Symbolism, Imagery, Allegory
Was there any doubt that feathers were going to play a major role in this story? We've got birds, we've got a story about an old man who talks to birds… yeah, feathers are going to be a symbol. Mainly, they are a symbol for Agatha and the way she floats on the wind, the way no one can tie her down—not Billy, not Grandfather Bolte, not her broken engagement with Mr. Olmstead. As Georgie says:
Feathers claw their way back into the sky, whereas leaves, after flying once, are content to rest on the earth. Agatha? She was a feather. She pushed higher, farther always. (3.2)
Do you notice the words claw and pushed? Feathers may be light and airy, but make no mistake about whether Agatha's tough. She refuses to give up on her dreams, reaching high time and again until she finally catches a breeze that carries her to where she wants to go.
Later, Georgie compares the moment she finds out Agatha is alive to a feather:
Pause a moment. Feel the air surround that moment. Push against it, and find that it truly exists. Blow on it, and see how the tiny barbs snag the wind and lift. Watch it fly. (24.36)
While it might seem a bit clichéd to say that Georgie's heart is light as a feather when she discovers Agatha is alive, it's also quite true. So there.