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Why are birds considered a clade while traditional reptiles are considered a paraphyletic group?

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A diagram showing the tentative evolutionary relationships among hominid species over the past four million years, with skull images and labeled species names connected by solid and dotted lines to indicate proposed evolutionary pathways. — phylogeny Tentative phylogenetic scheme for the evolution of the human lineage. Solid bars indicate the time ranges during which species are thought to have existed, based on fossil evidence. Dotted lines signify evolutionary relationships between hominin species that have been proposed on the basis of the fossil evidence.

In biological classification, the distinction between a clade and a paraphyletic group is based on whether a taxon includes all descendants of a common ancestor.

A clade, or a monophyletic group, consists of an ancestor and all of its descendants. Because birds (class Aves) are considered a lineage that evolved within the Archosauria, they represent a complete branch of the evolutionary tree.1234

In contrast, traditional "reptiles" are considered a paraphyletic group because the classification excludes certain descendants of their common ancestor. Specifically, while birds share a common ancestor with other archosaurs like crocodiles, they have historically been excluded from the class Reptilia. By leaving out birds—which are technically a lineage of reptiles—the traditional definition of "reptiles" fails to encompass all descendants of the group's common ancestor, thereby rendering it paraphyletic.234

Modern phylogenetic systematics, or cladistics, seeks to resolve these inconsistencies by ensuring that taxonomic groups reflect true evolutionary history, which acknowledges that birds are, in a phylogenetic sense, reptiles.35