Shorebirds adjust spring arrival schedules with variable environmental conditions: Four decades of assessment on the Yukon–Kuskokwim Delta, Alaska

Arctic summers are brief, and there has been strong selection for migratory birds to arrive in Arctic nesting areas as early as possible to time breeding with peak food availability and complete reproduction. The timing of emergence of nesting habitat in spring is, however, extremely variable in the Arctic, and few long-term studies have examined the ability of avian migrants to track spring conditions to assure optimal nesting. Such studies require long-term migration monitoring under variable spring habitat conditions. These conditions were met during our long-term (1977–2008) study of the timing of arrival of shorebirds to their nesting grounds on the central Yukon–Kuskokwim (Y-K) Delta, in western Alaska. Over this period, the timing of arrival on the nesting grounds of 12 species of shorebirds varied significantly, with the Black-bellied Plover (Pluvialis squatarola) generally arriving first (mean arrival of 4 May), the Red Phalarope (Phalaropus fulicarius) usually arriving last (mean arrival of 20 May). The Western Sandpiper (Calidris mauri), Dunlin (Calidris alpina), and Red-necked Phalarope (Phalaropus lobatus), the most common breeding shorebird species we studied, all arrived about the same time each year (7–9 May). From year to year, first arrival of all species varied significantly by more than 2 weeks, but there was no long-term trend in arrival times over the length of our study. Shorebird arrival was highly correlated with the timing of the break-up of ice on the Kashunuk River, which in turn was correlated with decreasing snow cover and increasing ambient temperature. The date of break-up of river ice also varied by year but did not advance significantly during our study. After arriving on the breeding grounds, Arctic-nesting shorebirds rely on local food resources, which on the Y-K Delta they achieve by timing their arrival to coincide with availability of snow-free habitat. Temperatures along the terminal portions of the spring migration route were significantly correlated with both conditions on the breeding ground and the timing of shorebirds’ arrival there, which suggests that shorebirds may use environmental cues during spring migration to regulate its pace. Ours is one of the few multi-decadal studies to evaluate the responses of a suite of migrant species to annual variation in conditions in their Arctic breeding habitat. Shorebirds’ adaptations to variable conditions on the Y-K Delta are probably rooted in frequent changes to the landscape since the last glacial maximum. Such inherent flexibility may serve them well under future scenarios associated with a changing climate.