Sylvia is a child who is different than other children. She tries to find solace in the Maine wilderness. She is walking along in the woods when she hears the whistle for the first time. She does not see where the whistle is coming from, and does not see the hunter, himself. She knows animal and bird sounds and, when she hears his whistle, she is immediately scared or alarmed. She is a child who has been terrorized by other children and who avoids people because she doesn't interact with them well, and has a hard time making friends. When she hears the whistle she knows that it is NOT a bird's whistle she is hearing and therefore it is coming from a person. This is something that is terrifying to her because a person could be an "enemy" or someone who could harm her, which is a great source of anxiety and fear. Sylvia is a person who is afraid of people. Her friends are the animals and creatures in the woods. They are where she finds comfort and security. According to the book, when she hears the whistle she is "horror-stricken". She is afraid of people, especially boys after she has been tormented by a boy at school. It is natural, then, that a young girl who is afraid of people and afraid of young boys in general, would be scared when she heard the whistle of a boy she doesn't know in a place where she generally feels safe and secure. It would be natural for her to see whoever the boy was as an enemy.
It's clear through the passage that there are no paved roads or cars to get the miners from place to place. They had to carry all of their gear and supplies through dangerous trails by foot, only being able to travel a certain amount each day.
