Ostrich: Bird or Dinosaur?
Photo by Michael E. Abrams

Aves ex Dinosauribus:
How Dinosaurs Evolved into Birds

By Harry Levin
Essay Number Seven

A seemingly trivial, childlike question: “Why do birds fly north to nest?” is indeed, a baffling question for an ornithologist. The correct response is that the bird is dinosaur-derivative. Birds are descendants of a therapod line of dinosaurs and, accordingly, have inherited the compulsive behavior patterns of the dinosaur; namely, periodic migration to the north for safe-nesting  and, in successive season, return to the south for abundant feeding,

Dr. Harry Levin

This is the seventh of Dr. Levin's list of essays with ground-breaking insights into the origins of flowers, birds and dinosaurs. He also explains  the philosophy of an engineer venturing into natural history.
The list of essays and his discourse are located

These essays are being published for the first time.

Florida  Wildflowers

Michael E. Abrams

Moreover, the bird is heir to the aquatic nature of the dinosaur. Theropods were deft swimmers, some among them aided by oily, hollow-shafted feathers, another bird legacy. Bird flight in due course followed feather inheritance. Finally, inheritance of endothermy was highly advantageous to birds in sustaining their swimming, feeding, and nesting activities.

Migratory instinct to seek out havens of safe nesting and feeding characterized the dinosaur and carried over to the bird.


With the complicity of a devastating asteroid, volcanism, and nest predation in a million-or-so-year span, extinction of the dinosaur was almost completely accomplished at the end of the Cretaceous about 65 million years ago. No dinosaurs were left on earth, except birds that had swam to polar lands and birds that had flown to safer havens than bare ground and had developed stratagems, in groups or in pairs, to protect their eggs and young.

The first birds on record appeared at the close of the Jurassic, from a line of  characteristically hollow-boned theropods, of coelurosaurian heritage, dating back to the early Jurassic. These first birds were aquatic -- swimmers, aided by oily, hollow-shafted feathers, which insulated their bodies in chilly waters and reduced drag resistance as they sped through the water to seek distant shores or to aggressively pursue their prey. Endothermy too was highly advantageous to these birds in their sustained swimming and feeding activities and particularly in their traverses of a widening western Tethys, which had become increasingly difficult to cross. Bird flight evolved soon thereafter, as a “spin-off,” as a serendipitous use of feathered bodies. 

The predominantly aquatic nature of the earliest birds is told by the fossils of the extinct Hesperornis that lived throughout the Cretaceous. Its wings were  feathered paddles, small and useless for flight. But Hesperornis was well suited for swimming and fishing. Its body was streamlined, strong of legs, long of neck, and agile.


The Penguin, Sole Survivor Today among Aquatic Flightless Birds

Worldwide, the asteroid collision contributed almost entirely to the discontinuance of flightless aquatic birds -- except for the penguin and its northern black-and-white flightless counterpart, the great auk, Pinguinus impennis, of the family Alcidae. (That great auk, now extinct, was killed off  by man after at least 50 million years upon the earth. Its mindless extinction, like the burning of an ancient library, diminishes all of mankind.)

As the Cretaceous neared its end, dense populations of flightless aquatic birds had inhabited the shorelines of South America and were in potential danger of flooding that could occur by incursion of coastal waters. Such was the setting for the calamity that did occur from the unimaginable fury and magnitude of the asteroid event. In that South American region, then so populated by flightless bird families, only one flightless bird has survived that collision to this day. It is the penguin. By its sole survival as a flightless form of bird, the penguin may well provide important insights into early evolution of bird from dinosaur. The penguin, or a close predecessor, in all likelihood was extant during the late Cretaceous. Fossils of penguins are found in Eocene diggings of 50 million years ago.

When the asteroid collision occurred, the western coast of South America was relatively flat. It was not yet buffered by the Andes (which later on, in the Cenozoic, were to rise in many places to 6,000-meter majesty). South  America was then attached to Antarctica beyond its Fuegian southern extremity (at about 65 degrees south latitude). The asteroid collision probably destroyed all South American dinosaurs and flightless birds right down to the Fuegian tip of Chile. Presumably, one or a few penguin species more than 66 million years ago had radiated even farther south of the equator to safe nesting and feeding grounds in Antarctica and  Australia. Hence that unique wedge-shaped, or sphenisciform, flightless bird survived the catastrophe and is alive today. Presumably, where some of the penguins had their nesting sites, the destructive energy of the asteroid collision was attenuated to safe levels – by distance of Antarctica from the source; by a Permian orogeny that then extended all the way from Antarctica to New South Wales; and by wave-suppressing ice floes.

The penguin, like the hesperornis, never flew; nor did its ancestors. Today it does not possess the light, hollow bones that help flying birds to stay aloft. Aided and protected by oily feathers, its hydrodynamic body wedges its way smoothly through water. It dives deeply; swims swiftly; and traverses hundreds of kilometers of ocean waters, including the deep waters off South Africa. It feeds adeptly on krill (most abundant in coldest waters). And even before the catastrophe, it followed the Antarctic Coastal Passage from South America to an Australia newly unbound from Antarctica.
Today the members of the penguin family Speniscinae are the only flightless aquatic birds. This singularity alone strongly implies that in South America no flightless aquatic bird survived the asteroid collision, although before then, a large and diverse population of flightless birds could have fished off the almost endless South American shores – shores that offered various degrees of natural protections from placental mammal predators. 


The Penguin: A Cretaceous Storehouse of Knowledge

To elaborate upon the remarkable penguin family, consider the largest, the emperor, some of them one-meter tall and weighing more than 40 kg. These birds have recorded dives well below 500 meters and they have the ability to stay under water longer than 11 minutes. In Antarctica, they exist for long months in community with snow and ice and with wind chill factors below minus 40 degrees equivalent. These values translate to unsurpassed intricate, efficient internal heat exchange and oxygen utilization systems. These adaptations are not recent. In emphasis, the very survival of the great auk and penguin from  asteroid-wrought calamity denotes that more than 66 million years ago they inhabited the colder regions of the earth beyond 60 degrees north and south latitudes – and that these inventive adaptations must, therefore, have been in place already 66 million years ago. 

The penguins are living proof that among dinosaur ancestors, and certainly the theropods, there were remarkabe swimmers and divers(1). Remarkable indeed, are the abilities of the penguin, e.g., to confront Antarctic cold through dark, long months and to dive deeply into dark, icy waters. Hence, the penguin has the gift of an eyesight reaching well into the infrared; possessing extremely high sensitivity in the optical (to detect the faintest bioluminescence of marine creatures); and even perhaps penetrating into the ultraviolet. Such exceptional endowments as these should be thoroughly looked into. Surely, they did not evolve abruptly or quickly. Indeed, they speak of more than 100 million years of dinosaurian evolution previous to the Cretaceous – perhaps even archosaurian evolution that began before the cold temperate climate of Gondwanaland.

For their sustained aquatic abilities, these dinosaurs had to be endothermic creatures with elaborate lung development --indeed, impressive creatures, in view of fossil testimony to the penguin that lived on oceanic shores at the time of the asteroid collision. Here today is that flightless bird itself, a living dinosaur from that time – in a sense, the oldest dinosaur today upon the earth, perhaps the oldest living repository of dinosaur genes. The penguin merits anatomical study mindful of that ancestral context. For example, the penguins, and the alcids too, have four-chambered hearts  and can provide (inferential) understanding of dinosaur endothermy and heat exchange and of the intricacy and efficiency of their lung tissue – which studies can shed light on dinosaur evolution, melding past and present. Here can be a rare source for bioengineering inquiry.


The Ratites

A discussion of flightless birds known as ratites – birds such as cassowary,  dodo, emu, ostrich, and rhea – seems odd preceding an inquiry into the  origin of bird flight; yet it helps to illustrate that some events make sense  in respect only to their historical background. Ratites have little biogeophysical significance. Ratites are ground birds that evolved polyphyletically on ecological opportunity from birds capable of flight. The penguin, on the other hand, never flew, nor did its precursors.

The ratite amply illustrates a course of evolution in birds that had already   evolved flight capability. Ratites, in fact, are a reversal –  “re-consideration” on the part of certain families of birds that flying is not all that great and is best avoided. The trend to flight is reversed in birds where food is plentiful upon the ground; nesting on the ground is relatively safe; and there is danger on the wing from predators. Under such conditions, the body tends to lose its streamlines; wings become abridged; and flight becomes short and eventually is lost. Freed from the constraints of flight, the bird body can become big again, like that of its theropod ancestors.

Thus, evolutionary simplification has taken place: a retrenchment in wing form and function. Even ratites cannot hide their kinship to dinosaur antecedents.  Cassowaries, for example, have a tall, blade-like keratin “casque,” ostensibly for running uninjured though the underbrush and perhaps for digging roots. This head feature is the only armor among birds, and it suggests the cranial (occipital) crests of the ancient coelurosaurian theropods – but not “the very casques that did affright the air at Agincourt.” Here and there among birds are indications of such atavistic morphology, throw-backs to the dinosaur. Occipital crests occur today in the here-and-there among species of diverse birds such as eagles, hawks, herons, ducks, cassowaries, and even penguins. They might offer protection in scampering through underbrush, or, in the case of penguins, scampering through seaweeds. 

Ratites, by their nesting habits, can provide a substantial look back at the dinosaur and its nesting habits, and, in fact, at their own theropod ancestry. Ratites such as the ostrich, the emu, and the cassowary portray the breeding habits of the dinosaurs by nesting features that that these birds share in common. The ostrich has been assigned to the order Struthioniformes; the emu and the cassowary, generally to the order Casuariformes. The ostrich is African; the emu and the cassowary are Australian. The latter is also a native of New Guinea. Nominally their weights run from about 120 to 70 kg., from ostrich to cassowary.

These ratites all travel in herds and colonize tightly during the breeding season. Emus and cassowaries are excellent swimmers. In fact, for nesting,  the cassowary prefers rainforests and swamps; ostriches and emus are  inclined to seek grassy ground. The clutches of the cassowary may contain 4 to 8 eggs; the emu, as many as 25. On the other hand, ostrich clutches may be communal, with as many as 60 eggs. By chicken standards, the eggs are of huge size. 

Reflecting the nesting behavior of dinosaurs, these ratites all nest on the ground. The clutches of the ostrich and the emu are bare and shallow, often shielded from predators by grasses and weeds. The cassowary clutch may be a slightly raised platform built of plant debris. Incubation of ostrich, emu, and cassowary eggs takes about 42, 55, and 65 days, respectively. The males have important roles, even predominant, in the incubation. Both parents and other herd members share in feeding and protecting the hatchlings in the nesting area for periods even as long as nine months. Nest predation by cats, dogs, foxes, feral pigs, reptiles, etc. is a concern that requires constant vigilance. Protection from nest predation is crucial to dinosaur history.

Thus, the nesting patterns of today’s largest ratites, ostrich, emu, and cassowary, reflect with amazing consistency the nesting history of the dinosaurs, especially the large clutches of eggs, the vulnerable nestlings, and the long brooding times essentially at ground level. For birds in general, characteristic dinosaur nesting patterns persist. Among these are two-parent, long-time incubation of eggs and nurturing of hatchlings and regurgitate feeding.

In context with the evolution of birds, the ground has provided opportunistic, but mixed, messages. To birds already capable of flight, the presence of trees and tall bushes promotes flight and enhances flying ability. Hard-to-access places on land offer survival advantage. On the other hand, when flying birds have found the ground to be to their advantage, they have shown a tendency to “short hop,” like turkey and peafowl, or even give up the ability to fly. They may perhaps be throwbacks to Archaeopteryx, which might have been the original “short-hopper,” 150 million years ago. 


The Origin of Bird Flight

The drifting apart of continents was the basis, to large measure, for the evolution of dinosaurs into birds. Bird flight evolved at sea, as theropods sought to cope with the hardships of lengthy water crossings. From the late Jurassic onward, a widening of the embayment between North America and South America occurred, making the annual back and forth crossings increasingly difficult. The first solutions were flightless birds, superb swimmers, with streamlining, increased feathering, and feathered forelimbs modified to wings to facilitate paddling. Witness the Hesperornis, flightless throughout the Cretaceous. Then flightless birds evolved into flying birds. Witness the Alcidae, evolving from great auk into puffin. 

The bird family Alcidae, order Charadriiformes, is used here as an exemplar of the transition of flightless birds into flying birds, although some among the theropods, for example, Archaeopteryx, 145 million years ago, may have made awkward attempts at flight. The Alcidae (alcids) are 21 living species of auks, murres, guillemots, and puffins. Also included is the great auk, whose skeleton is preserved, although it was made extinct by the hand of man by 1844. With some species exceptions, the alcids breed above or near the Arctic Circle and stay the winter farther south. All are streamlined birds, with short narrow wings and webbed feet. All are marine feeders, deep divers, and swift, graceful swimmers at the ocean surface and below. In the water, all use survival colors of black above and white below. An inference is in order, namely: in general, early birds, flightless or flying, had the same penguin-like pattern of black and white feathers.

The great auk, Pinguinus impennis, wore black and white and stood erect on land as do his counterparts, the penguins of the Antarctic – their vast separation a likely result of the asteroid impact that wiped out numberless flightless birds in between. Like the penguin, the great auk could not fly.

Its wings were no more than a mere 15 cm. long on a body about 75 cm. long. The great auk represents one end of the spectrum of family transition of flightless bird into flying bird which had essentially the same water skills and habits. The intermediates are lost to history. The living Alcidae all can  fly and still retain remarkable water skills. They still do much of their traveling by swimming, even over thousands of kilometers. The first Alcidae, more the 50 million years ago, were at least as large as the great auk (or the ancient penguin). But smaller size was good for flight. The largest of the living auks, murres, guillemots, and puffins is about 40 cm. in length. Their wings are short and narrow, yet adequate for flying. 

Some of the aspects of the remarkable metamorphosis of dinosaur to bird    are tenuously retrievable, conjectural, yet persuasive. Especially fascinating is the bird order Charadriiformes, 16 families of marine and fresh-water birds, web-footed, ancestrally feathered black and white – like dinosaurs in general, nomadic, gregarious, breeding in colonies. The family Alcidae (above), of order Charadriiformes, was among the first flyers.  


The Rationale of Bird Flight

Flying birds evolved from flightless birds of theropod descent. Bird flight began at sea, where, from the Cretaceous until the present, the bird  family Alcidae are paradigms of the transition from flightless bird to flying bird. Birds, already hydrodynamic and swift in the water, evolved smaller, lighter bodies and larger wings with which to depart the water and become airborne and eventually attain sustained flight. With flight came the enhanced ability to prey and to escape predators; the ability to accomplish annual breeding trips over larger distances; and the ability to seek out safer breeding places.

Surely the fact that numerous bird species today breed and feed in wetlands and deltas, on seashores, and among lakes, ponds, and waterways reflects a long Mesozoic theropod affinity with water.

Midway from Pre-crocodilians to Birds: the Dinosaur

Remarkably, a rewarding effort to flesh out a portrait of the dinosaur can be made by conceiving that the dinosaur is intermediate in form and behavior in many ways in the phylogenesis from pre-crocodilians to present-day birds. 

The basis for this conception is that pre-crocodilians, dinosaurs, and birds, in that order of origin, can be uncritically regarded in subclass Diapsida, superorder Archosauria, or “ruling reptiles.” Archosaurs are defined as both thecodontic (i.e., they, or their forebears, have teeth in sockets) and diapsid (i.e., they have upper and lower temporal openings in the skull). Today, the only living archosaurs are crocodilians and birds. Consider that between the pre-crocodilians and birds, in the course of earth history, the dinosaurs dominated the earth for 160 million years (from early Triassic).

Gizzards and gizzard stones are primitive features of archosaurs, appearing in both crocodilians and birds. Implied here is their use by all dinosaurs, with possible exceptions. Moreover, crocodilians have four-chambered hearts (three chambers being usual for Reptilia). Four chambers come also with both birds and dinosaurs; and both have the major improvement of a single systemic aorta. Crocodile-like in form, were two piscivorous  saurischian dinosaurs, Spinosaurus and Suchomimus.

Dinosaurs may be described as a genetic continuum from pre-crocodilian to bird. For example, Archie Carr in 1994, (2), describes the alligator, of pre-crocodilian descent, in its native habitat. Carr wrote that the alligator has “strong homing urge and direction-finding ability,” traits certainly also true for both dinosaurs and birds in their north-to-breed, south-to-feed behavior. 


Carr noted that the alligator has small front legs compared to powerful rear legs. (Incidentally, short forelimbs and long hind limbs are a general  characteristic of archosaurs and, especially, of the  theropods.) Again, the reader may think of prosauropods among the earliest dinosaurs; and he may leap in thought to the powerfully hind-legged, tooth-jawed Hesperornis, among the earliest (flightless) birds; and, even among the dinosaur order Ornithischia, to the bipedal ornithopods. Carr wrote: “... the crocodile may have had marine, estuarine, or fluvial ancestors for a long way back.” Here again is the very theme of the aquatic nature of the dinosaur, which manifestly continued on, from theropod to great auk and penguin and to numerous other aquatic birds. Then too Carr wrote: “Another special crocodilian feature is an arrangement of respiratory plumbing that allows the creatures to hold on to large prey in the water without flooding the breathing passages.” Undoubtedly, a feature carried over by the dinosaur to the aquatic bird.

And Carr revealed more.

He observed that crocodilians protect their eggs and young; that they build their nests on land, sometimes burying their eggs beneath a sandy surface and sometimes covering them in a mound of vegetative debris – suggesting probable nesting habits of dinosaurs and flightless birds. For the alligator the nesting time is two and one-half months. For the dinosaur, therefore, envision similar long months of nesting. Today raccoons, otters, herons, and other predators raid the alligator nest. They find the young to be both feeble and succulent. I infer that that situation was the same for the dinosaurs during the late Cretaceous when they confronted the precursors of those raiders.
Voices are not prevalent among reptiles.

Yet Carr described the voice of the alligator as “a vast rumbling growl – like something half sound and half shaking of the earth”; and he added the comment: “In post-Pleistocene Florida ... three magnificent voices still sound ... . They are the jovial lunacy of the barred owl, the ethereal bugling of the sandhill crane, and the roar of the alligator.” From alligator to bird, it seems likely that dinosaurs too had voices. Probably some could shake the earth with the thunder of their roar, the stamp of their feet, and the crack of their tails. Did havoc reign amidst the roar of the rutting brachiosaurs?

What about the sex life of the dinosaur? Carr remarked: “There are no reliable external signs by which to tell the [alligator] sexes apart.” – except as he added, among mature adults, the males being somewhat larger. On sight, this inability to distinguish has excellent reason, as Carr noted: “The sex organ of the male is internal and is everted only at the moment copulation begins.” The same as with birds; and certainly sharpens the imagination for a graphic picture of mating dinosaurs, Carr added: “During courting time there is a great deal of swirling and splashing about. Part of this is fighting among rivals, but part is certainly sexual play between male and female.” Thus, here is the interpolated dinosaur, whose borders lie between pre-crocodilian and bird.


The Strongest Evidence of Dinosaur Heritage

Many anatomical reasons (some in dispute) have been offered for linking birds to dinosaurs; but the strongest evidence – given in this paper – is circumstantial evidence, namely: birds fly north to nest.

Why do birds fly north to nest? Birds fly north because of dinosaur heritage. North flight is the remnant of an ancient scheme. Birds evolved from predatory theropod dinosaurs that swam in attendance with the herds of herbivores on their periodic treks across the Western Tethys (in a companion essay) and that reared their own young at the herd periphery. The dinosaur had begun 230 million years ago to be genetically wired for a seasonal trip to the north – about 90 million years before a theropod line evolved into the first bird. Marine migrations of ancient dinosaurs are no more; but the genetodynamic compulsion to fly north to nest in season lingers in numerous bird genera. The herd was father to the flock.  

Today, birds fly north to lay their eggs and to fledge their young in relative safety. Flocks (in analogy to herds) fly back and forth in season over the Mediterranean, the remnant of the western Tethys. William Bartram, in 1790, (3), having roved the eastern wildernesses of North America, melded birds with poetry as he observed that “... most of those beautiful creatures, which annually people and harmonize our forests and groves in the spring and summer seasons, are birds of passage from the southward.” 


Yet time has retouched every bird; and some, like the scrub jay, stay at home. That jay is unmindful of an ancestry by Cretaceous waters where  the tooth-jawed Hesperornis of yore, feathered but flightless, did gracefully swim the salty current shore to shore – as do the oceanic flightless penguins and flying puffins today.

Migratory Heritage

When the swan, the summer gone,
On the nightfall has departed,
Born to follow south the swallow,
Still will stay the scrubby jay,
Lord of meadow, brae, and woodland,
Born to bide the wintertide.



Flying birds evolved from flightless birds of theropod descent. A very early stage of bird evolution – a transition stage from feathered theropod to flightless aquatic bird – goes back about 145 million years ago to the late Jurassic. Fossil records of 50 million years ago indicate a large penguin and another large flightless bird, the great auk. True flightless birds, neither one ever flew. The great auk, recently extinct, was patriarch of the bird family Alcidae, about 21 living species, all of them illustrating a smooth transition from flightless to flying bird. Like the penguin, they all are prodigious swimmers.

“And No Birds Sing” – Keats

On the Galapagos Islands, the effect of egg destruction is profound. No longer is there island sanctuary for birds and reptiles that breed upon the land. Once again, mammals are the intruders – mammals whose antecedents were a major factor in extinction of the dinosaurs by savaging their nests and their young. Over the years, dogs and cats have jumped ship, have become feral, and are threatening bird and tortoise eggs. 

On Anacapa Island, threatened too is Xantus’s murrelet, member of the family Alcidae, black above and white below, sea rover, diver, whose tiny (20 cm.) wedged-shaped body and webbed feet are somewhat reminiscent of great auk and penguin. Anacapa Island, one of its breeding havens, lies off the coast just north of Los Angeles. Like all other Alcidae, this murrelet is pelagic and for millions of years has come ashore only to breed. Non-native black rats have found home on Anacapa Island.

They nibble and crack the murrelet eggs with pin-sharp teeth and sometimes roll them down the bluffs to eat later. Foreshadowing doom, 100 to 200 pairs of murrelet are left on the island. Such tragedy is being repeated for birds and reptiles all over the world. Once more it illustrates the mammalian savagery of nest destruction.



1. Ezquerra, R., Doublet, S., Costeur, L., Galton, P. M., and Perez-Lorente,  F., Were non-avian theropod
     dinosaurs able to swim?   Geology, 35 (6) 507- 510 (2007) Geological Society of America.

2. Carr, A., A Naturalist in Florida, A Celebration of  Eden, pp.104-124, edited by M. H. Carr, Yale
    University Press, New Haven, 1995.

3. Bartram, W., Travels, p.235, edited by M. van Doren, Dover
    Publications, New York, ~    1962.