In this simile, Nat compares the experience of the birds' attacks to the experience of air raids during the war:
It was, Nat thought, like air raids in the war. No one down this end of the country knew what the Plymouth folk had seen and suffered. You had to endure something yourself before it touched you.
Nat encounters Jim, the cowman, and Mrs. Trigg in the morning after killing roughly 50 birds in his children's bedroom. Neither of them really believe him—Jim remarks, "Never heard of birds acting savage." The birds' attack is unreasonable and outside the bounds of daily life. As such, it is unimaginable for those who didn't directly witness it. Nat's instinctive comparison is to the air raids that devastated the British countryside, particularly the area of Plymouth, where, as the reader learns later, Nat's mother lived. Nat is repeatedly reminded of the time during the Blitz. Through this simile, du Maurier suggests that the senseless, absurd violence of the birds is not actually so unbelievable in a world where people are capable of destroying one another on such a massive scale. In fact, in some ways, the birds' senseless violence is a logical progression from the example set by humanity.
In the following passage, Nat uses a simile that compares the birds to people, thus attempting to rationalize their behavior:
“Perhaps,” thought Nat, munching his pasty by the cliff’s edge, “a message comes to the birds in autumn, like a warning. Winter is coming. Many of them perish. And like people who, apprehensive of death before their time, drive themselves to work or folly, the birds do likewise.”
Nat does this several times throughout the start of the story, trying to find equivalent human explanations for the birds' restlessness. He conjectures that maybe the birds are anxious in an attempt to distract themselves from winter—a thought that tries to rationalize their behavior, perhaps because not understanding why the birds are acting as they are is what terrifies him most.
Similarly, when a bird tries to get into Nat's bedroom window that night, he attempts to explain it to himself:
Frightened, he supposed, and bewildered, the bird, seeking shelter, had stabbed at him in the darkness.
Here, he projects human emotions on to the birds. As the birds become more and more dangerous, the comparison between birds and people takes on a dual aspect. In this case, the birds are frightening because they act more like people than they ought to, full of seemingly human malice rather than more animal emotions like fright and bewilderment. And yet, they are frightening precisely because they are not people; their motives are not understood. They might behave like people, but the simile is ultimately inaccurate; they are completely other than people.