According to this reference, for a long time, folks thought that birds evolved from "pseudosuchian archosaurs". Then that fell from favour and was replaced by a purported "direct derivation of birds from theropod dinosaurs".
When that was shown to be untenable, some folks moved to the idea that birds evolved from crocodile type ancestors. (Yes, crocodiles).
But then that idea fell from favour and the folks moved to some purported vague dinosaur ancestry. But they do not give any specifics so that it can never be evaluated.
Perhaps someone could do us all a favour and tell us what the current thinking is about the purported dino to bird lineage.
I have proposed a lineage. What is the alternative?