Regarding the physical difficulties of getting dromaeosaurs up a
tree, this is less problematical in the recently described smaller forms,
such as _Bambiraptor_ and the eagle-sized _Sinornithosaurus_. Even
smaller species may well have existed, and not been preserved, particularly
if their habitat was a tropical forest, and immature dromaeosaurs would
surely have been "small enough" to have jumped around in the trees if they
had wanted to, long arms, retroverted pubes, cat-like claws and all.
And the pivotal theropod ancestor of the birds need not have been a
dromaeosaur per se, but rather the most recent common ancestor of both
dromaeosaurs and early birds (such as _Archaeopteryx_).
Well, almost NOBODY claims that dromaeosaurs
are the ancestors of birds!
Sister group, yes, but not ancestors.
(I said "almost" because some recent evidence
suggests that "Dromaeosauridae" as traditionally defined is paraphyletic
with regards to birds, in which case the ancestors of birds (and thus birds
themselves) would indeed be dromaeosaurs. Will have to wait and see on