birds of the Cretaceous period had relatively small wings.

In our comparison of the Mesozoic atmosphere to the present day ocean environment something else comes to our attention: the amount of life swimming in the ocean compared to how much live is crawling on the sea floor. It makes sense that the denser the fluid the larger percentage of life that will be flying or swimming in the fluid as opposed to just walking on the sea floor. Likewise in the Mesozoic world with its extremely thick atmosphere, flying is much easier than what it is today and so we should expect a larger percentage of vertebrates flying about than what we see in the atmosphere today. As a rough guess during the Cretaceous period as much as twenty to thirty percent of terrestrial life mass was in the air at any given moment as opposed to just moving about on the land.

‘Feathered Dinosaurs’ That Were Actually Dinosaurs

Some scientists seem to overly emphasize whether a vertebrate has a beak when defining a bird. However, besides modern-day birds, many living and extinct species possess a beak, including turtles, swordfish, numerous dinosaurs, most pterosaurs, and some amphibians. The evolutionary preference for a beak over a jaw full of teeth in flying animals is due to its lightness and strength, which improve aerodynamic efficiency. This could explain why both pterosaurs and birds initially had teeth but later evolved to have a beak. However, while a beak is a favorable feature, it is not essential.

Feathers, on the other hand, are the defining feature of birds because they contribute significantly to their aerodynamic smoothness and lightness. Additionally, feathers provide superior insulation, enabling the evolution of the high-powered metabolism that gives birds their aerial superiority.

Paleontologists have caused confusion by labeling two distinct groups as feathered dinosaurs. The first group comprises of large birds that lived throughout the Cretaceous period, as described in the previous section. The second group consists of theropod Saurischian dinosaurs from the late Cretaceous period. While these late Cretaceous theropods may have had some similarities to modern-day ostriches and possessed beaks, they were unequivocally not birds.

Dinosaur Feathers Look hard and squint. The evidence of Ornithomimus dinosaurs having feathers is supposedly inside the yellow lines.

A large group of birds, regarded as the first group of 'feathered dinosaurs,' emerged in either the late Jurassic or late Triassic periods. They coexisted with dinosaurs and pterosaurs before surviving the K-T extinction event and persisting to the present day. Although many early birds had a mouth full of teeth instead of a beak, they shared crucial avian characteristics with modern birds, including an Ornithischia hip structure, a stiff tail for flight guidance, body feathers, asymmetric flight feathers, and fully developed feathered wings.

The second group of 'feathered dinosaurs' likely evolved from theropod dinosaurs and existed during the late Cretaceous period. This group included species such as Therizinosaurus, Conchoraptor, Gigantoraptor, Corythoraptor, Oviraptor, Anzu, Ornithomimus, and others. Despite being labeled as 'feathered dinosaurs' by paleontologists, the evidence of them having feathers is either sparse or nonexistent. Unlike the first group, these late Cretaceous 'feathered dinosaurs' lacked avian characteristics such as a bird hip structure, stiff tail for flight guidance, flight feathers, and feathered wings. There is no evidence indicating that this second group of 'feathered dinosaurs' were capable of flight or had ancestors that could fly.

Several Ostrich Like Theropod Dinosaurs None of these dinosaurs show evidence of having wings, they do not have stiff tails, and they may not even have feathers. So why are paleoartists drawing these vertebrates this way?

Why is it that paleontologists are grouping together the large flying birds of the Mesozoic era and the late Cretaceous ostrich like dinosaurs under the same heading of feathered dinosaurs? Why is it that after describing the wings of every large Mesozoic era bird, paleontologists feel obligated to give their highly questionable opinion that these large birds must have been using their fully evolved wings for some purpose other than flying? Why is it that paleoartists show numerous ostrich like theropod dinosaurs as having feathered wings when there is no evidence supporting these claims? Why is it that paleontologists want to blur the distinction between birds and dinosaurs?

To summarize, there are actually two groups of vertebrates that paleontologists are calling feathered dinosaurs: the first group is the exceptionally large flying birds of the Cretaceous period that paleontologists do not want to recognize as being flying birds, while the second group is the late Cretaceous theropod dinosaurs that paleontologists and paleoartists are presenting to the public as if they are the same as the large flying birds that make up the first group. As of now, none of these theropod dinosaurs that make up the second group and are being called feathered dinosaurs show any evidence of having wings, a stiff tail, or even evidence of well formed feathers. While it would not be shocking if paleontologists were to someday find one or more species of dinosaurs showing evidence of actual feathers, currently there is actually no such thing as a feathered dinosaur.

“A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.” Max Planck

The Evolution of Modern Birds: Feathers and High Metabolism

Protoavis Paleontologist Sankar Chatterjee claims that the earliest known bird flew during the late Triassic period. He calls this bird Protoavis.

In response to the classic question of which came first, the chicken or the egg, the answer should be obvious: birds evolved from egg-laying archosaurs. However, the more difficult question is: which bird features evolved first on the evolutionary journey from advanced archosaurs to modern-day chickens?

Before birds, there were flying reptiles known as pterosaurs. Pterosaurs, like all reptiles, were most likely ectothermic, or ‘cold-blooded,’ meaning that their internal body temperature was typically nearly the same as the external temperature. In contrast, birds are distinguished from pterosaurs by being warm-blooded, or one could even say hot-blooded, considering how high their internal temperature is compared to mammals. But how do we know this?

While there are clearly advantages from being warm blooded there is also a price to be paid for this upgrade, and that is, an endothermic vertebrate needs to consume considerably more food to maintain its elevated temperature. In fact endothermic animals use far more calories to maintain an elevated temperature than they do for their physical activity. Hence, having some form of thermal insulation to slow down the loss of body heat to the surrounding environment is critical to the survival of endothermic vertebrates. This is why birds have feathers covering their body. Feathers are ideal for trapping air next to the body, and this trapped air is a very effective form of thermal insulation. Hence, the presence of feathers on the predecessors to birds indicates that these vertebrates were evolving an endothermic or warm-blooded metabolism.

Enzymes and Metabolism

Warm blooded vertebrates are able to achieve a much higher power output with the help of enzymes. Enzymes work within cells to move chemical reactions along at about a thousand times faster than normal. So enzymes are great for achieving higher metabolism. The only drawback is that the body needs to be kept within a narrow band of pH and elevated temperature in order for enzymes to work.

Flying is a power-intensive activity and so it requires flying animals to have a high level of relative power. The relative power of a vertebrate strongly correlates with the animal’s metabolism. Most scientists divide vertebrates into two broad categories: ectothermic, or ‘cold-blooded’ like reptiles, and ‘warm-blooded’ endothermic vertebrates like mammals. However, considering how much higher the internal temperature of most birds is compared to mammals, it would seem appropriate to classify birds as being ‘hot-blooded’. These differences in internal temperatures strongly correlate with each group’s metabolism, relative power, and flying capability. In our present time, reptiles have too low a metabolism to fly, while the warm-blooded flying mammals – bats – can fly, but they are usually rather small, whereas the ‘hot-blooded’ birds are by far the largest and most capable fliers.

Hierarchy of Flying Vertebrates

Flying Vertebrate Metabolism Thermal Insulation Notes Flying Ability
Birds Hot-Blooded
104 – 107oF
(40 – 42oC)
best insulation
Higher metabolism required
evolution of unique respiratory system
for greater oxygen intake
Birds rule the skies
(Flying Mammals)
good insulation
Evolved echolocation
for night flying to
advoid predatorily birds
Much smaller than birds
and not as fast
Reptiles Cold-Blooded
Temperature same
as environment
poor insulation
Lower metabolism means
much lower food consumption
Currently reptiles are
incapable of flying

Like the parts of a well-tuned engine, every organ of an organism needs to make its full contribution so that the organism can achieve its maximum performance. The transition of birds from advanced archosaurs to the superb fliers of today required the evolution of many parts. Flying demands high power output, so birds need a high metabolism. A high metabolism necessitates maintaining elevated temperatures, which, in turn, require the effective insulation provided by body feathers. Additionally, a robust respiratory system is needed to keep up with the oxygen demand of the high metabolism.

The Evolution of Modern Birds: Oxygen Feeds the Fire

One challenge ancestral birds faced was providing enough oxygen to sustain the continuous high power output required for flight. Birds addressed this challenge by evolving a unique respiratory system.

Let’s first consider how the respiratory system works for terrestrial vertebrates that are not birds. When humans, as well as other mammals, reptiles, and extinct dinosaurs, take or took a breath, fresh air travels down the trachea to fill our lungs. Upon exhaling and contracting our lungs, the air in the trachea reverses direction to exit our body. However, since we can only partially contract our lungs, a significant portion, if not most, of the stale air remains in our lungs. This residual air then mixes with the fresh air that enters upon the next inhalation. As a result of this mixing of old and new air, the air inside our lungs has a lower level of oxygen than the air outside our body. This makes it more difficult for us, as well as other non-bird terrestrial vertebrates, to absorb oxygen into our blood.

Diagram of a Bird's Respiratory System A bird’s respiratory system differs from ours. In our system, air moves back and forth in our trachea as our lungs expand and contract. In contrast, in a bird’s respiratory system, air first travels down the trachea to the air sacs in the rear of the bird; it then passes through the lungs before moving to smaller sacs in the front or sides of the lungs, and finally exits the bird through its mouth. Because high oxygen air is constantly flowing through the lungs, the bird’s respiratory system is more efficient at extracting oxygen from the atmosphere.
Beautiful illustration by Lizzie Harper showing a duck’s anatomy. Red organ is the lungs while the blue organs are the air sacs. Waterfowl are able to float on water because air fills most of their interior.
swan Mute swan and two chicks.

In contrast to our back-and-forth breathing, a bird’s respiratory system utilizes numerous air sacs to ensure a continuous flow of fresh, oxygen-rich air over the capillaries in their lungs. When a bird inhales, air travels to the large posterior and abdominal air sacs near the rear of the bird. From there, it undergoes a one-way journey through the lungs, maintaining its high oxygen content. After exiting the lungs, the air passes to the forward anterior thoracic air sacs before being expelled back into the environment. This unique respiratory system supplies modern birds with the oxygen needed for their high metabolism, effectively filling most of their interior with air. Birds benefit from a constant unidirectional flow of air through their lungs, allowing them to extract a greater amount of oxygen from the atmosphere.

In addition to extracting more oxygen from the atmosphere, a bird’s unique respiratory system results in much of its interior being empty space. Consequently, birds have a significantly lower body density compared to other vertebrates. While the body density of other vertebrates is typically around 1.0 g/cm3 - about the same as water - most birds have a density that is about one half to one third of this value. This lower body density is the primary reason why waterfowl are capable of floating high on the water.

Surprisingly, there is a third major benefit that arises from a bird's unique respiratory system: the large air sacs located towards the rear of the bird assist in statically balancing the bird while it is in flight.

The Evolution of Modern Birds: Physics of Flying

Force vectors acting on an airplane while it is in steady constant speed flight. Force vectors acting on an airplane while it is flying level at a constant altitude and speed.

The Science of Flight Equations calculate the takeoff speed and power requirements of an airplane or flying vertebrate based on factors such as weight, wingspan, air density, and aerodynamic shape. However, determining if an animal or airplane is capable of flying involves more than just these calculations. These flying objects need to be statically balanced as they fly. Besides the upward lift from the wings being strong enough to overcome gravity and the forward thrust matching drag to maintain speed, these opposing forces either need to be directly in line with each other, or the torques that they create need to cancel each other out to prevent unwanted rotation. A statically balanced airplane will maintain stable level flight without the need for a pilot to make corrections.

Microraptor in flight showing its four wings Microraptor is just one of numerous late Jurassic and early Cretaceous birds that had excessive plumage on their rear legs and tail. These flight feathers were required to give the rear portion of these birds enough lift to keep the bird in the prone position while they were flying. Since these earlier times the center of mass of birds has moved forward and so modern birds no longer need this excessive plumage on the rear of their body in order to fly.

In the early evolution of birds, the feathered archosaurs that were gliding between trees were not statically balanced. Like most animals, the center of mass was near the center of their body or nearly centered between their rear and forward legs. This arrangement works well for an animal walking or climbing a tree but not for a vertebrate whose forward limbs are evolving into wings. If the center of mass is approximately midway between the rear and forward limbs while the center of lift is closer to the forward limbs—the wings—then the two opposing force vectors are not aligned. The consequence of this misalignment is that once the winged vertebrate makes a horizontal leap from one tree towards another, the rear of its body will begin falling in respect to the rest of its body. Because nothing is holding up the rear of its body, not long after leaping, the gliding animal will rotate from a prone position to nearly vertical. For short leaps, this was not a problem and, in fact, it was preferred: if the destination is the trunk of a nearby tree, it's much better to be able to stop the forward motion by landing on all four limbs rather than slamming into the tree headfirst. Only when the vertebrate attempts longer glides or tries to fly does this unwanted rotation become a serious problem.

The early birds solved this misalignment problem by evolving large tail feathers that provided lift to the rear of their body. Many of these early birds also had excessive plumage on their rear legs, in addition to the plumage on their stiff tail. These feathers on the rear of their body gave these early birds the extra lift needed to maintain their aerodynamic prone position while in flight. Reviewing the fossil evidence and pictures of these early birds, one can observe that all of them have excessive plumage on their stiff tail, and many also had plumage on their rear legs.

Early Cretaceous Bird Mistaken For Being a Feathered Dinosaur The plumage on the rear legs and tail would create air resistance drag working against their efforts if they were trying to run. But why would a vertebrate bother trying to run fast when it can fly?

The plumage on their rear legs is particularly revealing, indicating that these vertebrates spent more time flying or gliding rather than running on the ground. This excessive plumage would create significant air drag if they attempted to run, suggesting that if these birds needed to move quickly, they would certainly opt for flying rather than running.

What has changed so that modern birds no longer need excessive plumage on the rear of their body? It appears that modern birds no longer require this excess plumage for static balance. In other words, the center of mass for modern birds must have shifted forward, aligning the downward weight vector with the lifting force of the wings pointing upward.

But how did the center of mass move forward? As explained in the previous section, birds possess a unique respiratory system that occupies a significant amount of space within their bodies. Although this unique respiratory system has existed since the late Triassic period, it has undergone some changes over time. Initially, the air sacs were likely much smaller, but as vertebrates evolved the ability to fly, these air sacs began to enlarge, particularly the rear air sacs. Consequently, in addition to modern birds having a lower overall density, this lower density is primarily concentrated in the rear portion of their bodies. With the majority of the high-density muscle mass and organs located near the front, and the low-density air sacs in the rear, the bird’s center of mass has shifted forward.


The Evolution of Modern Birds: The Supreme Flyers

Superior respiratory system, robust cardiovascular system, insulating feathers enabling a hot blooded metabolism, all these parts working together to maximize power output per unit weight is why birds are exceptionally strong flyers. And still there are so many more features about birds adding to their flying superiority. Aerodynamic form, hollow bones, incredible eyesight, superior intelligence, a chapter could be written on all the amazing ‘engineering’ aspect of just feathers.

The flight equations guide us in understanding of the evolution of the birds.

PT = 1/2 A C ρ v3 + W2 / (ρ b2 v)

The strongest flyers are those that the greatest power to weight ratio. This is the reason for why birds desire a hot blooded metabolism, a superior respiratory system supplying their muscles oxygen, hollow bones, and light yet strong feathers. The equations also show that smooth aerodynamic form is critical to the birds that wish to specialize in flying fast. A large wingspan can also be helpful in reducing power requirement, but these large wings are not necessary if the atmosphere is extremely thick. It is easy to fly on even modest wings when the atmosphere is extremely thick.

Head of Bald Eagle Beak of a Bald Eagle. Just like the advanced archosaurs from whom they evolved, most of the earliest birds had a mouth full of teeth. However modern birds evolved from the Avialae birds and Avialae birds had beaks, so all modern birds have a beak. A beak is better than teeth because a beak is just as strong as teeth and it is lighter; in most cases minimizing weight maximizes flying ability.
Image by Anrita

From the evidence presented in this chapter and much more that is readily available, it is now known that many of the flying birds of the Cretaceous were every bit as large as the largest pterosaurs. And not only were birds capable of growing just as large as pterosaurs, when it came to flying, birds were clearly superior. There are good reasons for why the smaller pterosaurs went extinct at the end of the Jurassic period, while the larger pterosaurs went extinct at the end of Cretaceous period: they had difficulty completing with the birds.

Since their humble beginnings, birds have evolved considerably to become the flying marvels that they are today. Their unique and highly efficient respiratory system delivers the high volume of oxygen their muscles need to deliver the high power required for flying. This revved up metabolism elevates their body temperature such that birds run at a distinctively higher temperature than mammals: birds are not just warm-blooded; they are hot-blooded. To maintain this warmth, birds have fluffy feathers covering their body that is superior to the hair found on mammals for conserving energy. The excellent lift of their properly shaped wings along with their light aerodynamic bodies, give birds a lift to drag ratio that nearly every airplane designer envies. A high powered metabolism supported by a highly efficient respiratory system, extremely aerodynamic and highly maneuverable, lightweight and strong, able to float and thus adapted to wetland environments, it is clear that birds evolved into being the most nearly perfect flying animal. No other group of flying animals - bats, pterosaurs, or anything else - is comparable to the flying supremacy of birds.

Large Modern Birds and Flightless Birds of the Cenozoic Era

Up until the last few million years of the Cenozoic era there were flying birds that were much larger than largest flying birds of today. Also approaching the end of the Cenozoic era there were large flightless birds and many of these similar species of these large flightless birds are still with us today. So how was it possible for flying birds to be so much larger than today’s flying birds and why would many of these large birds lose their ability to fly?

Before answering these questions let’s get a better perspective of flight limitations by looking at how our current thin atmosphere limits the size of the largest flying birds. Once again the flight equations enable us to calculate flight capability of the largest flying animals. Generally, as the weight of a bird goes up its flying capacity goes down until the heaviest birds can no longer fly.

Flyer Mass
Speed for
least Power
Minimum Power
for Flight
Available Power
Power Ratio
Great Bustard 19 2.5 15 0.44 0.38 0.89
Andean Condor 14 3.0 12 0.20 0.32 1.6
Mute Swan 13 3.0 12 0.18 0.30 1.7
Dalmatian Pelican 15 3.5 11 0.18 0.33 1.9
Bald Eagle 5.6 2.0 11 0.08 0.17 2.2
Wandering Albatross 13 3.6 11 0.13 0.30 2.3
Marabou Stork 9.0 2.9 11 0.10 0.24 2.3
Blackston's Fish Owl 4.6 1.8 11 0.067 0.15 2.3
Bar-headed Goose 3.0 1.6 10 0.039 0.12 3.0

Comments Regarding the Largest Flying Birds

Great Bustards – Males are typically three to five times larger than females and this size difference between the sexes has caused some confusion about the flying ability of Great Bustards. Researchers have tracked Great Bustards migrating thousands of kilometers, but these bustards were all females that only weighed about 3.5 kg. Because most bustards are heavier their preference is to spend nearly all of their time on the ground while at most only occasionally making short flights. It is questionable if the exceptionally large 19 kg male birds are capable of flying at all.

Andean Condor – With the exception of when they are taking off, these large birds rarely flap their wings. Instead they typically get their lift by soaring over warm rising air.

Mute Swan Landing in Water Photo of Mute Swan dragging its feet in the water to slow itself down as it lands on the water.

Mute Swans – While most swan species migrate, Mute Swans do not migrate. Perhaps it is because they do not need to migrate or fly that often that some individuals have grown extremely large: up to about 23 kg. These overgrown Mute Swans may be too heavy to fly.

Dalmatian Pelican – Many pelicans are surprisingly large and strong flyers.

Albatross – These are excellent fliers. They will usually go on migrations for multiple years where they are often flying continuously for days before taking a break to eat or float on the water. However, because they are large birds with narrow wings, when they do return to solid ground they need to land at high speeds and these high speed landings often give comical results.

Bar-headed Goose – These birds are noted for their ability to fly at high altitudes as they fly over the Himalayan mountains. Flying at such high altitudes test the limits of these birds’s ability because the thinner atmosphere requires the expenditure of more power to produce lift and yet the bird’s ability to produce this power is less because there is far less oxygen in the thin atmosphere.

Having established the fact that in today’s atmosphere 20 kg is about as heavy as a bird can be and still be capable of flying; let’s now investigate how it was possible for flying birds to be much larger than this during the Cenozoic era. Six million years ago a giant bird known as the Argentavis Magnificens flew in the Andes Mountains of South America. This giant soaring bird had a seven meter wingspan, but how much did it weigh?

Because Argentavis is relate to the Andean Condor in that they are both New World vultures and they even flew in the same geological location, we can get a good estimate of the Argentavis’ mass by simply scaling up the mass of the Andean Condor. Multiplying the 14 kg mass of the Andean Condor by the cubed of the wingspans (7m / 3m) gives us a value of 180 kg for the estimated mass of Argentavis Magnificens.

If we insert the Argentavis’ wingspan and mass estimates into the flight equations while assuming no change in the atmosphere’s density we get the 0.43 power ratio value thus validating our gut feeling that such a large bird would not be capable of flying in today’s atmosphere. But then we realize that because Argentavis is so similar to the Andean Condor we can match the determine the atmosphere during Argentavis’ time adjusting the atmosphere density until Argentavis’ power ratio matches the Andean Condor power ratio. In this way we discover that six million years ago the Earth’s atmosphere was over ten times thicker than what it is today.

Flyer Air Density
Speed for
least Power
Minimum Power
for Flight
Available Power
Power Ratio
Argentavis (Thin Atm.) 1.2 180 7 18.6 3.95 1.7 0.43
Argentavis (Thick Atm.) 17 180 7 4.9 1.06 1.7 1.6

Besides Argentavis, about 37 mya there was a seabird with a seven meter wingspan named Pelagornis sandersi. There are more examples besides these two but unfortunately the remaining specimens are only known by a few fossil bone fragments. In all likelihood during the Tertiary period there were many more flying birds that were much larger than the current largest flying birds.

Large Flightless Birds

With an estimated mass of between 220 to 500 kg, Utahraptor and Dakotaraptor are two of the heaviest birds that ever flew and yet they are not quite the heaviest birds that ever existed. That title goes to either Stirton’s Thunderbird or the Elephant bird, two of the heavier flightless birds of the Cenozoic era.

Ostrich The ostrich is the largest and heaviest living bird. It cannot fly. The ostrich avoids predators by outrunning them.
Elephant Bird The elephant bird existed on the islands of Madagascar and went extinct around 1100. It is often considered the world’s largest bird.
Giant Haasts Eagle Attacking Two Moa Drawing shows a Giant Haasts Eagle attacking two Moa. Both of these species were native to New Zealand and both went extinct around 1400 following the arrival of the Māori.
Drawing by John Megahan

The K-T extinction event brought an end to the dinosaurs and pterosaurs. It marks the end of the Mesozoic era along with the beginning of the Cenozoic era, but it was not end of the Earth’s thick atmosphere. The entire Cenozoic era has been a transition period from the atmosphere being extremely thick to the thin atmosphere that exists today. As new life bounced back from the K-T extinction vertebrates began to grow large again as much as the thick atmosphere would allow, while simultaneously the Earth’s atmosphere was thinning. For many of the larger vertebrate species, almost as soon as they reached their peak size they felt their increased effective weigh pressuring them so as to restrict their behavior. No vertebrate felt more burdened by this feeling of being too heavy than the largest flying birds.

Flightless Bird Mass (Kg) Extinction Date Island or Region
Stirton's Thunderbird 500 30,000 years ago Australia
Elephant Bird 650 1100 Madagascar
Moa 250 1400 New Zealand
Dodo 13 to 20 1600 Mauritius
Great Auk 5 1844 North Atlantic
Ostrich 130 Still Alive Africa
Emu 45 Still Alive Australia
Cassowary 60 Still Alive New Guinea
Rhea 40 Still Alive South American
Emperor Penguins 25 to 45 Still Alive Antarctica

The effect of the thickness of the atmosphere is much more apparent to flying vertebrates than it is for the vertebrates that walk on the ground. For example flying birds can be nearly twice as large, weighing nearly eight times more if the atmosphere is ten times denser, and yet this ten times thicker atmosphere will be insignificant in reducing the effective weight of vertebrates and so there will be no noticeable difference in the size of terrestrial vertebrates. The atmosphere would have to be least a hundred or perhaps two or three hundred times thicker before it is apparent that the terrestrial vertebrates are larger.

As the Cenozoic atmosphere gradually thinned, a greater demand was placed on the heaviest birds in their effort to achieve flight and so increasingly these birds would find themselves spending a greater percentage of time on the ground much like present day wild turkeys or bustards. This would go on until eventually they would not be able to fly at all.

Besides this forced evolution to becoming a flightless bird there was also the possibility of birds being encouraged to lose their ability to fly. By the middle of the Cenozoic era flying birds surely would have spread to the most distant and remote locations on Earth. No matter if they were large or small if these flying birds found themselves on a remote island where there are no predators and plenty of food on the ground then they could easily evolved into flightless creatures. These large or small birds that had earlier flown to islands and other remote locations all over the Earth were now finding themselves spending nearly all of their time on the ground until they completely lost their ability to fly.

Great Auks The great auk was a large flightless bird native to the North Atlantic. It went extinct on July 3, 1844 when fishermen killed the last confirmed pair of great auks.

Being grounded does not necessary mean that a species of bird is destined for extinction but that is the usual outcome. The record of these extinctions leads to some interesting observations. Throughout the Cenozoic era large flying birds were being grounded on almost every continent and numerous islands. The grounded birds that were the first to go extinct were mostly those on the large continents since they were most accessible to predators. The exception to this is the large birds that evolved the ability to outrun their predators, and these are the ones that were most likely to avoid extinction. Another interested fact is that the birds that were grounded on the islands had to have flown there before they lost their ability to fly. Most of the bird species that were grounded on islands or isolated land masses survived and thrive because they were isolated from predators. But this peaceful existence would come to an end the moment a predator, usually man, reached their isolated location. Sadly, it is easy to track the expansion of mankind with the extinction of each of these species of grounded birds. Humans have caused the extinction of hundreds of bird species; over a hundred bird species in just the last few centuries.

External Links / References

Are Birds Dinosaurs

Early Birds

Endothermic and Ectothermic (Warm and Cold Blooded) Vertebrates


Enzymes and Metabolism

'Feathered Dinosaurs'

Cretaceous Raptors


Bird Mimic Late Cretaceous Dinosaurs

Ancestral Birds

Taxonomy & Phylogeny

Bird Respiratory

Physics of Flying

Flightless Birds

Birds that Float

Largest and Heaviest Flying Birds

Geological Ages


Ecological Niche

Comments, Questions, and Answers

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