Correlates of Annual Stopover Counts in Two Species of Arctic-Breeding Shorebirds: Roles of Local, Breeding, and Climatic Drivers

Abstract

Long-term surveys at stopover sites provide measures of abundance (counts) that are commonly used to assess the status of shorebird populations. We analyzed a 29-year time series of counts of Western Sandpiper (Calidris mauri) and Least Sandpiper (C. minutilla) conducted during southward migration from 1990 to 2018 at a small stopover site on Sidney Island, British Columbia, Canada, and examined correlations between counts and conditions local to the site, on the breeding grounds, and large-scale climatic indices. Annual counts varied from 0 to ∼4,000 birds, and did not show strong long-term trends over the study period. Counts were most strongly associated with conditions on breeding grounds or large-scale climatic indices, rather than local weather variables, suggesting counts of juvenile shorebirds could serve as an index of reproductive success. Further, counts of juveniles and adults were positively correlated in the same year for both Western and Least sandpipers, indicating a common underlying process determining the abundances of the two age classes at this site. Across the 29-year study period, Sidney Island has remained a consistently used and locally important stopover site for both adults and juveniles of both shorebird species.