The birds are attacking because they had a "leader bird" that was immortal. However, just because its lifespan is supposed to be forever, that doesn't mean it can't die from a bullet or a disease. That's what happened to the leader bird. A human came along and shot it, not knowing what it was. After the leader bird died, everything was chaotic. The birds wanted revenge.
One gain Nat has is, he realizes that nothing he can do will stop the birds. The only way they will stop is if they die trying to break in. Unfortunately, Nat does face a setback. Jill is outside when the tide is out and loses track of time. The tide and the birds come back in and start to attack her. She is smart enough to cover her eyes with her arms but her arms get pecked really badly.
It has been weeks, and the birds are still attacking. They haven't slowed down even the slightest bit. Actually, there seems to be more birds now than there was before for reasons Nat doesn't want to think about. While Nat is tending to the tea, his mind wanders. He thinks back to all the times he has ever seen a bird, ever looked into its eyes before all of this went down.