Are Birds Reptiles?

Have you ever wondered how birds evolved? With about 11,000 bird species, it’s surprising to learn that birds evolved from reptiles. But how are birds reptiles?

Are Birds Reptiles?

Scientifically, yes, birds are reptiles, but this fact can be confusing because scientists use two classification systems: the Linnaean, which classifies birds and reptiles as separate groups; and phylogenetic, which classifies birds as reptiles.

• The Linnaean system began during the 1730s, when Carolus Linnaeus categorized living things according to their physical appearance.

• The phylogenetic system was created in the 1940s and helps scientists discover the ancestry of different species. This classification system uses DNA and genetic research to identify similarities.

In the Linnean system, “we separated all the living things by their differences. So when we see a bird and when we see a reptile, we can see that they are different,” explains Thaís Thomazini, MS, an evolutionary biologist and ornithologist who has conducted research on the genetic population of burrowing owls in São Paulo, Brazil.

While the Linnaean system is useful for understanding an animal’s place in nature, it doesn’t help us understand their genetic and evolutionary history. To do that, scientists use phylogeny.  

“There is no way to explain without talking about genetics,” says Thomazini, “because that is what keeps phylogeny together.”

When humans discovered phylogeny and how we can access information through DNA, we started to categorize animals by similarity. Phylogeny shows us the genetic relationships between modern animals and their ancient ancestors, including the fact that birds are reptiles—but not all reptiles are birds.

Mark Pyle, reptile educator and former president of the DFW Herpetological Society, agrees. He explains that while birds and reptiles look different, they have a lot in common too:

• Feathers are made from the same substance as reptile scales.

• Birds still have scales on their feet.

• Not all reptiles are completely ectothermic (relying on external sources like heat lamps). Some monitor lizards have a degree of control over their body temperature and some pythons produce heat by rapidly flexing their muscles (shivering).

In genetics, DNA barcodes help identify species. Thanks to mathematical models, the barcode also gives scientists information about an animal’s evolutionary history—showing them which characteristic came first, such as flight or hollow (pneumatic) bones. 

Evolution of Reptiles to Birds

Evolution is a slow process of change, taking thousands to millions of years. The earliest reptiles appeared around 315 million years ago and dinosaurs descended from those reptiles, first appearing around 245 million years ago.

Pyle explains that as you move through the fossil record, you’ll find dinosaurs like pterodactyls and stegosaurus; but you’ll also find fossils with feathers and beaks, even rudimentary wings. But these features appeared at different times, in species like archaeopteryx.

Part of the bird to reptile evolution, archaeopteryx was a bird-like dinosaur that appeared during the Jurassic period, around 150 million years ago. It is an intermediate species that has features of both birds and dinosaurs, including teeth, a long bony tail, and flight feathers. For these reasons, some scientists disagree on whether it should be considered a bird or a reptile.

While we may think of evolution as a logical progression from dinosaur or reptile to modern bird, it’s a series of random changes in their DNA that, given enough time, make big changes.

“It’s all by chance,” says Thomazini, who explains that some animal groups get lucky with a helpful change or mutation that improves their species’ chance at survival, while others struggle or die out.

For example, maybe a bird happened to have a stronger beak. It may have been able to break open a seed that others couldn’t, gaining an extra food source. Because it had more food, it also had a better chance of reproducing—passing on a genetic trait for a stronger beak. Eventually, all the birds of that species would have a stronger beak thanks to a random mutation.

Evolutionary Timeline

• Earliest reptiles: 315 million years ago (MYA)

• Earliest dinosaurs: 245 MYA

• Crocodilians: 235 MYA

• Bird-like dinosaur (Archaeopteryx lithographica): 150 MYA

• Microraptor: 125–120 MYA

• Sinosauropteryx: 120 MYA

• Oviraptor: 75 MYA

• Earliest “modern” birds (Wonderchicken/Asteriornis maastrichtensis): 66.7 MYA

• K-Pg extinction event: 65 MYA

Why Don’t Birds Look Like Other Reptiles?

Although birds and other reptiles share ancestors, they took different evolutionary paths millions of years ago—plenty of time for minute changes in their DNA to combine and create new species.

Because evolution means change, the animals you see today probably looked different 10,000 years ago. Some species thrive and evolve, while others go extinct for a variety of reasons, such as extreme climate shifts and competition for resources.

Mass Extinction Events

However, many species disappeared after mass extinction events, like the K-Pg (or K-T) extinction. Scientists believe a massive asteroid from 6 to 9 miles wide hit the earth and caused the K-Pg extinction about 66 million years ago. This extinction event eliminated 75% of all plant and animal species on the planet, including almost all non-avian dinosaurs, many mammals, birds, plants, and all pterosaurs.

Even the species that survived likely had challenges like finding food or mates for breeding. However, those difficulties don’t necessarily mean a species will become extinct.

Genetic Mutations

Thomazini explains that you can start a population with a few individuals, but the limited genetic diversity causes mutations. Some mutations are unhealthy, but others are useful and increase the population’s diversity.

“So let’s say that you start to have a lot of cousins having kids, and the kids start to have seven fingers. At some point, all of the population will have seven fingers,” Thomazini says.

Geographical Divides

She says that another form of diversity happens when you divide animal populations. Maybe they’re divided by a river and don’t interact anymore. Along with that separation, they may have access to different foods and begin behaving differently. At some point, they’ll stop recognizing each other as the same species.

The Wonder of Adaptation

Birds and their diversity are a testament to nature’s remarkable ability to adapt to changing environments. Hidden under colorful feathers is an unmistakable reptilian heritage, a shining example of the endless possibilities that evolutionary change offers.