Unlike most of his poems, "Our of the Cradle Endlessly Rocking" by Walt Whitman has a bit of a plot. It was originally titled "A Child's Remembrance," and that is the essence of the poem's narrative.
The narrator tells us about his experience as a young boy who once watched a pair of mockingbirds on the beach located near his home. Their relationship is a fascinating to him, but one day everything changes. The "she-bird" does not return home to her companion, and the "he-bird" mourns for his lost mate, waiting for her to return but to no avail.
The male's mourning cries strike a chord in the boy's soul, and he is able to "translate" what the grieving male bird sings because he has the soul of a poet. The boy is so moved that he then asks nature to reveal to him one word, that word which is "superior to all." As he stands by the ocean and listens, he hears the word: "Death." Both this word and the song of the male bird are a consistent presence in the remainder of the poem.
In an earlier version of this poem, Whitman wrote "dusky demon" and later changed it to "messenger." The earlier term includes the word "dusky," which means dark-colored, so the first description is perhaps our best clue that the "messenger" is a the "he-bird." Line 146, the beginning of section 9, says this:
Demon or bird! (said the boy’s soul,)
Is it indeed toward your mate you sing? or is it mostly to me?
Clearly the boy/narrator is speaking to the bird who has been left behind and alone, and he goes on to say that listening to the mourning bird's cry has revealed to him his calling as a poet. He continues to describe his awakening:
For I, that was a child, my tongue’s use sleeping,
Now I have heard you,
Now in a moment I know what I am for—I awake,
And already a thousand singers—a thousand songs, clearer, louder and more sorrowful than yours,
A thousand warbling echoes have started to life within me,
Never to die.
The bird's song, his lament, has awakened a thousand poems in the boy, and he will never be the same because of this experience.
Next are the lines which reference the messenger. The boy/narrator again addresses the solitary singer in a way that is reminiscent of Poe's "Raven" with its repetition of "nevers" and the life-changing nature of the encounter. Again the boy speaks to the male mockingbird, saying:
O you singer, solitary, singing by yourself—projecting me;
O solitary me, listening—nevermore shall I cease perpetuating you;
Never more shall I escape, never more the reverberations,
Never more the cries of unsatisfied love be absent from me,
Never again leave me to be the peaceful child I was before what there, in the night,
By the sea, under the yellow and sagging moon,
The messenger there arous’d—the fire, the sweet hell within,
The unknown want, the destiny of me.
This encounter has changed the boy forever; his soul has been opened and released, and he is now incapable of being an innocent, unaware boy again. He can no longer be the "peaceful child" now that the messenger has awakened in him "the fire, the sweet hell within, / The unknown want, the destiny of me."
This entire poem demonstrates the maturation of a boy as he becomes a man and then, eventually, a poet. He was moved to that realization by his experience with the soulfulness of the grieving bird, the messenger.