We see the raven come up in the book a couple times at very tense moments. What does the raven symbolize? I believe that it is a warning and a conscience to whomever walks across the ledge. It is something that is watching your every move and it makes you rethink what you are doing because it feels like someone is staring at you all of the time. It is a constant reminder of the danger that comes with crossing the ledge and it is always suspenseful, as the reader, when you hear the raven being mentioned because you know that there is a chance that something bad will happen. It doesn't just have to do with the ledge though, it also is just showing how you should always act as if someone else is watching. Would what you are doing be acceptable or not harmful in anyway? The raven makes you do a double-take on life.
picture from: http://static.howstuffworks.com/gif/willow/raven-info0.gif