I would suggest that the problem is the much higher number of predators, especially avian predators than there were 30,40 or 50 years ago.
It's hawks. No DDT, number of raptors has increased how much exactly? As a kid in the 60s we would rarely see one hawk in a week and when we did it was a big deal.
Now hawks are everywhere and they do a hell of a number on bobwhite. I've watched a hawk work a covey of quail over a 5 day period and kill every single bird in the covey.
Fewer birds just means we are going back to a more natural equilibrium in some places that have all the other requirements. In others habitat won't allow bird numbers at all. Just the way it is I guess.