Audio problem: When Norman runs down from the Bates house to the motel and meets Marion, he says "Gee, I'm sorry I didn't hear you in all this rain. Go ahead in, please." But you look closely, it looks like Norman is barely moving his lips. In fact it looks like Marion is saying something instead. (00:27:50)
Audio problem: When Norman enters the cellar to kill Lila and is grabbed from behind by Sam, Mother's voice can be heard yelling, "I am Norma Bates." However, this was added in post-production, as his lips are not moving.
Chosen answer: Birds are just a favourite choice of spooky animal that Alfred Hitchcock seems to use hence the film "The Birds". Eyes show the window to our soul where our deepest fears originate and who isn't scared of getting knifed in the shower. The mirrors i can't explain but I can suggest that it just looks creepy.