Quail declined because times changed. I doubt 150 years ago if there were many more quail than there are today. Post civil small farms, 40 acres with a mule, with hedge rows and edge cover became the norm. Birds habitat peaked well before farm chemicals hit their stride. Ask old timers and you will hear by the 50's numbers were going down. At the same time small farms and small fields were being merged into larger ones.

Cover declined as crop changed. Dairy declined, pastures were plowed, ditches cleaned out or made deeper with no tree borders. Fast forward to today, fields are 40 acres and larger not entire farms of 40 acres split into ten or more fields. Mono culture crops, weed and insect control is a must. Small farmers are big investments and large farmers mid size corporations.

When I was a boy deer were fairly rare and turkeys were not to be found. Now you can see fifty to a hundred deer any day and turkeys are very common. Quail are down but other game has moved into their place taking advantage of the void.