Inter-species transmission dynamics
Anseriformes and Charadriiformes were responsible for 59% and 41% of IAV introductions to Iceland, respectively (Supplementary figure 5). Local host-specific phylodynamic analyses within Iceland demonstrated that ducks (BF= 6.05) and shorebirds (BF= 5.57) drive transmission as sources of IAVs to gulls, while inter-species virus exchange occurs bi-directionally between shorebirds and ducks (shorebirds to ducks, BF= 5.16; ducks to shorebirds, BF= 3.27) (Figure 4a). Although geese in Iceland carry IAVs, there is no quantifiable inter-species transmission between geese and other species, which suggests a limited interface for geese-derived transmission to occur in Iceland. The proportion of time viruses are hosted by a particular species, as derived from host-specific Markov rewards analyses, revealed that gulls host IAVs for the greatest proportion of time in Iceland (62.5% versus 22.8% (geese), 8.4% (shorebirds), 6.3% (ducks)) (Supplementary Table 3b). This differs from global analyses where Anseriformes host IAVs for the greatest proportion of time as compared with other host orders (Supplementary Table 3c). Additionally, Anseriformes are the most significant drivers of transmission to other avian orders globally, including to Charadriiformes (BF= 29,353) and Galliformes (BF= 29,353) (Supplementary figure 6). Median diffusion rates, a measure of how quickly hosts move viruses in space and time, were similar among shorebirds (577 km/year (95% HPD= 36.7-1844.2)), ducks (604 km/year (95% HPD= 20.1-2768.6)), gulls (483 km/year (95% HPD= 286-734.9)), and geese (386 km/year (95% HPD= 194.6-640.9)) within Iceland, with some extreme outliers among ducks and shorebirds (Figure 4b).