The Detritus-Based Trophic System 449 



scarcity may occur even during mid-July; however, such periods prob- 

 ably influence all habitats alike. Striking year-to-year differences in 

 growth rates and survival of wader young appear to be closely related to 

 weather conditions during this mid-summer period (Norton 1973, Myers 

 and Pitelka 1979). Interspecific overlap in diet is greatest during this peri- 

 od of the season (Holmes and Pitelka 1968). 



In August, after the period of adult insect abundance, dunlin and 

 longspurs move back to upland tundra. Longspurs take large numbers of 

 sawfly (Tenthredinidae) larvae and seeds, both items that are little used 

 by the wader species. Dunlin feed on Tipula larvae and, if drying and ex- 

 posure of pond margin sediments permit, on midge (Chironomidae) lar- 

 vae. Holmes (1966) noted a segregation between adult and immature 

 dunlin, the immatures making greater use of coastal, brackish lagoons. 



In semipalmated and Baird's sandpipers the two adults share in in 

 cubation; however, females (occasionally males in Calidris bairdii) 

 depart at or soon after the time of hatching, leaving only one adult to ac- 

 company the young. Adults, and then immatures, move to coastal la- 

 goons as soon as the young become independent, and their southward 

 migration begins soon thereafter. 



In the polygamous pectoral sandpiper, females incubate alone; males 

 form flocks in lowland marshes in late June and early July, and leave the 

 tundra before the eggs hatch. Thus, male pectoral sandpipers that have 

 migrated to arctic Alaska from southern South America remain on the 

 tundra for a period of less than 30 days. The females remain with the 

 young in July, but leave soon after the young become independent 

 around the first of August. Flocks of immatures remain on the tundra 

 throughout August, feeding mainly on pond-margin chironomid larvae. 



In some years large numbers of immature pectoral sandpipers, clear- 

 ly representing far more than local production of young, appear on the 

 tundra in the Barrow area in August. Similarly, in some years large 

 flocks of immature long-billed dowitchers (Limnodromus scolopaceus) 

 may be found in August, although dowitchers breed only occasionally 

 and sparsely in the immediate Barrow area. There is thus a premigratory 

 coastward movement of shorebirds from inland breeding areas that con- 

 tributes to the use of resources of the coastal tundra in August. 



In the sex-reversed polyandrous red phalarope, females form flocks 

 and depart soon after all nests are completed, and males incubate the 

 eggs alone (Schamel and Tracy 1977). Coastal lagoons and even the coast 

 of the open ocean are used as premigratory staging areas by red phala- 

 ropes, and large flocks may remain throughout August and even into 

 September. 



Although a variety of tundra arthropods appear in the diets of these 

 birds, the majority of breeding activities are supported by larval and 

 adult Diptera, especially of the three cranefly species, Tipula carinifrons. 



