It is impossible for birds to have evolved from theropod dinosaurs, and there is no mechanism to support such an illusory claim.
The theory of evolution claims that birds evolved from a small, carnivorous reptile known as the theropod dinosaur. In fact, however, a comparison of birds and reptiles shows that these classes are very different from one another and that no evolution can have taken place between them. (See The Origin of Birds.)
An examination of the anatomies and fossil records of birds and reptiles also shows no evidence that evolution ever happened. In an article titled "Demise of the ‘Birds Are Dinosaurs' Theory," the American biologist Richard L. Deem writes:
The results of the recent studies show that the hands of the theropod dinosaurs are derived from digits I, II, and III, whereas the wings of birds, although they look alike in terms of structure, are derived from digits II, III, and IV . . . The second study shows that the theropod dinosaurs did not possess the correct skeletal structure or lung structure to have evolved into birds. The evolution of theropods into birds would have required the introduction of a serious handicap (a hole in their diaphragm), which would have severely limited their ability to breathe. As Dr. Ruben said, such a debilitating mutation "seems unlikely to have been of any selective advantage."246
There are other problems regarding the "Birds Are Dinosaurs" theory. In comparison with Archaeopteryx, theropods' front legs are very small in relation to their bodies. (See Archaeopteryx.) Bearing in mind the body weight of these animals, the development of any proto-wing appears impossible. The majority of theropod dinosaurs have no semilunatic wrist bone (which is found in birds), and possess other wrist components that are absent in Archaeopteryx. In all theropods, the VI nerves leave the skull from the side, together with various other nerves. In birds, however, the same nerves leave the skull through a hole, which is unique to them, in the front of the skull. Another problem is that a great many theropods emerged after Archaeopteryx. 247
Another major distinguishing feature between theropod dinosaurs and birds is the structure of these dinosaurs' hip bones. Dinosaurs are divided into two kinds, depending on their hip bone structure: Saurischian (with reptile-like hip bones) and Ornithischian (with bird-like hip bones). In members of the Ornithischian group, the hip bones really do closely resemble those of birds, hence their name. However, in other respects they bear no resemblance to birds whatsoever. For that reason, evolutionists are forced to regard Saurischian dinosaurs (those with reptile-like hip bones), which include the theropods, as the ancestors of birds. Yet as can be seen from their description, the hip bone structure in these dinosaurs bears absolutely no resemblance to that in birds. 248
In short, it is impossible for birds to have evolved from theropod dinosaurs, because no mechanism exists that could possibly overcome the enormous differences between the two classes.
246. Richard L. Deem, "Demise of the ‘Birds are Dinosaurs' Theory," http://www.direct.ca/trinity/dinobird.html
248. Duane T. Gish, Dinosaurs by Design, Master Books, AR, 1996, pp. 64-65.