What does Nat forget to get at the farm?

Nat has turned his little cottage into a fortress to keep the marauding birds at bay. But the little blighters just won't give up. The seagulls relentlessly thud against the windows and doors in suicidal acts of dive-bombing, seemingly hell-bent on destroying Nat's family and all mankind. Nat has a plan: to fix barbed-wire to the window-boards. The problem is that he'll have to do it in the dark, during a lull in the birds' attacks.
In the meantime, the situation's getting really desperate, with hawks gradually breaking holes in the door with their stabbing beaks. In such a tense situation, Nat feels for a cigarette. It's the last one, as he forgot to bring a full pack back from the farm. Although, in fairness to Nat, he did have rather more pressing concerns on his mind.