What might the birds' restlessness and large number foreshadow?

In the opening paragraphs of the story, Du Maurier describes how the birds have become "restless" and "uneasy" as they flock to the peninsula. The purpose of this description is to foreshadow the events which take place later in the story. Specifically, Du Maurier is using the birds' strange behavior to hint to the reader that something strange and unusual will happen. As we see later on, it foreshadows the birds' violent attacks on people and property.
By using foreshadowing in this way, Du Maurier not only creates interest for the reader, as they wait to see what unfolds, but she also creates a sense of tension. This tension is effective in establishing a foreboding and uneasy atmosphere—an atmosphere which Nat Hocken, the main character, is the first to notice. Nat knows that something is not quite right, and this functions as another clue to the reader that strange events are about to happen.