What is the speaker's explanation of the ravens one response?

The speaker's explanation of the raven's one response changes drastically throughout the poem.
In the beginning, the speaker thinks that, like a parrot, Nevermore is the only word this bird can utter. However, over time, he starts to change his mind about the bird, thinking that it is no coincidence this bird says only that one word. In the end, he starts thinking of the bird as the Prophet of death, and asks it whether he will ever see his beloved Lenore, to which the bird answers, Nevermore.