I think it's gone past hypothesis to consensus.
yeah, it is still a hypothesis but it seems to have growing evidence and support.
So a friend of mine found a dissenting opinion expressed by a professor emeritus of zoology at a West coast university. This friend had expressed doubts that birds were avian dinosaurs, wrote to this professor, expressing his doubts, and received this reply. I can only note the dissent; I'm hardly in a position to judge one way or another:
"Unfortunately, museum politics, careers, and public sentiment have surely played major roles in the perpetuation of the dubious proposal that today’s birds are merely dinosaurs that flew off into the Cretaceous sunset. That notion is, understandably, popular with the lay public. Not surprisingly, museums were, and remain, quick to invest millions of dollars on splashy public displays consistent with that scenario. In the process, many paleontologists have serious career investments in the "birds-are-dinosaurs” story.
However, as you suspect, there is increasing evidence that current conventional wisdom regarding dinosaurs and birds may well be inaccurate: (1) the most recent and complete cladogram analyzing bird-dinosaur relationships indicates that birds are just as likely to been derived from early non-dinosaurian archosaurs; (2) the “dinosaurs” from which birds were supposedly derived (the “raptors”) may actually have been secondarily flightless birds!; (3) so-called feathers, and/or
"proto-feathers” in many dinosaurs (e.g., Sinosauropteryx) were very likely to actually have been misidentified, sub-cutaneous collagen fibers; (4) the old problem of birds having fingers 2-3-4 as opposed to the 1-2-3 fingers in dinosaurs remains….that is, dinosaurs with fingers 1-2-3 are hardly good ancestors for birds, all of which retain fingers 2-3-4. Weak, "just-so" stories to account for those differences have been proposed to account for the finger differences, but that problem remains.
You might ask, why have these inconsistencies with the "birds-are-dinosaurs" scenario not received more public attention? Simply put, they have been systematically ignored by dinosaur “experts,” and, in turn, by the popular press. Nevertheless, all of the points I’ve made in the previous paragraph have been put forth in heavily peer-reviewed, scholarly papers that appeared in highly prestigious scientific journals (SCIENCE, NATURE, etc.). Perhaps the best single source for you to more closely review all of these issues would be Dr. Alan Feduccia’s recent book, “Riddle of the Feathered Dragons,” Yale University Press (ISBN 978-0-16435-0)."