LOUSE FLY. Just ran into these on my grouse also. Special when bring them in the house and they warm up. Took me forever to figure out where they were coming from. Andy Vidovic Chisholm Minnesota.Google louse fly ... may be it.
Me tooo I’ve seen white worm 🪱 looking thing in the chest cavity.I have even seen ruffed grouse with white worms underneath when I cleaned them. Usually later in the fall where the birds were plentiful.