High Quality Content by WIKIPEDIA articles! Birds are generally believed to have evolved from certain feathered theropod dinosaurs, and there is no real dividing line between birds and dinosaurs, except of course that some of the former survived the Cretaceous-Tertiary extinction event while the latter did not. For the purposes of this article, a `bird` is considered to be any member of the clade Aves sensu lato. Some dinosaur groups which may or may not be true birds are listed below under `Proto-birds`.