Habitat has a whole lot to do with the numbers on any game bird. I own a 400 acre tract of land in AL. My father bought the first parcel in 1965 and I have been able to add other parcels through the years. We had a good turkey and quail population the first 15 years.
A big section of it was clearcut in 1979 and replanted in loblolly pine. The quail thrived until the pines got about 10' tall, and by 1985 they were gone. I haven't shot a single one on the place since then. We got a few turkeys around the edges for a few years after it was cut, but from 1983 til 1990 we didn't get a turkey either.
We started killing turkeys again in 1991 and have killed at least one on the place every year since. Once the pines got tall enough that the ground under them was fairly open, the turkeys moved back in, but the quail never have.
I have managed the land much differently than the way it used to be. I've got 50 acres of open land and 80 acres in longleaf pine that is burned often and looks more like a field than a forest. I burn all the rest of the timber land on a 3 year rotation, providing every kind of cover that game birds should need.
It has worked very well for the turkeys. We have taken a total of 10 gobblers off it in the past 2 seasons, and that's the most ever in the more than 50 years we've had it. But the quail still aren't there, even though it looks like great quail habitat. The land that surrounds is paper company land and poor quail habitat. My theory is that they just can't survive in isolation on small tracts. There was a lot of cutover land surrounding us in 1980, and we had 5 or 6 coveys; now I don't think I have a single one.
I don't believe that the presence of turkeys on the place is in any way keeping us from having quail. There are large areas 100 miles to the south where both are doing well on the same properties.
It may be different with grouse, but this is my experience with habitat change being the primary driver of population change of our game birds.
