Trends in timing of spring migration along the Pacific Flyway by Western Sandpipers and Dunlins

Long-distant migrants timing their arrival on the breeding grounds face a tradeoff between optimal timing for breeding and optimal timing for survival. For many shorebird species, the flyway northward spans thousands of kilometers, and both conditions encountered en route and priorities of individuals can affect the timing of migration. We used data from spring migration surveys of Western Sandpipers (Calidris mauri) and Pacific Dunlins (Calidris alpina pacifica) along the Pacific Flyway of North America to determine whether the timing of their northward migration changed from 1985 to 2016. We compiled survey data for both species from 6 sites of varying size along the northern portion of the flyway from Washington, USA, through British Columbia, Canada, to Alaska, USA, and estimated interannual trends in the timing of passage through each site. Peak passage dates at the sites closest to the species' breeding grounds in Alaska became later by 1–2 days over the study period, while dates of peak passage at sites farther south became ∼3 days earlier. A post hoc analysis suggested that local temperatures affected peak passage dates at most sites, with warmer temperatures related to earlier passage. Discerning patterns of movement by Dunlins at southern sites was complicated by the presence of winter residents. Simulation analyses of sandpiper movement through a stopover site highlighted both length of stay and timing of arrival as important factors shaping peak passage estimates. We suggest that Western Sandpipers appear to be arriving earlier at southern sites and staying longer at larger stopover sites, such as Alaska's Copper River delta. Our methodology generated specific predictions of peak passage dates on northward migration that may be useful in other systems for which historical count data are available.