452 S. F. MacLean, Jr. 



2 3 



Young Fledged per Nest 

 in Previous Year 



FIGURE 11-10. Relationship of nesting density and 

 breeding success during previous year. (Data from 

 Custer and Pitelka 1977.) 



of chicks were lost before fledging. Predation accounted for 73% and 

 72% of losses of eggs and nestlings, respectively, and for most of the var- 

 iance in reproductive success (Figure 11-10). The decline in nesting den- 

 sity from 1968 to 1972 was attributed to a sequence of years with heavy 

 predation. As with the waders, the major predators upon eggs and nest- 

 lings are pomarine and parasitic jaegers and least weasels. Since the den- 

 sities of these predators are primarily determined by density of lemmings 

 (Chapter 10), the reproductive success of insectivorous birds may be de- 

 termined primarily by events in the herbivore-based trophic system. 



Energetics and Impact Upon Prey Populations 



Norton (1974, West and Norton 1975) used gas exchange techniques 

 to study the bioenergetics of sandpipers breeding at Barrow. An alter- 

 native time-energy budget approach was used by Custer (1974) on 

 Lapland longspurs and by Ashkenazie and Safriel (1979b) on semipal- 

 mated sandpipers. Energy budgets calculated for semipalmated sand- 

 pipers using these aUernative approaches yielded virtually identical 

 estimates. 



Norton found that the temperatures ordinarily encountered by the 

 birds during the summer season are below thermoneutrality, so that 



