What is the theme of "A White Heron"?

Several themes are developed in the story, but the primary theme is one of self-discovery. "A White Heron" is an initiation story, a narrative in which the protagonist changes in a significant way as the result of an experience. In an initiation story, the main character comes to understand something previously unknown about himself or herself or about life itself. In this story, Sylvia's experience with the white heron leads to an understanding of herself and her own values.

When she comes to live with her grandmother in the country, Sylvia is a shy, lonely child of the city. She spends her time outdoors, by choice, fitting into her new natural surroundings. The arrival of the hunter brings a new element into her life. She likes him, feels the first stirrings of young love, and wants to please him. Those emotions and his offer of ten dollars to help him find the heron (money her grandmother needs) motivate Sylvia to seek the heron's nest. 

Sylvia climbs the tallest pine in the forest, watches the sun rise, sees the ocean, and spies the nest in the forest below her. The white heron itself appears, a singular sight of tremendous beauty, floating free and safe in its own surroundings. By climbing the pine tree, Sylvia has left her own limited world and entered one unknown to her, the world of the heron high above the earth, one of nature's grandeur. In those moments, she becomes a part of all she sees and feels.

Returning home, Sylvia disappoints the hunter and her grandmother. She remains mute when asked to reveal the location of the nest:

The murmur of the pine's green branches in her ears, she remembers how the white heron came flying through the golden air and how they watched the sea and the morning together, and Sylvia cannot speak; she cannot tell the heron's secret and give its life away.

Sylvia has changed in a profound way. Through her experience in the pine tree, she has become aware of truth she did not know existed, and it has become a part of her. Sylvia's values are now clearly defined. She will not be one to endorse or embrace destruction; she will, instead, value life and beauty.