It seems to be an accepted fact that Theropod dinosaurs evolved into present-day birds, but I am still leery of this theory. Can biologists and paleontologists really compare these distinct animals and conclusively show that they are "one-in-the-same" creature, based upon certain morphological similarities? Is this idea "set in stone?"
The 'smoking gun' evidence linking birds and dinosaurs is the evolution of feathers. Feathers are a feature that set the birds apart morphologically from the rest of the tetrapods, and indeed from the rest of all animals. All birds have feathers and nothing that isn't a bird has feathers. So a logical place to start looking for the origin of birds is the origin of feathers.
Since the dawn of modern paleontology, there had always suspected links between dinosaurs and birds due to the fossils of Archaeopteryx found in the 1860s. Thomas Huxley claimed the two groups were related as early as 1863. But it wasn't until recent decades that the fossil record started being filled in with definitive links between Archaeopteryx and therapod dinosaurs, such as more species intermediate between birds and therapods and fossil impressions indicating that some therapods had feathers.
The rest is just filled in by logical deduction, or Occam's Razor. There is clear evidence that all birds have feathers, and no other animals alive today have feathers. There is now clear evidence that therapods had very similar skeletons to birds with feathers about 150 million years ago. There is also evidence that many therapods had feathers, a trait otherwise confined to birds.
Let our null hypothesis be that the origin of birds is unknown, and our alternative hypothesis be that birds evolved from dinosaurs. Given the similarities noted above (and, keep in mind, the evidence is much stronger for scientists who specialize in this area), do you think there is enough evidence to reject the null hypothesis and conclude that birds evolved from dinosaurs? If not, are there any plausible alternative hypotheses?
The great thing about science is that you never have to take someone else's word for it. Everyone has the right to be a skeptic, just keep an open mind and look at the evidence and you can draw your own conclusions.
One factor that helps you survive mass extinctions is small size becasue there are more of you and you can repopulate faster. Early birds are among the smallest dinosaurs.
Other indicators of their relationship besides feathers which Kingledon covers are eggshells, dinosaurs and birds both have hard shelled eggs with identical microstructure. most dinosaurs have round spherical eggs but theropods have asymmetric elongate eggs just like birds.
The shape of the hand and forearm of early birds and maniraptoran dinosaurs are almost identical. both theropods and aves also have air-sac based breathing. Honestly in anatomy maniraptoran dinosaurs and early birds blur together so seamlessly there really is no compulsion other than a direct relationship.
what throws most people off is the tail, but early birds have long tails (and teeth). we see a similar evolutionary path in pterosaurs, flying vertebrates start with long tails for passive stability but as they become better at flying shorten it to gain maneuverability and use better active control for stability.
I was hoping to post this comparison of the skeletons of late maniraptorans and early birds done by Scott Hartman but the resolution sucks. Hopefully you can zoom in and see how seamlessly one blends into the other.