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Why are birds considered a clade while reptiles can be considered a paraphyletic group?

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Illustration showing examples of vertebrates: a turtle for reptiles, a songbird for birds, a bluegill fish, a salamander for amphibians, and two mammals-a whale and a groundhog. — Vertebrate The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals.

In biological classification, a clade is defined as a group that includes a common ancestor and all of its descendants. Conversely, a paraphyletic group—often referred to as a grade—is one that includes a common ancestor but excludes some of its descendants.123

The distinction between birds and reptiles in this context arises from their evolutionary history:

Birds as a Clade: Birds (Aves) are considered a clade because they represent a complete lineage of descendants originating from a common ancestor.2
Reptiles as a Paraphyletic Group: The traditional class Reptilia is considered paraphyletic because it excludes certain descendants of the common reptilian ancestor. Specifically, scientific evidence confirms that birds evolved from within the archosaurian lineage of reptiles. Because the traditional definition of "reptiles" does not include birds—despite birds being modified dinosaurs—the group fails to encompass all descendants of the common ancestor, rendering it paraphyletic.24

Consequently, modern phylogenetic classification often recognizes that birds are, in fact, reptiles, even though the term "reptile" is still frequently used in a traditional, paraphyletic sense.42