Capture histories for radio-tagged Western Sandpipers at four stopover sites on the Pacific Flyway, 1992 to 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.