Things That Annoy Me

"Lines 1-4: I was the shadow of the waxwing slain, etc.

the image in these opening lines evidently refers toa bird knocking itself out, in full flight, against the outer surface of a glass pane in which a mirrored sky..." (73)

Thankyou Captain Obvious! I can read a poem. On a more serious note, I just love how Kinbote says that lines 1-4 evidently refer to a bird knocking itself out. What else would these lines refer to? I can't figure it out, and I have re-read these lines multiple times trying to figure out any other possible meaning. I can't find any.