What do the birds in the movie Psycho represent in Norman's life?

There is the scene where Norman is talking to Marion in his parlor. Norman is trying to impress Marion but not doing a very good job. In the midst of discussing his taxidermy hobby, Hitchcock has a large stuffed owl filling the background of the frame. It seems to be looming over Marion: a symbolic reflection of how Norman, and ultimately Norman's mother, will swoop down on Marion a few frames later.