I don't ever recall running into sickly wild birds while hunting, so I'm not sure that WE, in North America should be concerned about it?
Again, I was simply mentioning two separate studies that showed nesting success went up in hens that where void of certain parasites (parasites we have here in the U.S. and abroad--as you know) vs. those that were not.
Could the findings be rigged, faulty, sloppy science, or carelessly performed? Could numbers/tags/collars on birds been marked wrong or crossed over? Could it be that our birds don't have problems with parasites like in other counties? Maybe our strain is immune to such things? Sure. Though I'm a bit doubtful that is the case. But you never know.
We don't know why nesting success goes up in birds that are void of parasites. Again, the theory (just a theory) is that the birds become "gassy" and in turn put more sent in the air, hence, making it easier for predators to pick up on nesting hens. This is despite their ability to become virtually scentless (externally) while on the nest.
Do these studies apply to birds raised in the U.S.? If we didn't have these parasites here in the U.S. I'd say no. But we do have them here and God knows we have a number of issues with predation on the nesting birds (for multiple and some very obvious reasons).
Can pen and wild hens nest successfully with these parasites living in their guts? Absolutely they can. This is particularly true in areas with prime, large amounts of habitat for birds to nest in successfully. There's simply so much space and so many birds predators can't cover all the ground and hit all the nests. Or, in a case with great predator control. This obviously works too.
If all birds were some how miraculously rid of all parasites would we see a jump in nesting success within pen birds (in the wild)? I believe so, but this is just my two cents and we all know where that leads us
P.S.--I have shot wild birds that look very sick. So have others within my group. It's rare, but it is out there. As you said, there is no need for concern within our wild stock. --At least I hope that's the case-- It could be an issue for our wild birds here in Illinois. For now, it's genetics and inbreeding that are being looked into as to why our birds (even in areas with the proper habitat) are not increasing as they where prior to 1996.
God only knows why my friends. I just hope we figure things out soon. I miss the days of flushing a few hundred grouped up wild birds during a late winter hunt around here!