On what does the raven perch? What is the only word it speaks?

When the raven flies into the narrator's study, he alights on a bust of the Greek goddess, Athena.  The poem's narrator says that the raven entered "with mien of lord or lady, perched above my chamber door— / Perched upon a bust of Pallas just above my chamber door" (lines 40-41).  Thus, the bird conducts itself with somewhat of a noble bearing, acting like a lord, and flew to the statue of Athena that sits above the door to this room. 

The bird will only utter the word "Nevermore" to whatever question the narrator asks.  Now, Athena was the goddess of wisdom, among other things, and so this is a bit ironic because the bird -- the narrator knows -- does not speak out of any wisdom of its own.  It speaks only because its master must have taught it to say this one word.  The narrator says, "[...] what it utters is its only stock and store / Caught from some unhappy master [...]" (62-63).  He assumes that the bird's owner must have met with some unfortunate circumstances and uttered this word so often that the bird picked it up.