My experience in Kansas is it is 50% habitat, (change in land use bigger fields less edge-mono culture corps) 30% predator population explosion, 20% disease/weather patterns. I say the last factors of disease and weather as that is what KDWP friends say, but I cannot be sure to what real degree they influence the decline.

Bird hunting populations have declined significantly in the last 15 years, though I am told by friends last fall was slightly improved (I missed last season-deployed)

Michael Dittamo
Topeka, KS