A biologically informed threshold for distinguishing between modern and historical migration patterns of northern pintail ducks

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Brooke L. Berger, Graduate Degree Program in Ecology and Department of Biology, Colorado State University, Fort Collins, CO, Andy Ramey, Alaska Science Center, US Geological Survey, Anchorage, AK, Ryan S. Miller, Center for Epidemiology and Animal Health, USDA-APHIS, Veterinary Services, Fort Collins, CO and Colleen T. Webb, Department of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO

Background/Question/Methods
Waterfowl are the primary reservoir of influenza A viruses (AIV) and can move Eurasian lineages of the virus between Asia and North America during long distance migrations. In order to assess the risk of spatial transmission between continents we need to characterize movement patterns of bird hosts. Northern pintail ducks (Anas acuta) commonly migrate between Asia and North America and are likely sources of Eurasian AIV. Mark-recapture data from the USGS Bird Banding Laboratory offers a wealth of information on bird movement spanning 106 years, but due to shifting climate and breeding habitat loss, historical migration patterns have likely changed. In North America, annual heterogeneity of spring pintail migration paths are largely driven by the availability of wetland breeding habitat in the Prairie Pothole Region. We hypothesize that agricultural activity, which decreased available breeding habitat in this area in the mid-1970’s, and subsequent restoration efforts in the mid-1980’s, caused a shift in large scale migration patterns of these ducks that persists today. To investigate if there is a difference between historical and modern movement patterns we used banding data for all of North America to construct weighted, directed migration networks. In these networks nodes are defined as sub-regional watersheds and edges are the number of birds moving between them. The “historical” network included observations from 1914-1984, and the “modern” network included observations from 1985-2019. To identify distinct migratory populations of pintail, we used a community detection algorithm developed by Buhnerkempe et al. (2016), which identifies groupings within a network by maximizing modularity. To ensure that differences in detected communities were not due to different sample sizes, we took subsamples of historical band records, and generated ten networks with the same number of records as the modern network.
Results/Conclusions
All ten subsampled historical networks showed similar migratory communities of pintail within North America, notably identifying three distinct groupings on the West Coast. In contrast, the modern network grouped these three communities into one large West Coast community. These results suggest that modern migration patterns differ significantly from historical patterns, and that the mid-1980’s is a biologically relevant threshold to distinguish between the two. Additionally, long term land use change and wetland availability in the Prairie Pothole Region is a possible driver of this shift. Researchers interested in characterizing large-scale spatial transmission of AIV in North America should consider how environmental factors that influence the migration patterns of hosts, can impact transmission dynamics.