Why are birds considered a reptile lineage by many scientists?

It is a common misconception that birds arose from reptiles as a separate, distinct class; in reality, scientific evidence confirms that birds are a lineage of reptiles.12

Modern biological classification, which prioritizes phylogeny—the branching evolutionary history of organisms—demonstrates that birds evolved from a group of dinosaurs known as theropods. Because dinosaurs themselves are a branch of the Reptilia class, birds are technically classified as dinosaurs and, by extension, reptiles.132

The classification of birds as reptiles is supported by several key factors:
* Common Ancestry: Birds share a common ancestor with crocodiles within the subclass Archosauria.2
* Evolutionary Descent: The transition from theropod dinosaurs to birds involved the development of feathers and other specialized structures, marking birds as a specialized branch within the reptilian family tree.3
* Shared Biological Traits: Like other reptiles, birds possess a cloaca and internal testes.3

While birds are treated as a separate class (Aves) for traditional and practical purposes, they are phylogenetically nested within the reptile lineage.12