Stories that have a spring setting tend to be hopeful. Spring is a powerful symbol of rebirth and renewal. Before the final two paragraphs of "Barn Burning" we might not even realize it's springtime. Everything seems to be so grim and bleak. For Sarty that moment on the hill is probably the first he's had peace and quite and calm in a very long time. No wonder spring is revealed to him, finally.
This sudden awareness of spring coincides with freedom, the freedom to make his own choices. Because that freedom comes at great cost – the loss of his family and childhood – spring symbolizes for Sarty both the sadness of loss and the joy of finally being free from his father's tyranny. The whippoorwill, whose song is beautiful but also mournful is the perfect symbol of the blend of emotions inside Sarty as he takes in the spring morning on the first day of his new life. The description of spring is also soothing imagery to the reader. After the harshness of six days with Abner, we, like Sarty can gain relief from imagining the cool, dark woods, and the singing birds within them.