The entire story is permeated with images of shadow, light, and woods. Cite examples of how each influences the story.
Two references to shadow occur early in the story, prior to the arrival of the stranger, and they seem to foreshadow the danger to nature he poses. In the first line, the narrator says, "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." Foregrounding the darkness present, even during this beautiful sunset, helps to give readers a sense that some darkness is coming. Then, as Sylvia walks her cow home, she notes the "gray shadows," just prior to becoming "horror-stricken" by the stranger's "aggressive" whistle. Her first instinct is to think of him as "The enemy." These descriptions helps us to understand that this man, however nice, poses some kind of danger; he is the shadow to Sylvia's light.
At least twice, Sylvia is compared to light, and she is almost always associated with it. As she climbs the great pine tree, she is described as a "determined spark" and "[her] face [...] like a pale star," both sources of light. It is notable that the stranger arrives at night, but Sylvia seems to come most alive in the morning, as she ascends toward the sky (she is even not used to being out as late as she must be that first night to find her cow, and she is feeling sleepy by the time she finds the cow).
The link between light and the woods is strengthened by the climbing scene. "Yes, there was the sea with the dawning sun making a golden dazzle over it, and toward that glorious east flew two hawks with slow-moving pinions [....] and Sylvia felt as if she too could go flying away among the clouds [...]; truly it was a vast and awesome world." She imagines herself in the bright air, and it is, later, her memory of "the golden air" that renders her unable to "tell the heron's secret and give its life away." The light and the sight of the sunrise from the woods impressed Sylvia (whose name comes from the word "sylvan" and means of or associated with the woods) with the beauty of the bird's life, and she cannot tell the hunter how to take it now. It is as though the light dawning on the world represents a new perception of the stranger and his wishes that dawns on Sylvia. Moreover, her fantasy of herself flying away with the hawks and the comparison of her to a bird "with her bare feet and fingers, that pinched and held like bird's claws" to the tree renders her incapable of betraying the heron, who now seems one of her fellows. She is of the woods and the light in the same way that the heron is.