Birds evolved from a reptile, but they are not reptiles. Mammals also evolved from reptiles. If people want to call birds reptiles because their ancestor is a reptile, then we should also call mammals reptiles since mammals evolved from a therapsid reptile. Birds evolved from an archosaurian reptile, most likely Longisquama insignis, a small tree climbing reptile with feathers. No dinosaur has ever been found with feathers. The so-called feathered dinosaurs are the biggest hoax in the history of science. They are either birds or they are dinosaurs with collagen fibers preserved around their skin. Some of these nut cases who call birds reptiles also call Archaeopteryx a "feathered dinosaur" and claimed that it was NOT a bird because their inane cladograms told them Archaeopteryx is a dinosaur, not a bird. LOL
BTW, people who call birds reptiles also call therapsid reptiles and anapsid reptiles "stem mammals," and exclude them from class Reptilia. That is ludicrous, because most therapsid reptiles have nothing to do with mammals, since most of them are evolutionary dead ends, becoming extinct without leaving any living descendants. Only a single branch of synapsid reptiles led to the therapsid reptiles, and only a single branch of therapsid reptiles led to mammals. calling all the other branches of synapsid and therapsid reptiles "stem-mammals" would be similar to calling all archosaurian reptiles birds, and calling crocodiles and alligators "stem-birds." Not only are some scientists engaging in hoaxes, they also look silly when they call birds reptiles.