I don't think there is any doubt that if your goal is to be a great bird shooter, you had better hone your skills on a lot of wild birds. Endless assortment of subtle angle as mentioned by Onpoint, is sure one reason, followed by the fact that targets slow down fast, as they go, wild birds accelerate, and you have to deal with conditions of the hunt, brambles, obstructions, uneven ground, as well as simple conditioning, huffing across the uplands and being ready for a shot from any direction, at anytime. I am convinced that this is the reason so many "famous" trick shooters routinely look ordinary, to put it kindly, missing pheasants on outdoor shows, but can shoot 7 quarters thrown in the air off the cuff. Even if not ideal target shooting is worthwhile for upland shooters, as is almost any shooting, to develop maintain and sharpen the reflexes, while re-enforcing mechanics. Great wild bird shooters have simply had experience with a whole lot of wild birds. I speak this as another upland bird shooting bum, for most of my late teens, 20's, and 30's. My hunting partner in 1978, delayed surgery for a torn retina in his non-dominant eye, risking further damage, bleeding, and sight loss, because it would have interferred with the sharptail opener. We limited all three days and the surgery was successful, enboldening, rewarding us for our bad and reckless behavior! What's the old saying? God loves and protects children and idiots!