Driven grouse shooting is a pretty horrendous 'sport'. It's entirely reliant on the illegal persecution of birds of prey including Hen Harriers, Golden Eagles and Peregrines, with the result that those species are much rarer than they would be otherwise.
Grouse aren't reared and released like Pheasants, but they are maintained in far higher numbers than their natural population level by burning moorland on a massive scale and slaughtering all the predators both legally and illegally.
Grouse are also provided with medication to control parasites which build up because they're living in such unnaturally high densities.