Each state is different. It is something that would have to be brought up with your states game commission. Here in PA, a rimfire is legal for all small game, including birds. I usually pot 2 or 3 pheasant a year with a 22 handgun, on the ground, usually moving. In my opinion, its no less sporting then shooting one with a shotgun on the wing with a 30 inch pellet spread, and just as safe as shooting at a rabbit.
Larry