I've never attributed what I call the "dead bird flying" syndrome to shot size. Rather, at least from my observations, it's a function of where you hit the bird. Quite obviously, if it doesn't drop immediately, you haven't broken a wing. And the ones I've recovered don't show any head wounds.
But it's something I've seen numerous times with pheasants. Little or no visible reaction from the bird to the shot. But I've learned that if you think you hit the bird, you need to keep watching it as long as you can. (Much easier to do on open country birds, like pheasants, than on grouse and woodcock.) People often talk about the "towering" phenomenon, when you hit a bird and it flies straight up, then dies in the air and tumbles back to earth. Haven't seen that often, but I have seen pheasants continue flying as far as 2-300 yards, then just fall from the sky. What I've found with birds that behave in that manner: 1. They are dead, won't move from where they hit the ground; and 2. Their chest cavity is full of blood. Seems to be a function of severing a major blood vessel. Heart keeps pumping, bird keeps flying until it bleeds out, at which point it crashes.
Haven't often seen it with grouse or woodcock, mainly because you lose sight of them sooner. But a few years ago, my partner took a shot at a grouse with his 28ga, thought he'd missed. No feathers. But I told him I thought I'd seen a very slight "jerk" from the bird after the shot. We followed the bird's flight path as best we could. Maybe 150-200 yards farther on, my Brittany went on point. Big log in front of us. Didn't know whether it was the bird we were following or another one, or whether the bird we were following had been hit--so approached at the ready. Diesel vaulted the log, spun around, and pointed back at us from the far side. It was my buddy's grouse, stone dead, lying behind the log. When we cleaned it, we found the chest cavity full of blood--as I have on numerous pheasants. (And most of them have been hit with 6's, or even 5's.)
With grouse and woodcock, our rule is to always follow a bird we've shot at, if you have any notion at all that you may have hit it.
I have had the same experience with bobwhite.
Many times, after the bob we were aiming at in the covey rise is bagged, we have gone two or three hundred yards and the dogs found another dead bird, freshly killed.
Another indication of a hit on otherwise healthy flying bob is a dropped leg. Upon having the bird in hand the leg won't be broken, just loss of hydraulic pressure from the internal bleedout I suppose.
