| Summary: | Migratory birds are experiencing widespread global declines. Shorebirds appear to be experiencing some of the most dramatic declining population trends, with the long-distance, Arctic-breeding migrants apparently losing ground the fastest. Challenges arise in monitoring these populations as they undertake impressive circumpolar migrations between breeding and overwintering areas. The ability to trace individual-level movements and assess the consequences of those decisions is critical as migratory connectivity is known to influence population demographics. I used both stable isotopes (δ 2H, δ13C, δ 15N) in winter-grown feathers and the Motus Wildlife Tracking System to track a northward migratory population of Sanderling (Calidris alba) in the Western Hemisphere. I examined geographic variation in feather isotope values from a northern population of Sanderling staging at Chaplin Lake, Saskatchewan over seven years and compared these to feathers collected at three disparate wintering locations separated by approximately 4000 km to classify individuals based on their overwintering moult origin. Feather isotope values in northward migrants were expansive (δ2H: −0.08 to −98.2; δ13C: −7.4 to −25.3‰; δ 15N: 5.9 to 24.5‰) and differed across known-origin wintering latitudes (27oN to 42oS). The range in δ2H values in birds captured in the Gulf of Mexico (GOM) was wider (δ2H: −1.3 to −98.2‰) compared to Peru (δ2H: −17.7 to −58.8‰) or Chile (δ2H: −46.0 to −77.1‰). Using discriminant function analysis, 85% of the sample population at Chaplin Lake could be classified to one overwintering area while the origins of 15% remained unknown. The majority (70%; range 52-70% across years) were assigned to the GOM, suggesting this is an important overwintering area for Midcontinental migrants. Northward departures of nanotagged Sanderling (Chile n = 23; Peru n = 11, GOM = NA) were primarily influenced by site with individuals from Chile departing on average 18 days earlier than those in Peru, but with no detectable differences in ...
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