Three decades of western sandpiper stopover dynamics during northward migration on the Pacific Coast of North America, 1992–2022

Shorebirds typically spend more time fueling and refueling during migration than they spend in actual flight, and consequently their stopover ecology has important implications for fitness and conservation. We examined spatial variation and long‐term changes in stopover duration of radio‐tagged western sandpipers Calidris mauri over three decades on the Pacific Coast of North America. Birds were monitored during 10 periods of northward migration at four key stopover regions including: San Francisco Bay, CA; Willapa Bay/Grays Harbor, WA; Vancouver Island/Fraser River Delta, BC; and Copper River Delta, AK. We used Cormack–Jolly–Seber models to estimate the daily probability of staying at a stopover site (φ) corrected for daily detection rates ( p ) in relation to annual and seasonal variation. Daily probabilities of staying were then used to calculate length of stay (LOS) in days. Mean LOS decreased at stopover sites as birds progressed northward and varied from 23.1 days at San Francisco Bay to 3.6 days at the Copper River Delta. The duration of length of stay did not show a strong temporal trend by year or decade from 1992 to 2022. Effects of covariates on LOS varied by stopover site, with locally tagged birds staying 2 to 4 days longer than birds tagged farther south, females staying longer than males, and LOS being negatively correlated with mass of the bird at the time of capture with lighter birds having a longer LOS. We suggest that western sandpipers on northward migration may be limited in their ability to reduce LOS beyond the current duration over much of northern parts of the Pacific Flyway. Covariate effects demonstrate that stopover decisions of individual birds are impacted by their energy reserves, but stable lengths of stay over three decades suggest that western sandpipers may have limited flexibility to cope with environmental change in the northern part of their migration.