Breeding Ecology and Annual Cycle Adaptations of the Red-Backed Sandpiper (Calidris alpina) in Northern Alaska

Abstract

Ecological and behavioral specializations in the annual cycles of sandpipers of the inclusive genus Calidris nesting in the Arctic reveal adaptive adjustments to the environment, particularly to the shortness of the summer. In the comparative study of this general problem, the breeding ecology of Calidris alpina, was investigated in 5 consecutive summers, 1959 to 1963, in northern Alaska; for the nonbreeding phases of the annual cycle, wintering populations in California were observed from 1960 to 1964. At Barrow, Alaska, latitude 71[degree] 20'' N, the arctic summer lasts from early June until late Aug. Red-backs arrive throughout a 2 week period singly, in small flocks, or in formed pairs, depending on local weather conditions. Self-contained territories of 12 to 17 acres are established on upland tundra. Egg-laying is relatively synchronous, most clutches being completed between 12 and 18 June; later clutches mainly represent attempts at renesting following early-season nest loss. Both sexes incubate, the male''s role increasing as hatching approaches. Hatching occurs primarily in early to mid-July, after which the precocial young, accompanied by their parents, move into lowland marshes. Growth of young proceeds rapidly; the immatures fly within 3 weeks of hatching. Survival of young to fledging is relatively high but varies in different seasons, with weather and predation being the major mortality factors. In late July and Aug young red-backs move to coastal or lake-margin habitats, and adults flock to uplands; both depart by late Aug. or early Sept. In fall red-backs apparently stay in the Arctic or sub-Arctic until mid- to late Sept., drifting slowly across the tundra or along the coast. Their arrival in California winter quarters occurs in late Sept. and early Oct., reaching a peak on San Francisco Bay in late Oct. and Nov. Numbers then remain relatively constant until late Jan., after which the population declines. Spring migration probably consists of a gradual northward shift of the wintering population, although this is not well understood. Postnuptial molt of C. alpina is compressed into the short summer at high latitudes, beginning in mid-June concurrently with egg-laying and terminating by mid-Sept. before the species reaches its coastal North Temperate wintering quarters; most other Calidris sandpipers breeding at Barrow molt after the fall migaration. Although food supply in terms of total available biomass is higher in June and Aug. than in midsummer, the number and variety of surface insects required as food by precocial young are maximal in mid-July when young sandpipers are growing most rapidly.