I don’t see many scarecrows any more. So what does keep crows, and also deer, rabbits, etc., from eating the crops in modern farms?
If you mean a garden, most are well-fenced. Today’s farms are extensive. Several hundred acres of corn, cotton, soybeans is the norm. There aren’t normally enough critters to do significant damage, although sometimes a farmer may have to get a “Varmint Permit” and shoot deer out of season. They can become pests.
Nothing. The DNR will on occasions give special permits to shoot or trap animals, when they destroy the crops badly. There are also programs that pay farmers when they loose a lot of crops to an animal. Wild animals are considered a state property for the purpose of the law.
Farmers in the area have used super tall fences, and noise cannons in the past. The noise cannons don’t do much after a few days, and the neighbors want to kill you. I don’t know if they even would allow them now, since they are very annoying to residents for miles around. They go off every five minutes. The fences and guns are the most effective way to deal with animals. Deer can leave a field almost empty of corn. There was one stretch of road that for a mile length you could see hundreds of deer in one farmer’s fields. The fields were next to a large wetland. You would see them as groups of about fifty animals.
Harmonious Discord, what is that address again?
To the OP… nothing. It’s part of the loss of running a farm. Or you get some permits as mentioned above, or encourage hunting on your property.
I’d love to get friendly with such a farmer, and help him with this horrible problem.
This is in my case and on many other farms/ranches around town.
Crows: housemartins and swallows. I allow two broods per season of martins to nest in the barn. The swallows cover the arroyo east of the barn. I’ve seen both types of birds hassling crows.
Deer: human activity and horses. We left the arroyo wild so the deer can travel easily without going near our good grounds.
Rabbits: we have many fox in the area. Also, our cats keep them back, as does the irrigation pipe at least twice per season. 
There’s a farmer near here who also raises flower and vegetables starts for the local stores. I asked him if he had any problems with rabbits. Nope. “How do you keep them off, then?” “Lead shot.”
Some farmers keep a greyhound or two. Greys are literally death on rabbits.
You mean the road with all the deer?
The heard was whittled down and you only see the number that gather in all fields now. The herd was that huge before the DNR started trying to eradicate CWD. Not that that herd was a CWD target. I just know it was before CWD was something to worry about.
Just to echo what others have already said, you don’t do anything, you just live with it. When you are dealing with a 160 acre field of corn or beans you have to figure that there will be some loss to deer along the wooded edges of the field, but that land is usually the least productive. However when just the seed to plant three acres costs $120 and you have the expense of fertilizer and herbicide and equipment and fuel and hired labor, all of which has to be paid from the sale of the crop, you can understand it when some farmers take a little private action in addition to actively hunting in season and encouraging other hunters.
House gardens are constantly prey to critters. Racoons seem to have some special sense to alert them when the sweet corn is just a day or two from ready. There is a fatal cocktail for racoons, however – flat Pepsi and yellow fly bait. Sometimes you find them dead with their heads still in the pan, sometimes they move off a step or two. You don’t want to do that, however, if you have pets around. Next best protection for the house garden is to keep a dog. It may not catch anything but it will run them off.