Molt Cycle of the Red-Backed Sandpiper (Calidris alpina) in Western North America

Abstract

Pterylography, molts, and criteria for age determination in C. alpina are described preliminary to the detailed analysis of molt schedule in redback populations along the west coast of North America. The method of scoring molt, modified from that used by Pitelka (1958), is based on the stage of molt in selected flight and body feathers and on relative dry weights of major feather groups. In northern Alaska, the prebasic molt of adults is condensed into the short span of the arctic summer and overlaps with the breeding effort; however, the major portion of energy expenditure on molt comes in late summer when breeding is over and the young sandpipers are fully grown. At arrival in their north temperate winter range during late Sept. and Oct., adults and immatures are in completed basic plumage. Pre-alternate molt occurs between late Feb. and mid May. Redbacks molt during slow phases of migration, but in periods of rapid migration, molt is suspended. The specializations in molt schedule ofC_ alpina relate in part to the fact that this sp., which winters in nortJTtemperate regions, remains in the arctic through the end of summer during which time it exploits tundra food sources in the absence of possible competitors.