That’s a question I don’t have an answer for. It was my understanding that the mutation rates in the benign state (in the animal host) were not as high as in the pathogenic state. If that is not true, then I personally don’t know the answer. Dr. Carter might, but I know this is a very mysterious topic where more research is needed.
EDIT:
I should have realized this, but Dr Carter did already address this question in the box at the bottom of creation.com/fitness (he wrote that box part!); he is saying that the waterfowl apparently have regulatory mechanisms in place to maintain the integrity of the virus, which is beneficial for them.