Trivia: In the opening scene, Marion Crane is wearing a white bra because Alfred Hitchcock wanted to show her as being "angelic." After she has taken the money, the following scene has her in a black bra because now she has done something wrong and evil. Similarly, before Marion steals the money, she has a white purse; after she's stolen the money, her purse is black.
Trivia: Alfred Hitchcock makes a cameo during the scene when Marion Crane walks into her office - he is standing outside the office on the right. (00:07:00)
Continuity mistake: When the Highway Patrol Officer hands the driver license back to Marion, in the interior shot he's holding it out with his right hand and Marion takes it with her left hand, but in the next exterior shot he's handing it to her with his left hand and Marion is taking it with her right hand. (00:15:50)
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.