In "The Birds," what are two rational explanations that Nat offers to explain the birds' behavior?

From the beginning of the story, Nat is baffled by the sudden change in the birds' behaviour and tries to explain it rationally. The first of these is a change in the weather which Nat takes from Mr Trigg. When the birds attack Nat's house for the first time, for example, he reiterates this explanation to his wife:

"It's the weather," repeated Nat, "I tell you, it's the weather."

As the story progresses, Nat becomes convinced the tide is also responsible for their changing behaviour. While observing the "turning tide," for example, Nat notices that there are no birds around. This idea is reinforced after the birds have attacked his house:

Then he remembered the tide. The tide would be on the turn. Maybe the lull in battle was because of the tide.

For Nat, the tide has some secret power over the birds. It controls their movements, prompts them to attack and then calls them to retreat. Nat is thus convinced that the elements (the weather and the tide) are to blame for the change in the birds' behaviour.