Lengths of Stay and Stopover Strategies of Western Sandpipers During Migration at Two Sites in British Columbia, Canada

An accurate estimate of length of stay is necessary to derive passage population size for birds using a migration stopover site. In this study, we used VHF tags and a Motus automated telemetry array to estimate the length of stay of 385 Western Sandpipers (<i>Calidris mauri</i>) migrating through two stopover sites in British Columbia, Canada (Tofino and Fraser River Estuary) over the course of seven migration periods (three northward and four southward) from 2018 to 2021. The average length of stay of Western Sandpipers at the Tofino site on the west coast of Vancouver Island varied from 2 to 6 days and was shorter than the length of stay at the Fraser River Estuary, where the average length of stay varied from 4 to 8 days. At both sites, adult birds had shorter stopovers during southward migration, juvenile birds had longer stopovers compared to adults, and birds with lower mass at capture had longer stopovers. Morphology of adults and juveniles varied between the two sites. Birds captured at Tofino had shorter tarsi, as well as higher mass during southward migration compared to Western Sandpipers captured at the Fraser River Estuary. We also assessed prey availability at the two stopover sites, and we found that invertebrate density was greater in Tofino compared to the Fraser River Estuary during northward migration. Variation in minimum stopover length and morphology between sites suggests that individuals from different overwintering populations may use different routes along the west coast of North America. Western Sandpipers stopping at Tofino have a shorter length of stay during both migration periods and arrive heavier during southward migration, characteristics typical of "hop" migrants who travel shorter distances between stopover sites. Different stopover sites offer a unique set of site characteristics used by birds exhibiting varying migration strategies, highlighting the importance of conserving a diversity of migration stopover locations.