Lack of Population Structure of an Extreme Migratory Shorebird: Evidence of Gene Flow Between Geographically Disparate Populations

Gene flow affects the distribution of genetic variation of a species over time and thus can be crucial for a population's persistence and adaptive capacity. Given the importance of gene flow, it is key to understand the connectivity and genetic differentiation between populations of species with small and segregated breeding populations that are facing population declines, such as many long‐distance migratory birds. In this study, we explored population structure in Hudsonian Godwits ( Limosa haemastica ) from two geographically distinct breeding areas in the North American sub‐Arctic and two nonbreeding areas in South America using nuclear microsatellites. Despite being spatially and temporally segregated during most of the annual cycle, our results indicate no evidence of population differentiation between breeding populations, nor clustering between individuals from breeding and nonbreeding populations connected by migration. Considering the phenology of the species, godwits from both breeding populations could co‐occur during southward migration and/or throughout the oversummering period, likely in the Las Pampas ecoregion of Argentina. As with many other long‐distance migratory shorebirds, immature godwits stay in their nonbreeding areas until sexual maturity is reached, during which time they can explore, interact, and follow flocks of adults to different nonbreeding areas, thus increasing the chances of mixing between populations. This highlights the importance of recognizing the key role of early life period within the full life cycle of migratory birds for understanding their demography and evolutionary potential.