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.”

Du Maurier gives human emotions to the birds:

“There are birds in there,” he said, “dead birds, nearly fifty of them. Robins, wrens, all the little birds from hereabouts. It’s as though a madness seized them, with the east wind.”

...“Yes, they’ve all gone now,” said Nat. “It was the east wind brought them in. They were frightened and lost; they wanted shelter.”

...“Had any trouble with the birds?” asked Nat.

“Birds? What birds?”

“We got them up our place last night. Scores of them, came in the children’s bedroom. Quite savage they were.”

As the story progresses, the bird take on not only human qualities, but even superhuman and supernatural qualities:

Then he saw them. The gulls. Out there, riding the seas. What he had thought at first to be the white caps of the waves were gulls. Hundreds, thousands, tens of thousands . . . They rose and fell in the trough of the seas, heads to the wind, like a mighty fleet at anchor, waiting on the tide. To eastward and to the west, the gulls were there. They stretched as far as his eye could reach, in close formation, line upon line.

The birds start to behave like an army, like an air force:

Something black rose from behind them, like a smudge at first, then widening, becoming deeper, and the smudge became a cloud, and the cloud divided again into five other clouds, spreading north, east, south, and west, and they were not clouds at all; they were birds.

...The gulls were copying the rooks and crows. They were spreading out in formation across the sky. They headed, in bands of thousands, to the four compass points.

The birds surpass the attributes of humans and become a force of nature that seems to come and go with the tide:

He glanced at his watch. Nearly eight o’clock. It must have gone high water an hour ago. That explained the lull: The birds attacked with the flood tide. It might not work that way inland, upcountry, but it seemed as if it was so this way on the coast. He reckoned the time limit in his head. They had six hours to go without attack. When the tide turned again, around one-twenty in the morning, the birds would come back...

In time, even the smallest of the birds become superhuman machines which Nat, his family, and other humans are unable to overcome:

The smaller birds were at the window now. He recognized the light tap-tapping of their beaks and the soft brush of their wings. The hawks ignored the windows. They concentrated their attack upon the door. Nat listened to the tearing sound of splintering wood and wondered how many million years of memory were stored in those little brains, behind the stabbing beaks, the piercing eyes, now giving them this instinct to destroy mankind with all the deft precision of machines.