Okay, this one's almost too easy. Mattie is at her mother's grave considering everything that's tying her to home, remembering her mother's death. And then she sees:
[…] the body of a young robin half hidden in the grass. Its wings were twisted and bent. Its body stiff and bloodied.
A hawk's work, I thought, wondering if the robin had seen the brilliant blue of the sky and felt the sun on its back before its wings were broken. (23.glean.23-24)
Typically in literature, birds and wings represent freedom, the future, possibility—all that jazz. So here we see a bird whose life has been cut tragically short, just like Mattie's mother whose grave it's right by. The hawk, in this case, can be understood as the cancer that killed Mamma.
On a subtler level, though, the dead robin represents Mattie and her reservations about breaking her promise to Mamma and heading off to school. The promise is breaking Mattie's wings, keeping her home though she dies a little more inside each time she works to accept this future. The promise—like Mamma's death—is like a hawk to Mattie's robin-self, trapping her in a life with little hope.
Unlike the robin and Mamma, though, Mattie gets away from her hawks in the end. As she prepares to leave Eagle Bay, she thinks of the people she's leaving behind, but also remembers the robin. What significance do you think this has? We think you can interpret it a couple of different ways, and so we're going to go ahead and leave it up to you.