I think that du Maurier is trying to convey a sense of doom with the destruction at the Triggs' farm. For Nat to see the Triggs' bodies is a reflection of his worst fears. At the same time, he sees the other houses around the farm without chimney smoke. This indicates that the birds have wreaked havoc on these families. The fact that the Triggs doubted Nat's version of what the birds have been doing also adds to the foreshadowing element. The fact that human life is becoming extinct around him helps to foreshadow Nat's own precarious position. It is here where I think that du Maurier creates a condition in which Nat is able to understand the full implications of what he is going to experience. The foreshadowing element brings about a sense of inevitable doom and destruction that he and his family will endure. The feeling that arises is that he and his family are next. This becomes the dominant feeling that is illuminated out of the situation with the birds' destruction and how Nat perceives it. The ending that is developed is one in which Nat and his family do all they can, with the inevitable mass of birds descending on their home in attack mode. This helps to bring out the sense of helplessness and destruction that Nat and his family are about the endure.