Walt Whitman's poem "Out of the Cradle Endlessly Rocking" is essentially a coming of age narrative about a boy. The boy grows from possessing the mentality and understanding of a boy to that of a man through his interactions with nature. Specifically, the boy becomes very fond and observant of a family of birds, watching the father go out every day in search of food while the mother keeps the eggs warms. Sadly, the boy even observes the mother bird leave the nest and not return, perhaps having been killed. Even more sadly, the boy observes the remaining male bird keep calling out to his lost mate, and the boy says he treasured every note and understood its meaning. It is due to this observation of loss the boy learns what it means to die, a revelation that makes him think more like a man rather than a boy, helping him mature.
It is towards the end of the poem that the boy refers to a messenger, saying that the messenger aroused within him "the fire, the sweet hell within, / The unknown want, the destiny of me." It is most definitely a bit ambiguous as to who the messenger is. However, since up to this point the boy has been listening to the male bird that has lost his mate, we can argue that the messenger is the bird. We can see the messenger being identified as the bird earlier in the stanza because the bird is the only speaker, as we can tell when the boy asks so many questions and says so many statements about the bird's singing, such as, "Is it indeed toward your mate you sing? or is it really to me?" Towards the middle of the stanza, he addresses the bird as a solitary singer and states that now he has heard the solitary singer he will never be able to escape the bird's song, nor escape knowing what it is to feel "unsatisfied love," nor be the same child he used to be. Then finally he says that the messenger aroused the "sweet hell within," which refers to all of the emotions the boy felt due to the bird's song; hence, we can say that the messenger is the bird.
However, the identity of the messenger becomes ambiguous when we reflect that he is standing by the sea listening to the bird and in the next stanzas he hears the waves of the sea whispering the word "death" as if it is at this moment that the boy understands exactly what death is. Nevertheless, it is also the bird's singing for his deceased mate that made the boy feel these feelings, so it is really the bird's loss that made the boy understand what death truly is. Hence, while the ocean waves do speak next to the boy, it seems it can be argued that it is only the messenger who aroused the feelings of understanding within the boy, so the messenger is the bird.