In the poem "Byzantium," the speaker compares the creative process to the journey that the soul takes after the body has died. The speaker uses the symbol of a "golden" bird to represent the creative process. The bird is a common symbol of freedom and transcendence. It seems free to fly where it chooses, and it seems to transcend the human realm by flying closer to the heavens than to the earth. The speaker thus suggests that the creative process is also characterized by freedom and transcendence. The fact that the bird is "golden" attributes to it, and thus to the creative process, a sense of value and importance.
In the third stanza of the poem, the speaker says that this "golden" bird can "like the cocks of Hades crow." In Greek mythology, Hades is the god of the underworld. The speaker's comparison evokes an image of cockerels in the underworld, crowing to signal the passing of another soul from life to death. Thus, just as the "cocks of Hades crow" to signal a spiritual conversion, so, too, a poem or any work of art might, as it were, crow or sing and, in so doing, communicate a spiritual transformation. The speaker's point here seems to be that art can be spiritually transformative.
Another interpretation, equally valid, might be that the process of creating art must necessitate a death of some kind. The completion of a work of art perhaps signals a death, just as the crowing of the "cocks of Hades" signals that somebody has died. Perhaps the speaker is suggesting that once the work of art is complete, the artist dies a metaphorical death. Indeed the artist can no longer influence the work of art, which thereafter must exist on its own.