Nothing is 100 percent in science. A prime example is Newtonian mechanics. They were thought to be as close to 100 percent as any scientific theory, yet Einstein and experimental results made with light show that it is only accurate at low speeds. Nevertheless they are good enough to get us to the moon and back safely and allow us to send space probes to Mars and beyond. Although all scientific theories are provisional and subject to falsification, the theory that organisms evolve is as close to 100% as anything in science is. And so are the facts that birds and mammals evolved from reptiles and that reptiles evolved from an amphibian and amphibians evolved from a fish.
The theory that birds descended from a dinosaur is not even close to 100%. In fact, it is a lot less than 50%. It is all but disproven and belongs in the round file (garbage can). There is a mountain of evidence that opposes the dinosaurian origin of birds.
1. Birds have fingers 2-3-4 according to embryological evidence. Theropod dinosaurs have fingers 1-2-3 according to fossil evidence, as primitive theropods have vestigial fingers 4 and 5. It is well known that something that is lost through evolution cannot re-evolve. That means theropods simply cannot be the ancestor of birds. Advanced theropods lost finger 4 so they could not have been the ancestor of birds that still have the 4th finger.
2. Theropod dinosaurs that look like birds lived 20-80 million years after the early known birds such as Archaeopteryx and Anchiornis. Since ancestors cannot live later than their descendants, it means there is zero evidence of a dinosaur that is close to the ancestor of birds in any museum collection. Unless and until a bird-like dinosaur older than the oldest known birds has been found, there is zero fossil evidence in support the claim that birds evolved from a dinosaur. In fact, some people have suggested that some dinosaurs actually evolved from birds. LOL
3. No dinosaur has ever been found with feathers. Feathered dinosaurs is the biggest hoax in the history of paleontology, worse by far than Piltdown Man. Some of the so-callled feathered dinosaurs are birds and these include Microraptor, Archaeopteryx, Anchiornis, Caudipteryx and Protarchaeopteryx. These fossils have many uniquely avian characters but no uniquely dinosaurian characters. Other so-called feathered dinosaurs such as Sinosauropteryx probably have collagen fibers preserved around their body. These fibers lack anatomical details that would support that they are homologous with avian feathers.
4.The only known reptile that has feathers that are anatomically similar to avian feathers is Longisquama insignis, which lived some 75 million years before Archaoepteryx. Stratigraphically, it is also more suitable as bird ancestor than any known dinosaur.
5. No dinosaur is known to have claws that are suitable for climbing trees. Biophysicists point out that it is implausible for flight to have evolved from the ground up. That means it is extremely unlikely for a ground dwelling dinosaur to evolve flight without first shrinking into a small, tree climbing animal. A dinosaur that tries to launch itself into the air in an attempt to fly will end up as a grease spot on the ground. A small animal that glides, such as Longisquama, can survive a failed test flight as it can glide to safety if its attempt at active flight fails.
For these and other reasons, the theory that birds evolved from a dinosaur is being abandoned by an increasing number of scientists. In fact most scientists who continue to claim that birds evolved from a dinosaur have been accused of ignoring evidence. They refuse to even cite evidence that contradicts their claim when they write up scientific papers. They are willfully blind to contradictory information in order to claim that birds evolved from a dinosaur. That is behavior that is shameful and unethical. It is behavior that we often see from creationists, not from scientists. That is why the cladists, the very people who claim that birds can only evolve from a dinosaur, have been accused of being religious by no less prestigious a scientist than Ernst Mayr.