Discriminating overwintering origins of a long-distance migratory shorebird staging in the Midcontinent flyway

Long-distance migratory shorebird populations are experiencing widespread global declines with populations largely being monitored at key migratory staging sites in the Americas. However, data gaps exist in accurately tracing individual winter or breeding origins, which have implications for monitoring population dynamics and species conservation. Stable isotopes, in concert with banding and tracking approaches, can be effective tools to infer animal movement and migration. We analyzed stable isotopes (δ2H, δ13C, δ15N) of winter-grown wing covert feathers of Sanderling (Calidris alba) collected from 121 individuals of known origin captured at three overwintering grounds in the Texas Gulf of Mexico, USA (GOM), Paracas, Peru, and Chiloe Island, Chile as well as 567 migrants of unknown origin captured at a major northern staging site at Chaplin Lake, Saskatchewan, Canada located in the northern Midcontinent flyway over a nine-year period (2012–2020). The objectives were to (1) quantitatively determine if the Chaplin Lake migrant population originates from multiple overwintering locations, (2) assess whether birds from known sites of winter origin show overlap with the Chaplin Lake isotopic clusters, and (3) assess if these isotopic clusters have distinct morphometric and sex-specific distribution patterns. Sanderling captured at Chaplin Lake on northward migration separated into five different isotopic clusters and relative proportions of each cluster varied significantly among years. Isotopic clusters with more negative δ2H feather values were associated with longer wing length, suggesting probable greater migration distance originating from more southern wintering grounds. Discriminant function analysis classified the majority (70%) of Sanderling captured at Chaplin Lake with possible winter origins in GOM region, 8% in coastal Peru, 6% in coastal Chile, and a further 15% were of unknown origin. Band resights from 13 Sanderling overwintering throughout the Americas further validated the isotopic assignment of GOM birds but were less reliable for longer distance migrants. With increased knowledge of range-wide isotopic differences in winter-grown feathers, wing morphology, and moult schedules, feather isotopic assignment of wintering origin may be a powerful tool to monitor shorebird populations sampled at large common staging site aggregations throughout the Western Hemispheric flyways. Understanding the isotopic connectivity of distinct shorebird populations using common staging sites would strongly complement population monitoring efforts.