Eleanor Catton cites Macbeth as a major influence on Birnam Wood, and this influence is most apparent in her thematic treatment of ambition and subsequent betrayals Just as Macbeth betrays and murders everyone between him and the throne, each character in Birnam Wood betrays either another person or their own ideals on their ambitious quest for their version of power.

Catton uses this theme to cast a comparison between Shelley and Lady Darvish. Shelley’s declaration that she will sleep with Tony and Lady Darvish’s cozy confiding are equally self-serving, and Catton presents their ambition as the original sin. Shelley and Lady Darvish both betray another person to gain what they see as position and power. For Shelley, it begins with wanting to leave Birnam Wood, but actualizes in her desire for power and position over Mira, who has always eclipsed her. Lady Darvish’s frustrated revelation of the farm’s sale when her friends don’t properly acknowledge her husband’s knighthood reveals that her desire is essentially the same as Shelley’s: Both are concerned with how they’re seen, and both take steps to ensure that they get the respect they want.

Likewise, Catton compares Tony and Sir Owen’s ambition. Tony lets his ambitions of being a lauded journalist cloud his otherwise strict morality.