Keep looking, they are really in there.
On one side of the lane to my house is a woodland I have restored. Under portions of that woodland are some pretty thick gooseberry thickets. I never try and walk through it, always around it. On any given sunny afternoon, I can normally count on seeing a covey of quail in the thicket closest to the lane. If we happen to find the quail on a walk or while in the timber, they rarely flush. If they do flush it is just a quick hop into the air before they go back down in the gooseberries further down the hill. Normally they just run away under the shrubs.
While you ponder the number of quail in this shrub thicket, consider that quail rely on dense shrubs every day. Without shrubs, the chances of having a sustainable quail population is slim. Like my gooseberry thicket, your shrubby cover should be dense enough that you don’t want to attempt walking through it. Yet it needs to be open enough that they can flush up through the canopy. It should be free of ground cover such as fescue or brome. It should not be overtopped by larger trees that can foil their escape attempts when they flush up out of the shrubs. Research in Missouri shows that quail stay within 75 feet of shrubby cover during the winter months. Texas research showed that quail will seldom use shrubs that are overtopped by larger trees. I think my quail read that study, because they adhere to that average!
Looking at the picture, do you wonder that quail like this type of cover? No self-respecting hawk or owl will attempt to enter it. It will greatly slow a fox or housecat enough to let the birds get away. It is perfect for them; it has no fescue or brome understory, it is still open enough for them to flush out of, and my woodland has few trees overtopping the shrubs. When quail flush, they can fly right over shrubs yet under the canopy of the taller woodland trees.
If you no longer see quail in the haunts they frequented years ago, is it because this type of cover is gone? If it is still there, has it been invaded by fescue or brome? Is it overtopped by trees?
Okay, there are four quail in the picture. Three of them are in the upper left portion of the photo. The other is about in the middle. Probably eight to 10 others were not captured in the photo, but were in the thicket, too. My point in posting the picture is to give you an idea of what quail need every single day. We refer to these areas as covey headquarters. Just like you need a home with a kitchen, bedroom and a place to relax, quail need shrubs. Period!