"Having said this, the clerk experienced a moment of extraordinary relief- a treeful of birds broke into song: but the song was silenced when Morris, his eyes heavy, said, "This I already know, you don't tell me nothing new."
This phrase is made in the middle of chapter eight, right after Frank Alpine confesses to Martin Bober his role in the robbery. For much of the book, Frank has been longing to confess his role in the robbery to Morris, but he has been afraid. Now when he does it, he hears a "treeful of birds" break into song. The imagery of the birds relates back to Frank's obsession with the figure of Saint Francis of Assisi. Saint Francis often spoke to birds and preached to them. The sound of singing birds signifies the goodness in the act of confession that Frank has just undertaken. His ability to hear birds signifies is transformation into a figure that increasingly resembles Saint Francis.