Shore birds are important trophic components of tundra communities during summer months, preying chiefly on insects. The Red—backed Sandpiper (C. alpina), studied in five seasons near Barrow, Alaska, 71°N, feeds largely on larvae and adults of the families Tipulidae and Chironomidae (Diptera). Sampling for availability and abundance of insect populations shows that the food source readily available and heavily used in early summer and again in late summer consists of tipulid larvae; as summer predation proceeds, important changes in numbers, and availability of different food species result from progression of their life—cycle stages and from changes in environmental conditions. Food supply and diet are most varied in July. Although feeding behavior and diet of redbacks change with prey availability, preferences are shown by adult sandpipers for tipulid larvae in early and late summer and to a lesser extent for chironomid larvae in midseason. Recently hatched young feed entirely on small—sized adult insects, mostly chironomid flies, which are obtained easily on the tundra surface. Differences between adult and young redbacks in habitat and food selection during late summer represent an intrapopulational means of apportioning food supply in a critical part of the season. With arctic insect faunas depauperate in variety of taxa, there is a limited diversity of food species for sandpipers. Adverse weather conditions characteristic of high—latitude climates can cause local food shortages. Interacting with the timing of generation replacement of important insect prey species, adverse weather promotes the widest variations in food conditions in July when young sandpipers are hatching and growing.