Sylvia hears the hunter before she sees him. He whistles, loudly and clearly, and she is "horror-stricken" because she immediately figures out that it isn't a bird's whistle she hears but a boy's. A bird would have sounded somewhat friendly to her while the boy's whistle sounds more aggressive. The narrator actually describes the whistler as the "enemy" because this is how Sylvia perceives him at the time.
When they come face to face, the hunter tries to speak with her, and she is quite alarmed. Though he tells her that there is no need to feel afraid, she clearly still does fear him, and she is even more worried her grandmother is going to blame her for bringing him to their home. As a result of her concern, Sylvia "hung her head as if the stem of it were broken [...]." Such a description helps us to understand the threat that the hunter, however friendly, poses to the nature that Sylvia so loves. The facts that he carries a gun and mentions "'hunting some birds'" are not lost on the little girl, and she instinctively feels him to be an antagonist.