Lots of 20+ covey days in SE KS in the 80's and 90's. Then one opening day I showed up and my farmer friend had a D6 parked next to a big brush pile that used to be a 1/2 mile hedge row. That continued until he ran out of hedgerows.
Additionally the corn, beans, fence lines were weedy. Seed tech made everyone a clean farmer. Milo turned into corn. As the px of farm ground, equipment, inputs went up, economies of scale kicked in and the bigger neighbor bought smaller neighbor, again and again.
Then the cows showed up, followed by the fescue.
I still hunt down there, but 1-2 covey a day is the norm for me and oftentimes don't shoot because it's a 5-6 bird covey.
Man...I had some good dogs back then!