The birds that form such an implacable enemy to Nat and his family are personified throughout this terrifying short story, but you might like to consider the following examples of adjectives that are applied to the birds that reveal what a powerful and violent force they are. For me, one of the key passages in this story comes at the beginning, when the narrator describes the huge flocks of birds that Nat loves to look at so much who do not migrate and stay in Britain for the winter. Note the way that they are described and the kind of adjectives that are used to present them:
Great flocks of them came to the peninsula, restless, uneasy, spending themsleves in motion; now wheeling, circling in the sky, now settling to feed on the rich, new-turned soil; but even when they fed, it was as though they did so without hunger, without desire. Restlessness drove them to the skies again.
Consider how the use of such phrases as "without hunger, without desire" and words such as "restlessness" present the huge flocks of birds as being incredibly dangerous and inexorable in pursuing their aims. Here, through this description, we see the birds personified as potentially very dangerous enemies indeed, and possessing the mentality that would make them incredibly difficult to beat.