Why is the bird perched on a bust of Pallas? Why is this symbolic?
Pallas is another name for Athena, the ancient Greek goddess of wisdom, which can symbolize a few things in this poem. First of all, it can be connected to the narrator; just as an ancient goddess alone, the presence of this bust in the narrator's home is a testament to his scholarly nature, and as the goddess of wisdom, it shows the importance he places on wisdom. But its more important connection is to the raven. The fact that it perches on the goddess of wisdom could mean that it speaks not just from "its only stock and store," but instead it speaks intelligently, so the narrator can trust its words rather than think it was just repeating a meaningless phrase over and over.