What are three examples of foreshadowing in the first six pages of "The Birds"?

At the beginning of the story, Nat Hocken is at work on break, watching the birds. He thinks about their feeding behavior, which he describes as "without hunger, without desire."

Perhaps, thought Nat, a message comes to the birds in autumn, like a warning. Winter is coming. Many of them will perish. And like people who, apprehensive of death before their time, drive themselves to work or folly, the birds do likewise; tomorrow we shall...

At the beginning of the story, Nat Hocken is at work on break, watching the birds. He thinks about their feeding behavior, which he describes as "without hunger, without desire."

Perhaps, thought Nat, a message comes to the birds in autumn, like a warning. Winter is coming. Many of them will perish. And like people who, apprehensive of death before their time, drive themselves to work or folly, the birds do likewise; tomorrow we shall die. The birds had been more restless than ever this fall of the year. Their agitation more remarked because the days were still.

Nat's impression of the birds foreshadows what happens later in the story. The birds on the last day of autumn are a warning to the people—many of whom will die after the birds start attacking. He even thinks that "tomorrow we will die," and that night is when the birds start aggressing the humans.

On the way home, Nat discusses the birds with Mr. Trigg, a farmer who dies later in the story. Mr. Trigg says:

“Yes,” said the farmer, “there are more birds about than usual. I have a notion the weather will change. It will be a hard winter. That’s why the birds are restless.” 

The farmer was right. That night the weather turned.

This foreshadows the upcoming problem with the birds by showing both that the birds' behavior has been changing and that there are more of them than usual. It also foreshadows Mr. Trigg's death when he discusses the hard winter. Because of the birds, it's harder than any of them expected.

Later, when he's lying in bed, Nat hears a tapping at the window and goes to investigate.

Then he heard the tapping on the windowpane. It continued until, irritated by the sound, Nat got out of bed and went to the window. He opened it; and as he did so, something brushed his hand, jabbing at his knuckles, grazing the skin. Then he saw the flutter of wings and the thing was gone again, over the roof, behind the cottage.

The aggressive action of the bird at the window foreshadows the aggressive attack of the birds later in the story. They're able to frame that night as a fluke when really it's the first volley in the upcoming attack. 

The increasing strangeness of the birds' behavior that the characters are able to explain away foreshadows the deadly events later in the story.