In "A White Heron," why does the author use shadows?

In "A White Heron," Sarah Orne Jewett uses references to shadows in order to foreshadow the coming of the hunter, a figure that represents a force which is hostile to nature, and establish the story's mood.  In the first line of the story, the narrator says that "The woods were already filled with shadows one June evening, just before eight o'clock, though a bright sunset still glimmered faintly among the trunks of the trees."  By describing the shadow so early in the piece, Jewett uses it to help establish the mood of the story: there is a darkness looming and we should prepare ourselves for it.

The "gray shadows" are referenced once again, immediately prior to Sylvia's hearing, and being frightened by, the hunter's whistle, the whistle he uses to lure birds to him.  She was not typically out this late, and so, again, the reference to shadow seems to establish some suspense as well as to foreshadow the danger approaching in the form of the hunter who kills and stuffs the birds he claims to love.