The young Sylvia is completely at home in her woodland environment, so much so that the story calls her a child of a nature. Her insider status is established from the start, when we learn that Sylvie (and her cow) know where they are going, even if they can't see in the growing darkness, for:
their feet were familiar with the path
The outsider who intrudes into this geography doesn't have Sylvia's intimate knowledge of the terrain, despite being experienced and knowledgeable in ways Sylvia is not.
The outsider, though kind enough, has his interloper status emphasized by remaining nameless throughout the story. He is called "sportsman," "guest," "ornithologist," and "hunter" but his lack of a proper name emphasizes that he has no fixed place or sense of belonging in the context of this geography.
The guest wants to trade his cash—ten dollars—for Sylvie's knowledge of where the white heron nests so that he can kill the bird, take it with him, and stuff it as a trophy. Sylvie knows exactly where to go to find the heron. Without telling the sportsman, she climbs the appropriate tree. She finds, with excitement, that the
heron has perched on a pine bough not far beyond yours, and cries back to his mate on the nest and plumes his feathers for the new day!
All of Sylvie's instincts are to preserve the integrity of her environment by not revealing the whereabouts of the bird, and she does not. She saves the heron—an insider, as she is, in the local eco-system—from becoming part of an alienated outer world that dips in and appropriates what it wants from somewhere else, then leaves again.