Technically, birds are indeed reptiles. Birds descended from the very first reptile. Birds are also dinosaurs because they descended from a dinosaur. This can all be a little confusing. Paleontologists (people who study all the different animals that have ever lived on Earth) call T Rex and others like him "non-avian dinosaurs" or "non-bird dinosaurs" because technically, if you say just the word dinosaur, you are including birds.
That depends upon how you define "reptiles", but the short answer is yes - dinosaurs are reptiles, and birds are dinosaurs, and therefore birds are reptiles.
The problem comes in with the old definition of reptiles, which many still use, that defines reptiles as vertebrates that have developed the amniotic egg (an egg with a membrane enclosing the embryo inside of a hard shell), but which are not warm-blooded. Thus, by this definition, birds and mammals both are descended from reptiles, and the common ancestor of birds and mammals was a reptile, but they themselves are not. Since then, a new definition has been created by which reptiles are defined by a hole in the bone of the skull for air passages and muscle attachments, as well as by other attributes that do not fossilize, such as four-color vision. By this definition, birds have all of the attributes that would make them reptiles, despite that they are warm-blooded.
Mammals, however, did not have this hole in the skull at any time in their evolution, and no mammal has four-color vision (even primates like us have only three), so by this new definition, mammals are NOT descended from reptiles.
Which of these definitions is better? There is no simple answer, and it depends upon whether the descendants of an animal living in the past are of the same group as their ancestor. Some biologists and paleontologists see this as necessary. Others do not. Myself I think this is a silly argument, provided that we understand the true relationships between groups. In this case, birds absolutely do have evolutionary adaptations that unite them with reptiles (and some reptiles more than others), but because they are warm-blooded they are in many ways unlike more conventional reptiles such as lizards or turtles.
Yes, birds are reptiles. We know that birds have a very similar skeleton to some kinds of dinosaurs, which are reptiles. We even know from fossils that some dinosaurs have long arms like birds, long beaks (with teeth), and feathers. So, when people say that dinosaurs are extinct, they are wrong - you see living dinosaurs flying all over your neighborhood every day!
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