If we are talking about partridge/ quail - their population in a given area depends on several factors.
Due to the huge pressure on land, several villages (across India) have had neighboring brush cleared for farm use. The term used here for such land which was left fallow permanently for grazing etc. (usually by the erstwhile feudal lords) is "kaalar". This "kaalar" is now being encroached in many villages (usually) by lower caste farmers, whose encroachments are ignored by the authorities, since it is a sensitive political issue. Anyhow, this drastically reduces "cover" for these birds, which I would imagine is a bit of a handicap for them, especially considering their nesting preferences.
Also, of course the ever present agricultural chemicals (pesticides, insecticides, herbicides, etc. etc.) can't be helping much either.
There is almost no hunting in our area (trust me, I know!), but still even I can observe a perceptible and steady decline in the game bird population! On the other hand, at my maternal uncles place, there is definitely hunting going on - mostly by army officers at the neighboring tank shooting range (the Indian Army is still above the law in certain aspects

)... but still there is no visible decline in numbers - year on year on year!
Before I forget, too much precipitation/ irrigation affects bird populations for certain game (like sand grouse) negatively and areas which were earlier semi-arid and are now irrigated will certainly see a decline in their numbers!
I think I should add here that I am hardly a "know all" on this subject, having been born a generation or two - too late!

(don't want the wrong people to get the wrong ideas

) ...
Cheers!
Abhijeet