Just my opinion and I'm not claiming to be an expert.
I don’t think WNV is the whole story.
The last grouse that I saw in the wooded areas that I regularly ran my dogs in N.E. Ohio was before 1996. WNV supposedly showed up in New York around 2000. I have worked my dogs in that area fairly regularly since the mid 80’s and always saw grouse or signs of grouse.
That last grouse that I saw in that area was about in the mid 90’s. It was a hen with chicks. I first noticed her as she did a fake broken wing routine to try and lead me and my dogs away from her and as I took another step it looked like the brown leaves on the ground were coming alive. There were about a dozen grouse chicks that were all grouped tightly together and began to move when I almost stepped on them. They were about the size of half of my thumb and a would be a perfect meal for the recently introduced invading army of vacuum cleaners of the woods in that area, turkeys.
I expect turkeys are pretty good at surviving and spreading WNV along with other diseases and ticks. I wonder if ticks are likely to spread WNV too. Turkeys very likely eat a lot of the same things grouse eat along with eating and disturbing grouse nests and chicks.
When I started seeing turkeys in Ohio I saw a significant and steady decline in grouse numbers. That goes for every place that I used to grouse hunt in Ohio. That seemed to be in spite of some areas having a good amount of grouse habitat.
I think it might be a good idea for the grouse biologist in PA. to select a region in northern PA., someplace like Elk County, etc.., and allow hunters to be able reduce the turkey population in that region and see how that affects the grouse.
I know there’d be a lot of push back form the Wild Turkey Federation and some turkey hunters but it’s not like the turkeys would go extinct.
California recently did a thorough wild pheasant study in that state and were surprised to find that the presence of turkeys had some negative effects on some pheasant populations. I expect that may be even more profound with grouse.
See; Long‐term and widespread changes in agricultural practices influence ring‐necked pheasant abundance in California
Interestingly, our results indicated a negative relationship between pheasant and wild turkey, another non‐native game bird species to California. This could be an indication that the two species respond differently to the reported land use changes, or could indicate a form of interspecific competition. To our knowledge, competition between these species has not been reported previously and warrants well‐designed experimental study.