The Grim Reaper
For this scenario, you will need a grim reaper costume. If you know that the crows in your area worship/fear a different death deity, go with that. The important thing is that this is not the place to save money. The crows need to think you are the personification of death, not some clown in a plastic cape and scythe from a drugstore. You will also need a picture of the crows you intend to scare.
The day you plan to scare the crows, you will need to arrive before they do and stand in the the middle of the field. When they arrive, they will see you and keep their distance. Because crows are very curious, they will stick around, trying to decide if you are indeed the angel of death. It is important that you pretend not to notice them. Once you are sure that you have their attention, pull out a picture of the offending crows and say, “Oh, I had hoped to take the souls of all those crows today, but I guess they are not here. At least I know where they hang out. I’ll just come back here when they least expect it and take them all to hell. I won’t even bother looking anywhere else for them.”
The crows will think that will be able to elude death as long as they avoid your fields. They are, of course, wrong. Death will find us all, on its own schedule, not ours. But at least they will be out of your fields.
If the crows are unafraid of death and continue to show up on your property, I recommend leaving bread soaked in strychnine strewn around your field.
The Medical Clinic
Start by setting up a crow-sized medical clinic in your field. They won’t trust a human, so you will need to staff your clinic with a crow. This might sound difficult but crows are very clever, and will be able to fake their way through a medical check-up with a little training, and you will be surprised and disgusted with how many crows are willing to sell out there brethren for a salary of bugs and seeds. He/she tells the other crows that they qualify for a free medical screening and leads them, one by one, towards a mysterious health scanner.
Once a crow’s scan is complete, the “doctor” looks at a printout and, with a concerned expression, tells the “patient” that his body is riddled with tumors. “If we had caught this earlier,” your crow doctor says, “we would have a lot treatment options. But as it is, we are mainly talking about making the most of the time you have left.” The crow does not believe it at first, then becomes sick with fear. Eventually, the crow will start to reflect on his life. He probably has dreams that he has never shared with anybody, and realizes that he has wasted his life, never living as the “real” him. He wishes it did not take dying to see this, and resolves to truly live the life he has left. He will most likely go traveling, or pursue a career in the arts in a major city. Either way, he will leave your crops alone. If it turns out that he is content with his life and chooses to spend his last days ravishing your crops, poison him with a mouse carcass filled with strychnine.
This would be a relatively inexpensive option, and the bulk of the expense would go towards the hire/training of the crow doctor. You would not need to invest in a real health scanner, because the results would always be the same. So you could probably just attach a printer to a shoebox with a black light in it. If there a lot of crows in your fields, you might want to consider opening several clinics, to keep wait time down.
The Forgotten Spirit
You will need to find out where your crow lives. Wait for him to go out for the day and then weather his house to look like about 20 years have past since he left. If his family remains, capture them and keep them in your house until the plan has played itself out. Yes, this plan requires kidnapping. If you can’t handle that, I can’t help you. Good luck. I hope you enjoy living with crow ravaged fields.
Again, you need to enlist the help of a crow. Have him wait in the house until the crow returns from a long day of ruining your crops. He finds that his keys don’t work, and then rings the doorbell or bangs on the door in frustration. Your shill then comes to the door and tells the crow that he is the lawful occupant of the home. The crow will become agitated and point to his name on the mailbox. But it is no longer his name, because you changed it. The shill says that he recognizes the crow’s name but nobody by that name has lived there for a long time. Your crow is already feeling like the world around him no longer makes sense, and will be pushed over the edge when your shill tells him there is something he should see. He will take him to a single tombstone in the backyard. When your crow gets close enough to read it, his heart will stop and he will drop dead. Here’s why: the gravestone has his name on it. (Note: you will need to find out the crow’s name ahead of time.)
If, by chance, the crow secretly always wanted to be a ghost and is indifferent to his own death, I recommend tossing a pigeon corpse stuffed with strychnine at him.