Thanks beskeptical, it sounds like you have a great deal of knowledge about this and are good at communicating it clearly. The thing I still don't understand is, if a virus that already existed in birds but wasn't so deadly can mutate to one that is deadly for birds, then why do human pandemics have to start with birds? If a bird pandemic can appear in birds all by themselves, why would it not be easier to get human pandemics from humans all by themselves?