What It Is
Birnam Wood, the promise the third apparition makes to Macbeth: he will never be defeated unless the Great Birnam Wood marches against him up to the hill of Dunsinane. The play treats it as an impossibility that ends up coming true to the letter, through the hands of soldiers who carry branches.
Where It Appears
In Scene 1 of Act IV, the third apparition fixes the condition, and Macbeth receives it as a guarantee: who could recruit the forest, or order the trees to pull their roots out of the ground?
Third apparition:
Macbeth shall never vanquished be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him.
In Scene 2 of Act V, the Scottish nobles march to Birnam to meet Malcolm's army. In Scene 3 of Act V, Macbeth scorns the barons' flight and says he will not fear death or ruin while Birnam Wood does not reach Dunsinane.
In Scene 4 of Act V, before the wood, Malcolm orders each soldier to cut a branch and carry it in front of him, to hide the number of the troop.
Malcolm:
Let every soldier hew him down a bough / And bear 't before him. Thereby shall we shadow / The numbers of our host and make discovery / Err in report of us.
In Scene 5 of Act V, a Messenger swears that, from the hill, he saw the wood begin to move. Macbeth, shaken, says he is beginning to suspect how the devil plays with words, lying as if he spoke truths, and runs to arms. In Scene 6 of Act V, the army drops the branches on the plain before the castle. And in Scene 7 of Act V, even with the wood already arrived at Dunsinane, Macbeth says he will still cross swords with Macduff.
What It Means
The marching wood fulfills the prophecy to the letter and undoes it in meaning. In Scene 1 of Act IV it sounds to Macbeth like an impossibility, and that is why it comforts him. In Scene 4 of Act V it stops being a prodigy and becomes a stratagem: there is no miracle, only soldiers carrying branches for a practical reason. The passage from one thing to the other is what Scene 5 of Act V records, when he begins to suspect the words that deceived him.
The wood is the symbol of the wrong reading more than of the prophecy. The promise had nothing supernatural in itself, and Macbeth reads it as a guarantee because he wants to. The play puts the difference on stage: on one side, the witches' words, and on the other, Malcolm giving a field order that, without mentioning any prophecy, fulfills it.
Relationships
- Standard themes: Fate and Free Will, the Supernatural, Power.
- Characters: Macbeth, Malcolm, the Witches, who make the promise, and Macduff, who fulfills the other half of it.