If Sylvia loved the young hunter, what life experiences did she have that made it impossible for her to tell the young man where the white heron was?
A good question.
She remembers being chased by a boy when she lived in town (as the bird would be chased).
Dan left, and so Sylvia can't trust those she loves (perhaps specifically men she loves).
However, those pale compared to the experience she has during the story itself, which is simply a feeling of intense connection with the birds and the surrounding life.