As I said to Dr Swamidass, it was my understanding that in the benign state they were not mutating as much, though I don’t understand all the reasons. This is an interesting topic for sure.
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.