I used to hunt them off our land year round. They ate holes into our watermelons, thus becoming my enemy for life!. They loved our pecan tree and would eat planted seeds right out of their newly planted rows. I am amazed at how smart they are as a group. They will scout an area, usually a lone bird or two and pretty high in a tree (loved the pine tops on our place). For your corn field you I'd bet they'll hit tall tree's on the edge of the field. Then the group will come, and they'll post sentries round about. They take turns coming in to feed. Don't know the best way to hunt them with rifle, I'd be afraid of where the bullets would land if shooting them from tree top, and I'd guess it's impossible to see them from any distance in a corn field?
I remember walking thru a pasture and seeing a couple ~20 yards away squawking at me from tree tops (they were likely scouting for some group). I wondered why they didn't fly. I picked up a stick and put it over my shoulder (imitating long gun) and they immediately flew off. From then on I hid my rifle or shotgun as much as possible when I was on the hunt. If they are hunted hard and/or successfully for a few days, they'll leave an area (stop feeding ) for about a week or 10 days ...
My dad could call them in with ridiculous skill, and I learned a bit. He busted a single crow one afternoon in a pine thicket (single bbl shotgun) and durned if he didn't give the wounded call. Ended up with two more dead (I could never do that). I did kill one that came to help a bird that I wounded once, if you have the skills to intentionally wound one you would likely kill several.
He knocked a hawk out of the sky one day, peppered his tail with a shotgun. We nursed it back to health. He never told us why, but we eventually got him to eat hamburger and the butt wound healed. Then he tethered him to a pole in the middle of the field and shot a mess of crows ... they literally couldn't help themselves.
For all the damage they do to crops, I'm shocked they aren't considered to be a pest and huntable at anytime! I still think about killing them ANY TIME I see anywhere!