Assessing migratory connectivity and genetic population structure in the Upland Sandpiper

Effective conservation of long-distance migratory shorebirds requires a comprehensive understanding of population structure and movement throughout the annual cycle. The grassland-dependent Upland Sandpiper (Bartramia longicauda) is a species of conservation concern, threatened by habitat loss, agrochemical exposure, and the cumulative effects of disturbance throughout migration. However, limited knowledge of migratory connectivity and genetic structure constrains targeted conservation efforts. In this study, I aimed to determine (i) the strength of connections between breeding and nonbreeding Upland Sandpiper populations (i.e., migratory connectivity) and (ii) the genetic distinctiveness of Upland Sandpiper populations across the breeding distribution. To test the hypothesis of weak connectivity, I analyzed satellite transmitter data from individuals tagged in six populations across the North American breeding range between 2021 and 2024: Delta Junction, Alaska; Whitehorse, Yukon; South Allan, Saskatchewan; Saint-Marc-des-Carrières, Quebec; Utopia, New Brunswick; and Deblois, Maine. Results indicate scale-dependent spatial separation on the nonbreeding grounds. Western (Alaska, Yukon; n = 18) and central (Saskatchewan; n = 9) breeding regions exhibited overlapping migratory routes through the Midcontinental Flyway to shared nonbreeding areas in Argentina and Uruguay. In contrast, individuals from the eastern breeding region (Quebec, New Brunswick, Maine; n = 15) undertook significantly shorter, nonstop flights across the Atlantic Ocean to nonbreeding regions in northern South America and central Brazil—areas almost entirely distinct from those used by other populations. This combination of convergence and isolation produced a moderate to strong overall migratory connectivity value (0.328 ± 0.305 including a Saskatchewan-breeding outlier; 0.724 ± 0.002 excluding the outlier), with regional pairwise comparisons varying significantly (-0.035-0.980). DdRAD sequencing of blood samples (n = 121) revealed subtle but significant neutral genetic structure, with a primary east-west division separating a western-breeding population (Alaska, Yukon) from those breeding from Saskatchewan eastward. Pairwise FST values ranged from 0.028 to 0.128 among breeding regions and from 0.035 to 0.061 among broad regional groupings (West, Central, East). Despite this structure, isolation-by-distance was weak (Mantel r = 0.139, p < 0.01), as fine-scale differentiation among eastern populations exceeded expectations based on geographic distance alone. Bayesian STRUCTURE analyses identified two primary genetic lineages, with additional substructure detected within eastern populations. As the first study to assess both migratory connectivity and genetic structure in Upland Sandpipers at a continental scale, this research provides key insights into the species’ natural history, revealing a species with regionally distinct migratory pathways that do not align with patterns of genetic differentiation. This decoupling of genetic and spatial structure indicates that western and eastern populations face fundamentally different conservation challenges across the annual cycle. Although genetic and spatial divergence alone do not warrant designation of separate Designatable Units, the combination of demographic isolation, distinct threat profiles, and contrasting population trajectories necessitates region-specific management strategies—particularly for rapidly declining peripheral populations in Alaska, Yukon, and Atlantic Canada. Further research in understudied nonbreeding areas of northern South America and Brazil is critical to fully assess population-specific vulnerabilities.