In "The Birds," du Maurier creates a mood of mounting tension and suspense. From the very opening, the readers get a sense of foreboding:
On December the third, the wind changed overnight, and it was winter. Until then the autumn had been mellow, soft. The leaves had lingered on the trees, golden-red, and the hedgerows were still green. The earth was rich where the plow had turned it.
Although the imagery given is of "mellow, soft" autumn, the first sentence primes the reader for change; the better autumn sounds, the worse the expectation for winter becomes. Descriptions of the cold winter weather shape the atmosphere of the story, too; the mood chills as du Maurier describes cold drafts. The subsequent description of the "restless, uneasy" birds creates a matching discomfort in the reader, a vague uneasiness: something is perhaps not right, but Nat and the reader are unsure what it is. As the strange behavior of the birds escalates, dread builds. The suspense climaxes in sheer terror with the birds' direct onslaught on the cottage. From then on, the mood is one of increasing hopelessness: the military aircraft falls; the neighbors are all dead; the radio goes silent. At the very end of the story, the terror of uncertainty is replaced by the terror of certainty: Nat and his family will die, and there is nothing they can do about it.