The shadow acts as a dual symbol in this story: it represents both the nature Sylvie feels part of and the threat of the invader.
Early in the story, leading the milk cow home, the narrator uses the word "shadow" twice in one paragraph, writing:
She [Sylvie felt] as if she were a part of the gray shadows and the moving leaves. She . . . thought of the great red-faced boy who used to chase and frighten her [which] made her hurry along the path to escape from the shadow of the trees.
Starting with the second use of shadow, we understand that shadows represent the threat of danger. In fact, danger does come out of the shadows of the trees in the form of a man with a gun. This stranger stays with her at her grandmother's home and offers ten dollars, a large amount of money in those days, if she can lead him to a white heron. He wants to shoot and stuff it.
This brings us to the first use of shadow. Sylvie, a few days after the stranger's arrival, climbs a tall tree at dawn and becomes one with nature, as she was with the gray shadows. She spots the white heron from the trees. But this child of nature will not betray a fellow creature and so keeps the whereabouts of the heron hidden in the shadows. Not even a large sum of money can cause her to betray the bird or her integrity.