This page should help you, Corali:
http://www.birds.cornell.edu/crows/crowfaq.htm#raven
He also has a great write-up on the difference between Fish Crows and American Crows. It's something that's always bugged me because the main difference most point to is the voice. However, I've heard what I thought were American Crows (especially young crows!) make similar noises, so I was confused and frustrated. His article clarified what I in fact was hearing... the begging noise of the American Crow, and how to tell the difference between that and the Fish Crow voice. Great Article!
