I really love Colbert's reports on bears, I watched a compilation of several clips some time ago. I actually at first only watched the Daily Show, but now I'm a big fan of the Colbert Report as well.
Ok, enough about that, talking about them birds:
Crows, and especially ravens, often feature in European legends or mythology as portents or harbingers of doom or death, because of their dark plumage, unnerving calls, and tendency to eat carrion.
Now this is strange, since it would seem that crows and ravens are to be seen more or less as belonging to a similar mythlogical category, evil carrion-eating war-bringing birds.
However, in the case of the ravens there is one special case I know of. Odin was connected to ravens in Norse mythology, I believe they served as his eyes and ears. As such, ravens were useful and good birds rather than bad ones. And isn't there a story that says that England will fall if the ravens leave the Tower of London? I believe I heard something like that whilst in London this summer on some tourist tour.
So, Tolkien may have had some reason to portray ravens as good birds and crows as bad.
I do however believe that there is a lot more to the whole mythological background of these creatures, I think the matter deserves a thread of its own which I'll be starting soon.
EDIT: xed with Morth and I missed that in Andsigil's post leading to the repetition.