Very helpful - thank you! The interesting thing I was thinking about was that if I never brought home any other birds from the outside (and assuming the wind/wild bird avenue is not possible,) it would seemingly be possible to completely prevent the flock from getting infected.
I'm glad you caught on with the possible Marek's disease. My mind went to ways of flock integration but you were spot-on with the age and symptoms. If the new birds she were to get were vaccinated against Marek's, would that be an option in your opinion?