When I was in high school we used to use a crow and owl fight tape to call crows. No decoys needed, but we would play the tape from a pine tree (lots of cover) that was next to a leaf-less alder.
When the crows first came in we wouldn't shoot, but would wait for them to add to the racket and call in more birds. When there were enough birds we would start sniping them from a blind with CB caps and pump .22s.
I don't know if they are just really focused on getting into a fight, or if they just can't hear they quiet report, but we could put a lot of birds on the ground before they ever got wise.
