Question

some bird are sitting__a tree​

Answer 1

Answer:

Some birds are sitting on a tree

Thanks

Answer 2

Answer:

Your case is special. If you were to say two people were "sitting on a tree" in this case, it would imply that the tree was on the ground—i.e., that it had fallen or been cut down. Sitting "in" a tree means sitting in among the branches, most likely at least partly hidden from view.