How does Birnam Wood arrive at Dunsinane?

This, of course refers to the prophecy by the third apparition summoned by the witches that Macbeth can never be "vanquish'd" until "Great Birnam wood to high Dunsinane hill/Shall come against him." Macbeth takes the prophecy literally, and since it seems very unlikely that Birnam Wood, a forest, will physically march up a hill against Dunsinane (his palace) he interprets the prophecy to mean he can never be destroyed. He is shocked, then, in Act V Scene 5, when one of his messengers comes with the news that the forest is moving up the hill against him. We learned in the previous scene that Malcolm ordered his men to hew off boughs of wood to use as cover as they advanced. The marching soldiers, each carrying tree branches, look like a forest. So it turns out that the prophecy was somewhat misleading, as was the prophecy that no man "of woman born" could destroy him. Macduff, it turns out, was born by Caesarian section. So the witches' prophecies have filled Macbeth with misplaced confidence that he can carry out his schemes with impunity.