I wouldn't claim to know what has caused the quail decline in the southeast, but my opinion is that it's habitat related. And habitat includes all the factors like invasive species such as coyotes and fire ants, and the switch to herbicides by the forestry industry.
We had quail in central AL when I was growing up in the 50s and 60s, but not in great numbers. I can remember finding maybe 2 coveys on a typical half day hunt. Our population was at it's highest in the late 70s when much of the forest land was clearcut and then site prepped with dozers. There were days back then when killing a limit of 10 birds in a day was a reasonable goal. When the pine trees got to about their 3rd year, that was the end of that tract being good quail habitat and for the most part it never was again. The next forest was site prepped with herbicides, and it wasn't as good for the quail.
Some things were the same as they are now, and we still had quail. We had fire ants for as long as I can remember, and the quail lived with them. There was very little agriculture in the area, but the quail still thrived. We hunted them, but that didn't affect the population.
But some things were different than today. Nobody site preps with dozers now. I'm looking to get 40 acres site prepped for planting longleaf this winter and I can't find anyone who still has a root rake. It's all done with herbicides, and I don't think that helps the quail.
I caught a coyote in a trap in 1980 and that was the first one I'd ever seen. Now they are everywhere and that's a change in the habitat. We definitely have more hawks these days, though there were some around in the 70s and the quail survived anyway.
I think all of these things work together to reduce the quail population. We still have some, and I hear them whistling in the summer, but I don't have enough to hunt them. I wouldn't have thought this would be true, but it doesn't seem that quail can thrive in small habitat areas of a few hundred acres. I can grow turkeys on my 400 acres, and actually have more of them than ever before. But quail seem to need a block of thousands of acres to do well. I have read this in research reports, and I have also experienced it trying to manage our land. They apparently move around a lot more than I expected.
The coyotes and hawks are not the only predators who have increased. The nest predators like coons and possums are a lot more abundant than they were back in the 70s. Stan wasn't the only one trying to catch and sell fur back then. I had 2 small kids and my wife didn't have a job, so the extra money I could make from trapping was very helpful. A couple of cats, an otter, and a big coon made this a very good day. Good hunting to all of you who still have quail to hunt!