In "The Birds," Nat keeps trying to find a rational explanation for the bird's behaviour. What is the explanation that he thinks of?

When the birds attack Nat's home, he develops a rational explanation as he tries to make sense of this violent and unexpected behaviour. As he tells his wife, Jill, he believes that a change in the weather is responsible for their behaviour. Specifically, an east wind has caused the birds to become disorientated and hungry, as he explains:

"It was the east wind brought them in. They were frightened and lost; they wanted shelter.”

Nat also believes that this east wind has brought new flocks of birds to the area which has added to the birds' desperation and confusion: 

"They aren’t the birds, maybe, from here around. They’ve been driven down from upcountry."

Nat's belief in this explanation never waivers, even though other people disagree with him. According to Mr Trigg, for example, there are rumours in town that the Russians are somehow responsible for the birds' attacks. Similarly, Mrs Trigg also suggests that the birds might be "foreign" birds which have flown down from the Arctic Circle. 
Du Maurier, however, never reveals what has caused this change in behaviour nor does she reveal the fate of Nat Hocken and his family.