Why is it significant that the bird remains perched on a "bust of Pallas" in Edgar Allan Poe's "The Raven"?
After listening to the repeated tapping at his chamber door, the speaker of "The Raven" flings open the shutter and a raven enters in a "stately" manner, perching itself upon the bust of Pallas that the speaker has mounted above his door. This bust of Pallas Athene, the Greek goddess of wisdom, is symbolically subjugated by the raven, a symbol of death, who dominates it with the "mien of lord or lady."
In the next stanza after the bird perches upon the bust, the speaker alludes again to mythology as he addresses the ominous visitor,
"...Ghastly grim and ancient raven wandering from the Nightly shore—
Tell me what thy lordly name is on the Night's Plutonian shore!”
Pluto is the Greek god of the underworld, the land of darkness called Hades, which is separated from the world of the living by several rivers. This is why the speaker mentions the "Plutonian shore." Of course, this allusion also increases the dread and foreboding that the speaker feels at the sight of the raven as well as the fear that the raven may possess the wisdom of the gods since it dominates the bust of Pallas.
Further, Poe calls upon the myth of Pluto, the God of the Underworld, the land of the dead in Greek mythology. The raven, therefore, may be thought of as a creature from the land of the dead, who responds to the speaker with "Nevermore," indicating that he will never depart, forever casting his shadow of death upon the soul of he who loves Lenore:
And my soul from out that shadow that lies floating on the floor
Shall be lifted—nevermore!
From the reaction of the speaker, it is apparent that his wisdom and rationality has become dominated solely by the thoughts of death. Thus, the symbolic perching of the raven upon the bust of Pallas is central to the speaker's mental unraveling.