How does Tobias Wolff use imagery in In the Garden of the North American Martyrs?

Careful readers will notice that "In the Garden of the North American Martyrs" begins and ends with images of birds. Mary likens her diminishing capacity for original thought to the sight of "birds flying away." In the final scene, when she delivers her prophecy to her stunned audience, one of the last things Mary hears before she turns off her hearing aid is "someone whistling in the hallway outside, trilling the notes like a bird, like many birds." According to Wolff's explanation in his interview with Jay Woodruff, "Language, especially the language which she speaks at the end, her own language, is freedom, is flight." In an earlier interview with Bonnie Lyons and Bill Oliver, Wolff explains that the sound of the bird in the hallway is "a sign of her own voice coming back to her. An image of the words she has lost, like birds flying away."