Wild birds make a dog that will work on wild birds.
I get mine on as many wild birds as possible as soon as they are big enough to handle the grass. I want to see what they've got and bring out their desire.
Training? Waiting 'til their old enough? Phooey! It depends on what you call training. IMO every time you're out with your young dog, you're training. Shooting over 'em, obedience, etc., that comes later. Heck, you can teach a cat to whoa. I want my dogs "out there" as soon as possible.
Do I ever use a pen raised bird at a young age? Yes, at some point when their size dictates, I put small pen raised quail with a wing clipped in with a few of the pups and let them maul it (and yes I have pointing dogs).
Formal training begins at a much, much, later date.
Only wild birds teach a dog about wild birds.
For all of you that answered this thread, you (we) aren't telling Shadow anything that he doesn't already know!
Point!