I assume you are refering to the beginning of this story, when Nat observes and comments upon the vast numbers of different species of birds that act increasingly erratically. We are given disturbing description of "great flocks" of all sorts of different birds, gulls, jacdaws, starlings, sea birds and waders are all mentioned and refered to. The narrator makes the following guess about what the birds are going to do and why they are acting so strangely:
The restless urge of autumn, unsatisfying, sad, had put a spell upon them and they must flock, and wheel, and cry; they must spill themselves of motion before winter came.
Nat therefore assumes that the birds are driven into a frenzy by the approach of winter and the way that this signals a time of hardship for the birds, just as it does for humans. Nat believes that the birds are going to get rid of their frenzy through a huge burst of energy before the onset of the harsh winter.