Ted,Mike,
I don't have a dog in this fight, but it's been my observation that Ted is correct with the following caveat. Changing land use is more damaging than anything else in my area.When the open pastures and fields with the edges created by the terraces,fencelines,and creeks/branches gave way to big pine tree "orchards"(plantations), the birds went away.What really happened, I think,was it took a much larger area of this kind of land to support a covey.However, when the trees get larger and are thinned a couple of times,the land can be agressively managed for birds.This can be pretty expensive and be beyond the means of many hunters.
Mike