Crows are tough birds to hunt believe it or not. Especially getting them to decoy and land. Spot and shoot would be the way to go.
Do you have a shotgun? You can kill some pretty good numbers of them with a shotgun and a mouth call, a few decoys made from wire coat hangers shaped into crows then wrapped with black tape. It is a lot of fun. An e caller and even more decoys helps too. Learn the calls of the crows and what they mean.
But.. If you don't have a shotgun and want to use your air rifle, get a call and set up in the woods. I've had birds land A LOT more rather than just flying by to check it out in a field.
Check out Crowbusters.com for some good tips and techniques, and to listen and learn the calls.