When the story begins, the mood is calm and serene. This is created by the description of Nat Hocken, sitting on the edge of a cliff and enjoying nature as he eats his lunch. The mood quickly alters, however, when Hocken observes the sudden changes in the behaviour of the local birds. Words like "restless" and "uneasy" signal the move to a tense and anxious mood.
This continues as the story develops and the birds begin to attack the humans. This is reinforced by the image of the birds "tapping on the window" when they attack Nat's house, for example, and the fluttering of their wings as they peck at his body.
As the characters try to make sense of what is happening, the mood also becomes fearful. This is shown most clearly through the reactions of Nat's family, especially his wife, Jill, who is terrified of the birds and in constant need of reassurance. At several points in the story, she asks Nat what he will do and what will happen next, for instance, as she tries to make sense of what is happening outside of her home.