A Fundamental Error

A fundamental error of the dino-to-bird idea, is thinking that the feathered creatures of the Late Jurassic and the Cretaceous had evolved from dinosaurs. They had not. They were primitive birds that had developed from the pterosaurs.
Flying (and secondarily flightless) birds already existed at the time of the very first dinosaurs. In fact, primitive birds may have even pre-dated the appearance of the dinosaurs.