I don't sense that turkeys are that common in the Grasslands or middle of the San Joaquin Valley are they? There aren't the riparian corridors that exist north of Sacramento allowing them to move from the foothills out into the valley. I've busted up flocks of 20+ turkeys on Gray Lodge for example.
There is a statewide decline for sure but on the public areas in the Sacramento Valley there is actually enough good habitat for pheasants to have a larger population than currently exists. There is virtually nothing on Gray Lodge or Upper Butte Basin except for birds that are planted or planters that wander onto the areas.
The class C area I described has no agricultural or mosquito abatement activity on it, is miles long and 1/4 mile wide, has great cover and water year around, located in the heart of pheasant country with a historic population of pheasants and the only thing that has changed is that turkeys have established themselves there. No pheasants.
All raptors are not bird eaters and even the ones that are struggle with full grown pheasant sized birds. I once saw a Northern Harrier glide in from a roosters blind side and try to turn himself inside out getting away from the pheasant when it turned around and confronted the raptor. They eat them but you don't see much evidence in the field.
Turkeys aren't the only reason but they are a significant part of the puzzle.
That is an interesting observation about how the turkeys expand their distribution. There is plenty of riparian habitat with trees in the San Joaquin corridor, but it is a no man's land of barren farm fields to either side. So if your argument is correct, they can't get to the trees along the rivers and sloughs of that drainage unless someone plants them.
I have watched the turkey population explode here in the Santa Cruz Mountains, along with the wild boar population over the last 40 years. Life is good for non-natives that don't really have many predators, but plenty of food, water and cover.
If I understand you correctly, then my idea of checking out the patchwork quilt of Sacramento NWR areas along the river will likely result in seeing only turkeys and rabbits, and I am unlikely to see any pheasants, since the walk-in areas are predominantly riparian vegetation of trees and vines.
I see the opposite problem in North Dakota, probably because of the lack of trees and the nasty winters there. The turkeys are struggling there compared to the pheasants, probably because it is mostly grassland and farmland with very few trees. Plus the pheasants are tough bastards and also hunker down in the tules when the days get cold and snowy.