Anyone who has observed a rooster strutting around a farmyard might notice its striking resemblance to a dinosaur. That’s no coincidence—birds are, in fact, dinosaurs. But this classification raises an interesting question: Since dinosaurs fall under the reptile category, does that make birds reptiles as well?
“Any modern biologist would, or should, classify birds as reptiles,” says Martin Stervander, an evolutionary biologist and senior curator of birds at National Museums Scotland, in an interview with Live Science.
This perspective, however, wasn’t always the norm. Prior to the 1940s, scientists used the Linnaean classification system, developed in the 18th century by Carl Linnaeus, to categorize organisms based on shared physical traits. Under this method, reptiles were defined by their scales and ectothermic nature—meaning they rely on external heat sources to regulate body temperature. Because birds are warm-blooded and covered in feathers rather than scales, they were classified separately from reptiles.
While the Linnaean system helped establish broad biological classifications, it overlooked a key factor: genetic lineage.
The shift toward DNA-based classification, known as phylogeny, emerged in the 1940s. This approach organizes organisms into “clades,” which are branches on the evolutionary tree that group species based on shared ancestry.
Birds’ Place in Evolution
Using phylogenetics, modern birds belong to the Neornithes clade, tracing back to a common ancestor that existed 80 million years ago, before the mass extinction of non-avian dinosaurs. Going further back, birds are also part of the Theropoda clade—a group of two-legged, often carnivorous dinosaurs—and ultimately, the Dinosauria clade.
Interestingly, even during the age of dinosaurs, the distinction between reptiles and birds was not as clear-cut as once believed. Some dinosaurs were warm-blooded, and fossil evidence suggests that certain non-avian dinosaurs had feathers. In fact, prehistoric birds coexisted with their dinosaur relatives 66 million years ago. One example is Asteriornis, also known as the “Wonderchicken,” a fossil described in 2020. According to Klara Widrig, a postdoctoral researcher at the Smithsonian’s National Museum of Natural History, if Asteriornis were alive today, it would look just like a modern bird.
The lineage of birds, dinosaurs, and reptiles extends even further. Dinosauria, along with crocodilians and extinct groups like pterosaurs, belongs to the larger Archosauria clade. This, in turn, shares a common ancestor with Lepidosauria—the group that includes snakes, lizards, and the tuatara. The common link between all these creatures is a distant ancestor called a sauropsid, which lived around 315 million years ago.
Sauropsida eventually became synonymous with the class Reptilia, as it includes all modern and extinct reptiles. “All of these—snakes, turtles, crocodiles, birds, and the dinosaurs before them—trace back to a single common ancestor,” Stervander explains. “Since birds belong to this group, they are, by definition, reptiles.”
Why Don’t Birds Look Like Other Reptiles?
One reason birds seem so distinct from other reptiles is the mass extinction event that occurred 66 million years ago. “If that asteroid impact hadn’t wiped out large portions of the reptile lineage, we might have had more transitional species alive today,” Widrig notes. With entire branches of the Archosauria group lost, only crocodiles and a few bird species survived.
Had these intermediate species endured, the connection between birds and reptiles might be more visually apparent. Instead, birds’ closest living relatives today are crocodiles, despite their vastly different appearances.
To put this into perspective, Widrig offers an analogy: “If all non-human primates suddenly went extinct, our closest living relatives would be rodents and rabbits. While they would still be part of the same larger evolutionary group, the connection might not be immediately obvious.”
Much of our hesitation in calling birds reptiles stems from the legacy of the Linnaean system, which shaped the way people understood biological classification for centuries. “It wasn’t until the 1940s that genetic research clarified birds’ place on the evolutionary tree,” Stervander notes. “But by then, the term ‘reptile’ had been used in a different way for nearly two centuries, leading to resistance in redefining it.”
Modern science, however, continues to reshape our understanding of life’s interconnectedness—reminding us that the skies are still ruled by feathered, flying reptiles.