"An often-fatal virus called psittacine beak and feather disease has also taken a toll. The virus’s origins are debated, but research suggests wild parrots may have caught it from captive birds kept in aviaries. "
The article states this about B&F disease (and of course blames it immediately on humans). I've read several articles about the fate of the Cape Parrots and the Beak and Feather disease is rampant, just like among the wild cockatoos in Australia, never have I seen deforestation as a cause of their decline, before good old Natioal Geographic.