Cladogram of Mesozoic Bird Relationships

A list of birds with approximate dates they arose in the fossil record. An excerpt from The Evolutionary History of Modern Birds edited by Dr. Gareth Dyke, Gary Kaiser

(Source)

Birds were living before and after the Cretaceous theropod dinosaur. Dinosaurs are not necessary to explain the evolution of birds.

See the fossil record for (true) birds in red and (true) dinosaurs in green. There is no need to "prove" an intermediate between dinosaur and bird. The only "intermediate" is to demonstrate gradual evolutionary transition from Archosaurs and modern birds to confirm Darwin's theory.. and the oldest known, Archeopteryx did share reptilian features but was in fact, a bird, in the modern sense. How this argument became corrupted with
1) the need to prove Archeopteryx was a cold-blooded reptile, or
2) that he branched off from dinosaurs in the Cretaceous (100 million years after he lived) is evidence of all the radicalism that surrounds the issue.

There is no definitive way to know with certainty whether or not extinct and fossilized remains were, with all certainty, "warm-blooded" or "cold-blooded." There are merely theories and hypotheses. Nothing more. Most of which, do nothing but cloud the issue of what is known with certainty about the origins of birds based on evidence.