The key to killing A LOT of crows is finding their major flyways. They usually roost in urban areas where they are protected and then in the early morning, they fly to rural areas to feed. The best time to catch them is early in the morning and just before dusk while they're flying to and from their main roost.
I went crow hunting with a buddy of mine a few years ago and he had permission to hunt a farm about 8 miles out of the city limits. We must have shot 4 boxes of shells that morning and killed a bunch of those black bandits.