The tone of "The Birds" is one of fear and foreboding. It takes on its protagonist, Nat's, voice, especially in moments of unease, urgency, or frustration. Both Nat's thoughts and the narration of the story are contemplative, prone to posing rhetorical questions that work to build suspense. For example, as Nat watches the birds out at sea, while not overtly indicating that it's something Nat is thinking, du Maurier asks, "Where, and to what purpose?" This slightly ominous curiosity progresses to bewilderment and helplessness later, like in the following passage:
He wondered if he should go to the call box by the bus stop and ring up the police. Yet what could they do? What could anyone do?
Descriptions of the atmosphere become more and more hostile as Nat reads threats in everything: the sky becomes "sullen, heavy"; the sea is "vicious." The scariest moments are conveyed through flat observation, as if Nat doesn't want to linger on them in his thoughts, leaving the horror to strike the reader plainly.
As the action progresses, the tone is urgent and grim:
He got everything inside the cottage. It could be sorted later. Give them all something to do during the long hours ahead. First he must see to the windows and the doors.
Here, when Nat is focused on survival, the sentences are short and the tone becomes brusque and pointed. By the end of the story, however, even as Nat is making further plans to fortify the cottage, there is an underlying despair, and his urgency feels hollow. As he listens to the wood splinter beneath the birds' beaks, the tone is one of hopelessness, as all of Nat's gestures amount to little more than an empty search for comfort, like switching on a silent radio.