I've been trying to think of a better name for a group of them. When I see crows near a garbage bin, I think a plunder might suit them. But then, hearing a huge number of them, as in the video below (shades of The Birds), I consider the word cacophony. A cacophony of crows -- it even sounds cacophonous. But since crows aren't always loud, that might be nearly as unfair as murder. How about a riddle of crows?