In "The Birds," why are the birds so destructive?
While it's never conclusively determined why the birds are acting strangely, the main theory mentioned in the story is a change in the wind and weather.
When the birds start attacking Nat's house, he has to fight them off. The next day, he and his wife discuss what happened. He says it must be a change in the weather that drove them down from the north. His wife points out that the weather only changed the day before, implying that they shouldn't have had time to reach Nat's home yet. She says that there's plenty of food for them in the fields, so the explanation for their attack can't be hunger.
He later tells her they went for Johnny's eyes because they were scared. He also says that it was the east wind that drove them.
Mrs. Trigg and Jim both suggest it might be the cold weather that brought the birds.
Before things get very bad, the Home Office releases a statement saying:
It is thought that the Arctic airstream, at present covering the British Isles, is causing birds to migrate south in immense numbers and that intense hunger may drive these birds to attack human beings.
As society falls apart, though, no one is entirely sure what's to blame.