Why are you threatened by people questioning this?
Before I frame this one I want to suggest you look at the timeline of bird evolution (at least 200 mya) and dinosaur extinction (65mya).
Or had they already evolved?
Yes.
But that poses a problem. If this did occur, how did these other species survive?
In at least one mass extinction case (at the Permian-Triassic boundary about 250 mya) 90 percent of all the species on earth were wiped out, but (so far) there hasn't been a 100 percenter. Why dinos, but not birds and little insectivore mammals? I not only don't know, but don't know if anyone does. Doesn't mean it didn't happen. Not even a real problem for the idea that it did happen, just an interesting question.