That's interesting. I scout a lot and cover a lot of ground, but I rarely find a kill sight of anything, turkey, deer, or anything. I'm sure it happens. I used to see a few deer kills on the lease I was in. I chalked those up to coyotes. I have heard for years that the biggest problem we have with ground nesting birds is fire ants. Supposedly that is what did in the quail populations.
I think populations ebb and flow over time in spite of human's best efforts to make things go the way we want them too. When I was a kid dove hunting was big. Opening day of dove season sounded like WWIII had broken out. Now, very little. The quail were plentiful too when I was a kid. Not anymore. I see a few coveys a year.