Your observations mirror mine. Turned 12 in '72. My Dad raised and hunted English setters and our area of Franklin Co. was loaded with wild birds. Their demise was very sudden. Habitat was the same, predators the same....there was one BIG difference (or two)….no-till farming. Instead of corn fields having fox tail grass growing in them and being knocked over by the picker about knee high then plowed under in the spring, they were now sprayed with round-up, seed drilled and when harvested, the whole stalk was taken leaving only stubble and no cover. Also, that was the time (as RSB pointed out) that they started cutting hay very early in the spring.
In my mind, those two farming practice changes were what caused the complete demise of our wild pheasants.
I grew up on a farm in SE PA and we had loads of wild pheasants, and so did many farms in our area.
I agree with most of your points. The downfall was very sudden. It was a CRASH. The numbers went from loads of pheasants to very few to none in a short while.
The landscape was changing, but it did not change that fast.
What you described about the corn is correct.
IMHO, the pheasants' main habitat was the corn fields. There were weeds between the corn which provided seeds to eat, and the bugs that lived on those weeds.
And even after the corn was picked there was enough stubble and corn stalks left to provide cover.
We did not use Roundup. We cultivated for weeds when the corn was still small. But that still left plenty of weeds in between the corn plants.
By summer time the corn got too tall to cultivate, so the weeds carpeted the ground under the corn. It was good habitat for pheasants.
The Roundup killing all the weeds under the corn is the main cause of the pheasants demise, IMHO.
Regarding early hay cutting. We were lucky to have a mild climate and well drained soil so we did 4 cuttings of hay most years, and one year we did 5. (In most of PA, 3 cuttings is typical.)
So we cut hay early and often. And yet we had pheasants galore. And also lots of rabbits.
We used a sickle bar mower on an old, small, SLOW Ford tractor, so that probably allowed many pheasants and rabbits to escape, that would not escape the haying machinery of today, which runs much faster.