How does the raven finally enter the chamber in "The Raven"?

In the poem The Raven , by Edgar
Allan Poe, the following stanza describes the encounter between the narrator (poet/main
character) and the bird which he still does not recognize, yet, has heard "tapping at
his chamber door"

Open here I flung the shutter,
when, with many a flirt and flutter,
In there stepped a stately raven of the
saintly days of yore.
Not the least obeisance made he; not a minute stopped or
stayed he;
But, with mien of lord or lady, perched above my chamber
door—
Perched upon a bust of Pallas just above my chamber
door—
Perched, and sat, and nothing
more.

What this stanza means is that at one point
the narrator opens his window (flung the shutter) and the bird crept in from the
outside. Under a more allegorical perspective, we can assume that the bird, representing
melancholy and the past, crept into the life of the narrator as surprisingly as his
reality crept into him, making him now a lonely man in the bleak climate of
December.