Discriminating Overwintering Origins of a Long-Distance Migratory Shorebird Staging in the Midcontinent Flyway

Long-distance migratory shorebird populations are experiencing widespread global declines. However, challenges exist in accurately tracing full hemispheric movements throughout the annual cycle, which has implications for monitoring population dynamics through time. Measurements of naturally occurring stable isotope abundance in avian tissues are an effective means to infer movement and migration, particularly when mark-recapture approaches are not feasible. We analyzed stable isotopes (δ2H, δ13C, δ15N) of winter-grown flight feathers of Sanderling (Calidris alba) collected from (a) individuals of known wintering origin throughout North and South America and (b) migrants of unknown origin captured at a major northern staging site at Chaplin Lake, Saskatchewan, Canada, in the Midcontinent flyway (2012-2020). The goal was to classify overwintering locations of the migrant population and assess whether winter origin was associated with morphometrics on the northern staging grounds. Feathers sampled from known-origin birds wintering at three latitudinally distinct sites (coastal Texas, USA; Paracas, Peru; Chiloé Island, Chile) were isotopically distinct but showed overlapping isotope values with Chaplin Lake staging migrants. Chaplin Lake migrant Sanderlings were separated into five unique isotopic clusters representing potential distinct wintering populations and the relative proportions of migrants from each cluster varied among years. Isotopic clusters were associated with wing length and probable migration distance. Discriminant function analysis classified the majority (70%) of Sanderling captured at Chaplin Lake with winter origins in the US coastal Texas region, while another 8% likely wintered in coastal Peru, 6% in coastal Chile, and 15% were of unknown winter origin. Band resights of overwintering birds throughout the Americas further validated isotopic assignments. With increased knowledge of range-wide isotopic differences in winter-grown feathers, wing morphology, and moult schedules, feather isotopic assignment of wintering origin is a powerful tool to apply throughout flyways to better understand the connectivity of distinct shorebird populations and complement population monitoring efforts