A similar thing happened to the pheasant population in Northampton Co, during the 70s . Within just two years our wild breeding population was wiped out and all we had left was a few stocked birds. The crash was not due to changing farming practices as the PGC claimed because the populations in residential areas were also wiped out. The only thing that could wipe out an entire population over a large area with diverse habitats is a bacteria or a virus.