This is a mystery that is never fully solved in the story. What Nat and others assume at the beginning, before the bird attacks become too serious, is that the birds are acting in such strange ways because of the harshness and severity of the winter, which is making them come inland and attack humans in scattered, isolated incidents, because of lack of food. However, when it becomes clear that something much more sinister is going on, and that the birds are turning themselves into killing machines intent on the destruction of the human species, this reason becomes null and void. The only suggestion that is made in the text is in the last page of the story, where Nat talks about the birds and their capacity to remember:

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.

This suggests that there has been some sort of latent evil or hatred of man that exists deep within birds and has only now found its full expression. The way Nat talks of "millions of years" hints at a kind of primeval antagonism between the two species that now makes them act as they do. They are hardwired to execute humans, but only now has this become apparent. In a sense, the story becomes more terrifying with this lack of certainty about the reason behind the behaviour of the birds.

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.

In "The Birds" by Daphne Du Maurier, the main character theorizes that the birds' destructive behavior is a consequence of a sudden change in weather, a "cold snap." The story is set in England, and the British Broadcasting Corporation's (BBC) news reports confer with the main character's speculation that the birds' sudden aggression is likely a result of the unnatural weather. This makes a great deal of sense considering the story begins by stating that "On December third, the wind changed overnight and it was winter."

When people hear of "The Birds," they most often associate it with Alfred Hitchcock's 1963 classic film The Birds, which is an adaptation based loosely on Du Maurier's 1952 story. In the film, several explanations are offered as to why the birds act so strangely, including the weather, but no one answer is settled upon as it is in the original story by Du Maurier. At one point in the film, a self-proclaimed expert on bird behavior even dismisses all the claims of the attacks, saying there is no logical, natural reason the birds would collaborate to injure or kill humans.