the mood of bleak uncertainty that has been growing in the story, leaving us pessimistic about the little family's fate. But apart from the conventions of the horrorgenre, which generally demand a tragic ending, there are three other reasons I think Nate and his family will not survive the prevailing conditions in "The Birds."

One, as you rightly noted, is the foreshadowing of human extinction in the narrative. By day 3 of the bird-attack, the Triggs have met a ghastly end as has the postman and also, it is suggested, the families living in the "council flats." The birds have crashed rescue airplanes and even the wireless has gone silent. As humans fade, the birds have only been increasing in number and power. They also seem to be evolving rapidly in intelligence, suggesting they will outmaneuver Nate soon:

The smaller birds were at the window now. He recognized the light tap-tapping of their beaks and the soft brush of their wings. The hawks ignored the windows. They concentrated their attack upon the door. Nat listened to the tearing sound of splintering wood and wondered how many million years of memory were stored in those little brains, behind the stabbing beaks, the piercing eyes, now giving them this instinct to destroy mankind with all the deft precision of machines.

Two is the story's implicit theme of the dangers of climate change. Time and again people in the story postulate that the change in the behavior of the birds is linked with "change" in the "arctic circle." Something has happened which has changed nature as represented by the birds from a nurturing entity to a dark machine of destruction. With humans at the receiving end of avian fury, Du Maurier is suggesting the crisis may have been propelled by human actions. It is significant that today, many critics read "The Birds" as one of the early prescient stories about the impact of climate change. The lesson of the story thus can be fully delivered only with the extinction of all humans, which sadly includes Nate and his family.

Lastly, "The Birds" is also an allegory for some of the events of World War II. Like the Triggs initially ignore Nate's warnings about the birds, people over Europe too were blind to the rise of fascism. The aerial attack of the birds recalls the air-raids over Britain during the war, and Nate's wife's query recalls America's role in helping end World War II:

Won't America do something….They've always been our allies, haven't they? Surely America will do something?

The reference to war and the continuing ignorance of everyone other than Nate point to a world which has not learnt much from its mistakes. Therefore, doomsday is at hand for the inhabitants of this world.

When Nat comes to the Trigg farmhouse, he finds a scene of carnage and the dead body of Mr. Trigg. This scene is described as follows:

Trigg’s body was close to the telephone. He must have been trying to get through to the exchange when the birds came for him. The receiver was hanging loose, the instrument torn from the wall. No sign of Mrs. Trigg. She would be upstairs. Was it any use going up? Sickened, Nat knew what he would find.

This does foreshadow the deaths to come to Nat Hocken and his family. Although he loads up the car with supplies from the Triggs and boards up his house in hopes of escaping the assaults of the birds, his wife finds all the radio stations, British and foreign, have gone dead. This suggests that the birds have attacked everywhere, all over Great Britain and Europe, leaving no hope for escape.

Nat has been sensitive about the birds from the start, never taking their assaults as a joke, as the Triggs did, but this will not save him or his family from Triggs's fate. The implication is that the humans can fight, but they cannot win against the birds. Reflecting this reality, Mrs. Hocken decides to have a last cigarette before the birds get into the house.

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.