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.