du Maurier develops Nat as focused and precise.  Though no one would ever wish to be in the situation in which he is placed, Nat is one of the best equipped to handle such a crisis.  Nat is focused on taking care of what he can to protect his family and allay their fears.  He is consumed with doing everything he can to help them avert the disaster that the birds wish to perpetrate upon the humans. When he puts his head down to sleep, Nat's dream speaks to his driven condition:

He lay down on his mattress and closed his eyes. He slept at once. He dreamt uneasily, because through his dreams there ran a thread of something forgotten. Some piece of work, neglected, that he should have done. Some precaution that he had known well but had not taken, and he could not put a name to it in his dreams. It was connected in some way with the burning aircraft and the stack upon the hill. He went on sleeping, though; he did not awake. It was his wife shaking his shoulder that awoke him finally.

Nat's dream foreshadows his impending siege that the birds launch against his home and his family.  The "burning aircraft" is symbolic of the fire that he needs to keep going so that the birds do not come down the chimney as a point of entry.  At the same time, the "stack" refers to the bodies of the dead birds, and could very well foreshadow the bodies of humans that have fallen prey to the birds. Nat's dream foreshadows the absolute difficulty that the humans face against this particular adversary.  When his wife wakes him up, it becomes clear to him what he had failed to do: "He had forgotten to make up the fire."