Shakespeare uses an extended bird metaphor throughout Act IV, scene two in Macbeth. The comparison of man to bird begins immediately as the scene opens and Lady Macduff despairs that her husband's "flight was madness" (IV.2.4).
She goes on to explain to Ross how parents should defend their children, using a wren as a metaphor and example:
From whence himself does fly? He loves us not;
He wants the natural touch for the poor wren,
The most diminuitive of birds, will fight,
Her young ones in her nest, against the owl. (IV.2.11-14)
In Lady Macduff's mind, her husband has acted against nature. She feels his abandonment keenly. Her use of the wren metaphor, however, also suggests that she identifies herself to the wren, "the most diminuitive birds," suggesting that she will not run away as her husband has.
Shakespeare's use of the extended metaphor continues as the son enters the conversation. When his mother asks him how he will survive without his father, he responds "As birds do, Mother" (IV.2.37). This comment opens their discussion into more word-play about birds and survival.