Evolution Of Birds | How Have Birds Evolved Over Time?

No matter where we look, we would undoubtedly see a bird. With nearly 10,000 species, they show diversity like nobody else. It naturally leads us to wonder about the evolution of birds. Do they all have one ancestor? If so, who is it?  

Intro Video - Backtobirds
Intro Video - Backtobirds

Here, we try to catch a brief glimpse of the origin and evolution of birds. Read on! 

How Have Birds Evolved Over Time  

Evolution is a topic that fascinates us all. We all love to know about our ancestors. The first person to talk about it was Darwin in his book, Origin of Species. Unfortunately, and surprisingly, the book never discussed the evolution of birds. It fueled our curiosity to trace the evolution of these diverse and amazing creatures.  

After two years of publication of Darwin’s book, the first clue of the mystery of the evolution of birds was unearthed in a site in Germany. It was the discovery of the fossil of Archaeopteryx. Post it, many other fossils were discovered.  

Plenty of information was collected through intensive information collection from fossils, molecular phylogenetic analysis of living birds, and finally, quantitative macroevolutionary analysis. It completely revolutionized the way we look at the evolution of birds.  

From the information, scientists believe that the evolution of the birds initiated in the Jurassic period, and it evolved from the bipedal or two-legged dinosaurs called theropods. The leading fossil of theropods is Deinonychus. Its fossil was first identified in the 1960s. It is this fossil that comprehensively convinced scientists about the evolution of birds from dinosaurs.  

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The theropods bore a strong resemblance to the birds or avians. Though they were giants and weighed about 100 to 500 pounds, they had big teeth, large snouts, and nothing much between the ears. Most of them are carnivores and two-legged. They show a striking skeletal structural similarity to birds. Its specimen, Velociraptor, is the closest relative to modern birds. 


We have already stated that the Archaeopteryx fossil was first discovered in a limestone quarry in Germany in 1861. Its scientific name, Archaeopteryx lithographica, translates as ‘Ancient Feather’ or ‘Ancient Wing.’ 

Today, we have about 12 fossils of this animal. All the fossils are approximately 150 million years old. For decades, archaeopteryx fossils remained the only fossil link between the birds and dinosaurs. This fossil is like a rock star to all those interested in evolution, be it experts or amateurs.  

Archaeopteryx was unique and fascinating for several reasons. It was approximately the same size as a crow but showcased the characteristics of not only birds but also reptiles. It flaunted well-developed feathers on its forelimbs and tail. It had bird-like feet. But it also exhibited a few reptilian features like a wishbone, pointy little teeth in its jaws, a long bony tail, and the ankle bone fused to the shinbone.  

Archaeopteryx was clearly neither a bird nor a reptile. It actually represented a missing link between the two groups. Hence it gave rise to two streams of thought. The first was the evolution of birds from dinosaurs, and the second was the evolution of birds from reptiles.  

Evolution of Bird from Reptiles  

Present reptiles can be divided into four major groups. They are turtles/tortoises, snakes/lizards, crocodiles, and dinosaurs. The dinosaurs and crocodilians are believed to be over 225 million years old and a very specialized group of reptiles, the archosaurs.  Birds are believed to evolve from this archosaur group.  

In the reptiles’ group, birds are most closely related to crocodiles. Walking through history, it was about 300 million years ago when the first group of reptiles evolved. After approximately 40 million years later, a group of reptiles, the therapsids, branched off to eventually become modern mammals. Over the next 120 million years, several groups of reptiles branched off. One such successful branch was that of dinosaurs.  

The dinosaurs are distantly related to many modern reptile groups like snakes, turtles, and lizards. However, 65 million years ago, the great extinction happened in which an asteroid smashed into our planet and wiped many lives from the earth, including most of the dinosaurs. Only one group, the feathered flying theropod dinosaurs, made it through.  

These dinosaurs went through a series of adaptive adjustments called adaptive radiation and an evolutionary explosion and process to evolve into diversified groups of modern birds. 

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Evolution of Birds from Dinosaurs 

The evolution of birds is a fascinating story to read and understand. The discovery of the Archaeopteryx was a ground-breaking breakthrough in understanding how have birds evolved over time.  

Post intensive studies of all evidence and fossils, paleontologists and scientists reasoned that Archaeopteryx belonged to a group of small carnivorous dinosaurs named theropods. It led to the belief that perhaps the theropods are the ancestors of the birds. As they built the evolutionary tree, they got more convinced that birds are merely a twig on the dinosaur branch of the tree of life. 

Furthermore, the studies revealed a deep revolutionary history of tens of millions of years of evolution. Their origin and diversification were not one burst of innovation.  

Looking at the evolution tree, we go back 250 million years ago when dinosaurs had not truly become dinosaurs. At that time, there was a group of reptiles called archosaurs. They were already becoming distinct from things like turtles, snakes, and lizards.  

After millions of years, one of its lineages branched off to become the dinosaurs. Yet another one branched off to become the crocodiles while one more branch emerged as the pterosaurs, famous as the flying reptiles of the Mesozoic.  

Early studies strongly suggested that birds might have evolved from the archosaur group, and particularly the pterosaurs. It was because pterosaurs had bat-like wings. However, it did not flap from feathers but from the membranes of skin that were stretched between long fingers. But it is interesting to note that pterosaurs are not the actual ancestors but only distantly related to birds. 

Right from the time of Darwin in the 1900s, paleontologists were discovering and studying a good number of dinosaur fossils. They made a critical observation with each fossil – the wishbone or furcula and clavicles were missing in the dinosaurs’ fossils. In contrast, the clavicle was present in the older group of reptiles and primitive pre-dinosaur reptiles-the archosaurs.  

From an evolutionary point of view, it was an almost impossible scenario to believe that the clavicles would get lost through natural selection, and species would live without it for millions of years and then suddenly re-evolve them. It was a certain thing to happen if dinosaurs were the ancestors of birds.  

Hence, came the theory that birds were not a descendent of dinosaurs. It was based purely on the grounds that birds have clavicles while the dinosaurs lacked them. It made the theory strong that birds have evolved from reptiles. The theory remained in vogue for decades till the 1960s. 

The 1960s marked the beginning of the Dinosaur Renaissance. During this time, several new fossils of dinosaurs were discovered with Archaeopteryx leading the pack. The Deinonychus fossil in North West America was another example as it was an astonishing bird-like dinosaur. Studies of these were now conducted with all modern techniques and statistical analysis. It drastically changed the viewpoint as it brought forth several similarities between the dinosaurs and the birds. 

New studies revealed the presence of the furcula bone in the dinosaurs. Being delicate bones, the furcula and clavicles don’t preserve well in fossils. Their presence cemented the thought that birds descended from the dinosaurs.  

In the 1990s, a few amazing dinosaur fossils were discovered in the Liaoning province of northern China. Many of these fossils were more than 130 million years ago and had feathers. They were of different shapes and sizes, and each had feathers. It was also evident that many of them were not direct ancestors of birds. 

It established the fact that feathers are not unique to modern birds. Moreover, they formed a vital link between the dinosaurs and the birds.  

These fossils also showcased many skeletal similarities. A few of these fossils had lung structures almost like that of modern birds, including multiple air sacs. A few of these sacs penetrated the skin in both birds and dinosaurs.  

A few other noteworthy evidences that established the link between the birds and the dinosaurs are: 

  • The sleeping posture of a few dinosaurs is a carbon copy of that of modern birds. These fossils show the head of the dinosaur tucked just so underneath the arm forming the sleeping posture.  
  • Strong evidence suggests that just like modern birds, dinosaurs brooded their eggs and took care of their young ones. It would sometimes happen in a communal herd setting. The behavior is exactly similar to modern birds.  
  • Heaps of genetic data from the DNA of these fossils provide molecular evidence of a relationship between the dinosaurs and modern birds. The most famous of them is the collagen protein extracted from the Tyrannosaurus femur in the early 2000s. Post it; several more research was conducted, which strengthened the claim.  

The belief of dinosaurs being the ancestors of birds is gaining strength with each passing day and is most prevalent today.  

The Evolution of Flight 

An essential characteristic of the birds is their feathers and flight. Now, over 30 species of dinosaurs flaunted feathers, though most of them belonged to the theropod group. But simple feathers also existed in non-theropods that belied the statement that birds are the only group to have feathers.  

Interestingly, all present evidence suggests that feathers evolved before the flight. Today, scientists do not believe that feathers evolved due to adaptation for flight. Instead, it is believed that feathers might have originated for insulation purposes or to signal other members of their own species. Millions of years later, feathers proved to be advantageous for flight. 

Many hypotheses have been suggested about how birds evolved wings from their front limbs. Amongst several hypotheses, we list the two most popular ones: 

  • Few bird ancestors used to leap into the air to capture the prey or avoid predators. Hence, their forelimbs became modified into wings over the years to help them leap higher.  
  • Bird ancestors might have been living on trees. Hence, their arms got modified into wings to help them glide from branch to branch.  

To date, no concrete evidence is available, but scientists are studying both fossils and living vertebrates like bats who also showcase adaptations for flight.  

Evolution of Birds’ Beaks  

The evolution of toothless beaks is another fascinating topic for scientists.  

The first fossils of the birds were discovered in 1870. However, most have their heads crushed and incomplete. The first breakthrough came when scientists managed to piece together the skull of a strange ancient bird, which had a primitive beak lined with beaks.  

The fossil that proved a breakthrough was that of so-called stem birds. The leading example of this is the bird-like dinosaur, Ichthyornis dispar. Its bones were first discovered by the US paleontologist Othniel C Marsh. 

The creature is best described as a half-bird, half velociraptor. It was the size of a modern seagull and had large eyes and long beaks just like them. But unlike the seagulls, they also had teeth and a muscular jaw to use them. They could also fly.  

This eclectic combination of teeth, beak, flight, and jaws make this creature a crucial link to understanding bird evolution. The study suffered a setback due to broken and smashed skulls but got a new lease of life when a complete skull fossil was discovered in 2014 in Kansas. 

Studies reveal that the Ichthyornis could lift its upper beak without moving the rest of the skull. This movement is similar to modern birds. The second finding was its brain which is also quite similar to modern birds. 

Scientists found these primitive beaks much smaller and seemed to be evolving to do a few of the functions of the hand in a better way as their hands were getting evolved as wings and making their original work impossible to do.

According to experts, these creatures probably flew around to search for morsels of fish and shellfish. Once spotted, they would grab them with their pincer beak and then throw them back into their strong dinosaurian toothed jaws to crunch them a few times and then swallow them.  

The fossil also helped cement the thought that the evolution of the birds from the dinosaurs was not an overnight thing but a long and gradual process.  

Few Common FAQs on Birds Evolution  

We hope that by now, you have a clear perception of the evolution of birds. To make things clearer, we answer some specific queries that run in most minds, interested in the evolution of birds.  

What group of reptiles led to the evolution of birds? 

Birds are believed to evolve from the Thecodont reptiles, the Diapsids. It had two major lineages. One developed into snakes and lizards and the other into dinosaurs, reptiles, and birds.  

From the lineage, birds have evolved primarily from the pterosaurs branch of the archosaur group.  

Presently, it is believed that the birds are most closely related to crocodiles, and they might be their ancestor.  

Have birds evolved from dinosaurs? 

Yes, birds have evolved from dinosaurs. In fact, strong evidence indicates that birds are nothing but modern-day dinosaurs.  

When did birds evolve from dinosaurs? 

Modern birds are believed to start evolving about 65 million years ago after The Great Extinction. But they have a rich evolutionary history before that also.  

Why does the archaeopteryx indicate the evolution of birds from reptiles? 

Archaeopteryx showcased the characteristics of both birds and reptiles. It had jawed teeth, clawed fingers, and a long bony tail with free caudal vertebrae and keel less sternum, similar to reptiles.  

Similarly, it also showed many bird characteristics like body feathers, forelimbs modified into wings, bird-like girdle and limb bones, and the V-shaped furcula.  

All these led scientists to believe that birds evolved from reptiles.  

How have birds evolved over time? 

According to scientists, birds did not evolve overnight in an evolution explosion. But they underwent a long series of evolution over millions of years called adaptive radiation to evolve to modern-day birds.  

How long have birds been on earth? 

According to fossil records, modern birds started originating 60 million years ago. It began when the Cretaceous period ended about 65 million years ago. It also marked the end of dinosaurs.  

But molecular studies strongly suggest that the genetic divergences leading to the diversity of birds and their lineages started happening during the Cretaceous period.  

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Wrapping up  

A significant amount of information has been accumulated about the evolution of birds but a lot more needs to be discovered. The story of the evolution of birds is still in its infancy state and an intense subject of study. Discovering it is an exciting road.  

We hope you enjoyed reading this write-up and now have quite a bit of information at your fingertips. Do get back to us if you have any further information, queries, or feedback to share. We would love to hear from you. Till then, do look at birds with new eyes!

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