Are birds reptiles?

Anyone who has watched a rooster stalking around a farmyard might agree that there is something very dinosaur-like about birds. That may come as no surprise, given that birds are dinosaurs. But in terms of classification, this raises a question: Since dinosaurs are reptiles, does that mean that birds are reptiles, too?

"I would say that any modern biologist would, or should, say that birds are reptiles," Martin Stervander, an evolutionary biologist and senior curator of birds at National Museums Scotland, told Live Science.

It wasn't always this way. Before the 1940s, biologists relied on a system called the Linnaean method to classify all life on Earth. This approach was developed by Carl Linnaeus in the 1730s, and it works by grouping animals that have similar physical characteristics. Linnaeus determined that all reptiles share two key features: They have scales, and they are ectothermic, or "cold-blooded," meaning they must rely on external sources of heat to regulate their body temperature.

Because warm-blooded, abundantly feathered birds lack these features, "birds were considered their own branch on the tree of life" under the Linnaean system, said Klara Widrig, a postdoctoral fellow in the Vertebrate Zoology Department at the Smithsonian's National Museum of Natural History in Washington, D.C. Linnaean classification accurately describes many relationships and helped to establish the broad branches of the tree of life we use today. Yet this classification system overlooks something that can reveal a lot more about an organism: its genes.

From the 1940s, the ability to examine genetic material gave rise to a new type of classification, called phylogeny. The phylogenetic method groups or separates organisms based on how much of their DNA they have in common. It also allows scientists to compare organisms' DNA to trace their evolutionary history and find shared ancestors.

Phylogeny for birds

Under this system, organisms are grouped into "clades," which are distinct branches on the phylogenetic tree that include all the descendants of a most recent shared ancestor. For example, modern birds are grouped into the Neornithes clade, because they all emerged from a common Neornithine ancestor that emerged 80 million years ago, before the nonavian dinosaurs went extinct. "Clades are a nested hierarchy," Widrig explained. So "if you go further back along the branches of the tree of life, you find that [Neornithes] also belong to the more inclusive clade Theropoda," a group of two-legged and mostly meat-eating dinosaurs that itself is descended from the clade Dinosauria.

Even in the dinosaur age, the defining features of scaly reptiles and birds weren't so distinct. Research suggests that some dinosaurs were warm-blooded and some nonavian dinosaurs sported feathers. Meanwhile, later on we know that birds that looked exactly like modern birds were already stalking the Earth alongside their dinosaur kin 66 million years ago. One of these was the "Wonderchicken" (Asteriornis), a fossil that was described in 2020 by a team that included Widrig. "Say if I got into a time machine and scooped up an Asteriornis and brought it back, everybody would be like, 'Oh yeah, that's just a normal-looking bird.'"

But back to the tree of life: birds' descent from Theropods and Dinosauria is what makes them dinosaurs. Meanwhile, Dinosauria, along with crocodilians, and other lineages like the pterosaurs, are contained within another clade, called Archosauria. This clade, in turn, shares an ancestor with a group called Lepidosauria, which gave rise to all modern lizards, snakes and the tuatara. That shared ancestor was a creature called a sauropsid that emerged about 315 million years ago, and launched the clade by the same name.

Sauropsida was one of two lineages that branched off from the amniotes (vertebrate animals that lay shelled eggs or carry fertilized eggs), the other being the group that evolved into the class Mammalia. Sauropsida, meanwhile, has become synonymous with the class "Reptilia," because within its network of clades upon clades, all reptiles — both living and extinct — are contained.

Related: If birds are dinosaurs, why aren't they cold-blooded?

"All of these guys — snakes and turtles and crocodiles and birds, and the dinosaurs when they were around — they all come back to one common ancestor," Stervander explained, and so under the phylogenetic grouping system, "that simply means that since birds are in [that clade], they are, per definition, reptiles." This is why phylogeny is valuable, because it can reveal that organisms that look very different are, in fact, very close to one another genetically and belong in the same group.

Why don't birds look like other reptiles?

The connections between birds and their scalier kin might have been easier to grasp if there hadn't been a mass extinction event 66 million years ago. "The reason why birds seem so strange compared to the rest of the reptiles that we have alive today is because all the evolutionary intermediate stages are extinct, so we don't have that to compare to," Widrig said. When the asteroid wiped out swathes of life on Earth, it obliterated whole sections of the Archosauria lineage of reptiles, leaving only crocodiles and some avian dinosaurs behind.

Without this void in the once-intricate tree of life, evolution might have ushered relatives into the modern age that would have made birds' reptilian roots more immediately clear. Instead, today crocodiles are birds' closest living relatives, even though they sit far apart on the phylogenetic tree.

To put it into human terms, "if all non-human primates were to suddenly go extinct today, our closest living relatives would actually be rodents and rabbits," Widrig said. They would be closest to us on the remaining branches of the tree of life, and our shared mammalian ancestry might be harder to accept.

If it still seems strange to think of birds as reptiles, it might partly be because of those Linnaean roots, which continue to shape our colloquial understanding of what a reptile is. "It's not until around the [19]40s where genetic evidence actually clarified where birds sit [on the tree of life]," Stervander said. "People had been referring to reptiles for almost two centuries before that. I think that's why there's been some unwillingness to redefine what we call the reptile."

But modern science is changing that, revealing unexpected connections in the tree of life — and opening our eyes to skies filled with modern feathered, flying reptiles.