Symbolism, Imagery, Allegory
Ten days before her disappearance, Agatha tells Georgie a story from the Seneca about a wise old man who dreams of a white pigeon. He tries to follow it, but can only do so feather by feather. Georgie says:
"This is a story about you," I crowed. Agatha never could tell a story that wasn't somehow about herself. (3.27)
Because Agatha chooses to tell Georgie this story just before she leaves, we think Georgie's right. Agatha is telling Georgie that she has to make her own way, step by step, but she does it through a story instead of saying, "I'm going to run away with some pigeoners. Don't try to stop me, okay?" Importantly, though, this means Agatha doesn't just up and leave—she offers her younger sister an explanation for her actions, even if Georgie doesn't understand this at the time.
After they receive Agatha's letter months later, Georgie says, "Feather by feather, she had made her way"(23.37). The realization that Agatha has actually accomplished her goals in the same manner the story describes makes Georgie realize that she, too, can change and grow. In fact, you might say Georgie's already started doing just that.