I did a little research and this article seems to sum up most of what I found in a lot of individual pieces. The crow and raven are from the same 'bird family' and in some myths they were considered good things, but in most they were not. I'll put the link to the entire piece, but this paragraph seems to sum up what the guys were going for:
"These birds were thought to have a special taste for the bodies of hanged criminals and to enjoy plucking out the eyes of sinners (Prov 30:17). Christians thought they carried off the souls of the damned and associated this bird with the Fall of Man and Satan who blinds sinners, dulls their moral senses, and feasts on their corruption."
http://ww2.netnitco.net/~legend01/raven.htm