Birds, they evolved from dinosaurs didn't they but that's a point subject to subject to controversy? No on both those counts, yes the evolution of birds is somewhat controversial but neither of the prominent theories state that birds evolved from dinosaurs. What one of those theories postulates is, is that birds are dinosaurs, the other competing theory is that birds evolved from a creature a bit like a pterodactyl. Not an actual pterodactyl because they'd have to grow their fingers back, but a critter somewhat similar that still had all the necessary digits.
But but but, pterodactyls are dinosaurs, I hear you say, and you know to me they are but that's not what the palaeontologists say and they're the ones who get to define such terms. Oh no, dinosaurs are a specific group of extinct or mostly extinct, depending on which theory you endorse, creatures. The thing is though, there are two major classifications for dinosaurs, ornithischian and saurischian, those are just fancy terms for bird-hipped and lizard-hipped respectively. Most dinosaurs fall into the second category, lizard hipped. Now here's where things start to get ironic, guess which group that, according to one of the theories, birds fall in. That would be, lizard-hipped, it's particularly ironic because the categories allude to the similarity in the structure of pelvic bones. So birds are dinosaurs excluded from a category, that was named after a similarity to birds, right that makes sense!
Now at one time, these categories, ornithischian and saurischian where somewhat deprecated within palaeontological circles but it's interesting to note that recently, so recently it's happened within the last couple of years, that these categories have made a bit of a comeback. Why that should be, I wouldn't know, i only take a passing interest in such matters and as for which theory I endorse, my bet goes on the birds are dinosaurs theory.