I agree that weather is a major influence on wildlife, but being from Northern Illinois, and being an avid pheasant hunter for 40 years, here is my experience; Northern Illinois - lots of habitat in the 60's, 70's, and early 80's.
Commercial and residential development, along with strip farming practices, decimated cover in this area. Used to shoot birds on every hunt back in the 70's, started to decline with loss of habitat, and pheasant hunting in this part of the state was over by the late 80's early 90's; Decided we had to try another state during this time, or stop hunting (not an option!). Went to Iowa, in the Grinnell area, hunted all private land, and had great success. This went on for roughly 8 or so years.Then multiple CRP fields started to come out, removing substantial habitat in the areas we were hunting. By the third year the bird numbers were so low we knew we had to make another move. I called Bill Smith with SDFG, as he was the one helping put together the SD Atlas (remember the days when no road numbers were printed!!). He suggested trying the Watertwon area, due to lower hunting pressure; we gave it a try and did not see the numbers we were used to seeing, so after 2 days moved southwest. Found the birds, and have enjoyed great SD hunting for about 12 years. Seeing the amount of habitat loss in early November, and the huge tracts of cattail burn offs occuring was depressing. From the posts read since our trip, this has continued. Again I agree that weather has a huge influence, especially spring nesting and winter kill offs, but to me there was no doubt that the great pheasant numbers we have witnessed in SD was mainly attributable to the awesome habitat available to the birds. I do not want to come off as a Debbie Downer, just passing along what my 40 years of chasing this great game bird has seen first hand. 15 days and counting!