Hoo boy. Here we get into the “Grades are useful” vs. “Clades are the only usable group” mess among taxonomists. Cladistically birds fall within Dinosauria, because some non-avian dinosaurs are closer to modern birds (e.g. Troodon) than others (e.g. Ornithimimus). However, if one defines “dinosaur” more like “something with diagnostic characters of Dinosauria, but not one capable of powered flight, or descended from something capable of powered flight”, then modern birds are not dinosaurs, and “dinosaur” refers to a grade. Grades may be quite useful, especially for paleontology, when in some cases the group wasn’t yet a grade.