The BSP analyses ( Figure 3E-3H) indicate that the reassortant viruses follow a logistic-like growth curve, which is typical for virus invasion and maintenance, especially in a structured population [24, 25] . In contrast, the group 2 viruses followed a more continuous and relatively slow growth in diversity. There was insufficient data in our samples to definitively discriminate between alternative population growth models and provide narrow confidence intervals for parameter estimates, but our results are suggestive and future sequencing will add to the needed statistical power.