The actor playing Macbeth would have a dove on his shoulder as he enters to show his innocence (doves are a symbol of innocence). The bird would fly off of his shoulder right before his final speech that portrays his sinisterness.
"Sparta has no walls because you, her warriors, are her walls. Sparta’s boundaries are undefined because these your spear-points define her borders. So when the Barbarians come, let them find nothing but death on the frontiers of Sparta!