| Quote #4
He was a hunter of the hills,
The speaker's middle brother is an outdoorsy "hunter." Because he's so used to the freedom of the outdoors, he can't stand being imprisoned and is the first to give up hope and die.
| Quote #5
I saw the glimmer of the sun
The "glimmer of the sun" that "creep[s]" into the dungeon is like the first glimmer of hope the speaker sees after his youngest brother dies. The bird, too, could represent the power of the natural world to restore hope.
| Quote #6
And it was come to love me when
Here the speaker explicitly says that the bird had come to bring him back from the "brink" of despair. Yay, nature!