The Semipalmated Sandpiper (Calidris pusilla) is a small shorebird, most commonly seen on migration along the coastlines of the eastern United States. It was historically one of the most widespread and numerous shorebird species in the Western Hemisphere, breeding across the North American Arctic tundra, but major population declines have been documented in the core of the nonbreeding range in northern South America. Breeding populations have also declined in the eastern North American Arctic, but appear to be stable or increasing in the central and western Arctic. To help understand what is causing the declines and work toward conservation of this species, we set out to track migration routes and stopover sites using light-level geolocators, a relatively new technology which determines the bird’s position on earth by measuring the length and timing of daylight throughout the year. The major challenge to use these tags is that you have to catch the bird once to put on the geolocator, and then you have to recapture the same bird the next year to retrieve it, which requires finding the same bird again in the vast arctic tundra. Luckily, they tend to return to the same breeding areas the next year.
Our large group of 18 partner organizations worked collaboratively to carry out the study across the entire North American Arctic from Nome to Hudson Bay, and we attached 250 geolocators to birds by mounting expeditions to 8 different field sites. Our field crews faced challenging conditions, working in the Arctic where the weather is always unpredictable and where both Grizzly and Polar Bears regularly visit field sites. We repeated expeditions the next year to each site, and recovered 59 of the units by recapturing birds. The treasure trove of data showed migration routes and stopover sites from the entire year in the life of each bird, and showed that birds breeding in the eastern Arctic wintered in northeastern South America. Birds from eastern Alaska and far western Canada wintered from Venezuela to French Guiana. Central Alaskan breeders wintered across a very wide range from Ecuador to French Guiana. Birds that bred in western Alaska wintered mainly on the west coasts of Central America and northwestern South America, outside the nonbreeding region in which population declines have been observed.
Our results confirm that Semipalmated Sandpipers that breed in the eastern Arctic and use the Atlantic Flyway also use the areas in South America where population declines have been detected, suggesting that declines may be concentrated in populations along the Atlantic Flyway and in the eastern Arctic. However, because some birds from sites as far west as Barrow also used the areas in northeastern South America where declines have occurred, further work is needed to localize the geographic areas used by declining populations, and therefore the potential causes for the declines. We identified several new stopover and wintering areas, where implementing conservation actions to preserve the habitats used by Semipalmated Sandpipers could contribute to protecting the species. We measured a larger impact of geolocators on return rates than has been observed for larger shorebirds, indicating that caution should be used when working with small shorebirds, and that potential new information gains from additional geolocator studies should be weighed against expected impacts on individual survival. Our data also provided new insights about how long birds stay at migration stopover sites, which will be useful to studies that measure and monitor the total size of populations using these sites. Understanding the connections between breeding, migration, and wintering areas for these populations of a widespread yet declining shorebird can help future studies to identify the causes of declines and ensure the effectiveness of targeted conservation efforts.