We can find the answer to this question by considering the overall message and theme of this story, which involves the young, lonely girl having to choose between betraying nature in the form of the white heron and losing her one and only friend in the form of the hunter. The way in which she chooses to remain true and loyal to nature and to disappoint the hunter results in the melancholic tone that dominates this excellent short story. Note how the final paragraph establishes this tone:
Many a night Sylvia heard the echo of his whistle haunting the pasture path as she came home with the loitering cow. She forgot even her sorrow at the sharp report of his gun and the piteous sight of thrushes and sparrows dropping silent to the ground, their songs hushed and their pretty feathers stained and wet with blood. Were the birds better friends than their hunter might have been, --who can tell?
The tone is one of loss, sadness and grief, as the girl has been forced into choosing between nature and companionship with men, and now is forced to face the consequences of her decision and the uncertainty of whether she did the right thing.