Yes, scientifically, birds are reptiles. Although they look and behave differently from other reptiles, modern classification based on genetics and evolutionary lineage confirms that birds descended from dinosaurs, which were themselves reptiles.
Historically, the Linnean system of classification grouped organisms based on physical characteristics. Reptiles were defined as cold-blooded and scaly, while birds were warm-blooded and feathered, placing them in a separate category. However, with the development of phylogenetics in the 1940s, scientists began to classify organisms based on their evolutionary history, rather than simply their external features.
Birds belong to the clade Neornithes, which includes all modern birds, and are directly related to theropod dinosaurs, a group of mostly bipedal carnivores within the larger clade Dinosauria. Since dinosaurs are part of the archosaurian clade, which also includes crocodiles, birds share a deep evolutionary relationship with modern reptiles. This means that, according to phylogenetic classification, birds are by definition reptiles.
Fossil evidence further blurs the distinction between birds and reptiles. Some non-avian dinosaurs were warm-blooded, and many had feathers. The 66-million-year-old “Wonderchicken” (Asteriornis), a fossil, looked almost identical to modern birds, reinforcing their connection to dinosaurs. Even before non-avian dinosaurs went extinct, the differences between scaly reptiles and birds were not as clear-cut as once thought.
The classification of birds as reptiles stems from their common ancestry within the clade Sauropsida, which includes lizards, snakes, turtles, and crocodiles. About 315 million years ago, a group of amniotes split into two evolutionary branches: one led to mammals, and the other, the Sauropsida, led to all reptiles, including dinosaurs and birds. Despite their differences, birds and crocodiles are each other’s closest living relatives, even though they look very different due to millions of years of evolution.
The reason birds look so different from other reptiles today is the mass extinction 66 million years ago, which wiped out most dinosaur species and many other reptile groups, leaving only birds and crocodiles as the survivors of their lineage. If more intermediate forms had survived, the reptilian heritage of birds would have been more apparent.
Although the traditional Linnaean system still influences how we perceive reptiles, modern phylogenetics clearly places birds within the reptile evolutionary tree. Science continues to uncover surprising evolutionary connections, making today’s skies filled with flying, feathered reptiles.
