Norman Bates: A boy's best friend is his mother.
Mother: No! I tell you no! I won't have you bringing some young girl in for supper! By candlelight, I suppose, in the cheap, erotic fashion of young men with cheap, erotic minds!
Norman Bates: Mother, please...!
Norman Bates: She might have fooled me, but she didn't fool my mother.
Norman Bates: She just goes a little mad sometimes. We all go a little mad sometimes. Haven't you?
Marion Crane: Yes. Sometimes just one time can be enough.
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.