Unknown reasons for the collapse? What a crock o' $h!t.
Change in habitat from farming practices and building, protection given to hawks and owls and huge decline in trapping.
It doesn't take a brain surgeon to figure out why the decline in wild birds. Pheasants Forever is restoring habitat which will help but they'll find also for a truely sustainable wild population there will be a need to have someone in the area to do some trapping. Skunks , Racoons , Fox et al do a number on nests and broods. Here in Monroe and Pike counties pheasant stocking is like ringing a dinner bell for winged and fourlegged preditors.
I've attended field trials and watched hawks come in and take birds out in front of the dog, two guns and 2-4 judges.
Pheasants can take hold if conditions are right. One can look at the Meadowlands from Hackensack to Newark in NJ and see a wild pheasant population that has sustained itself for over 50 years now.