Pheasants can seem almost armor plated when hit going away. There's a lot of bone and feathers to penetrate to reach anything vital if you don't hit the head. There's also a gizzard in the way. I agree that quail aren't that hard to kill going away, although I think most people who've hunted both bobs and scalies would say that the latter seem to take more killing. Not sure why. Maybe the average distance at which they're shot is longer.