Again, things like this might happen in the Spring, but it only seems really foreboding in the late Autumn air...there's just something about this time of year...
(Oh, in case you don't know, crows usually only live in small groups of two or three during the "mating/family raising" time of year, and travel in large groups like this in between mating seasons--its not terribly common to see them in this large a group). :)