The Detritus-Based Trophic System 451 



nest in the most exposed sites. Given this strong selection pressure, the 

 continued use of exposed nesting sites by Baird's sandpipers at Barrow 

 seems paradoxical. The solution to the paradox may lie in the lack of ten- 

 acity in this species, indicated by the paucity of return sightings of the 

 many breeding birds and juveniles banded in the Barrow area. 



Soon after hatching, adults may lead the chicks a distance of up to 2 

 to 3 km from the nest site (Ashkenazie and Safriel 1979a); thus, broods 

 are difficult to follow, and data on survival of young are rarely collected. 

 Safriel (1975), however, was able to follow the fate of 39 broods of semi- 

 palmated sandpipers, and found a mean of 1.74 young fledged per 

 brood, for a success rate of 44<^o. As with eggs, most losses of juveniles 

 were attributed to predation. 



The average density of a population of longspurs studied over a 

 seven-year period (1967-1973) was 0.47 nest ha"' (Custer and Pitelka 

 1977), a value well above the wader species. Density was highest (0.82 

 and 0.88 nest ha') in 1967 and 1968, but dropped steadily to a low of 

 0.12 nest ha"' in 1972. The population showed some recovery in 1973 and 

 1975 (Seastedt and MacLean 1979). In longspurs, year-to-year variation 

 in productivity of nesting habitat cannot be detected at the time of terri- 

 tory establishment because most of the ground is still covered by snow; 

 hence, territory size and nesting density are related to average or "ex- 

 pected" productivity of the habitat (Seastedt and MacLean 1979). Nest- 

 ing success should be more responsive than nesting density to year-to- 

 year variation in habitat productivity. The decline in nesting density of 

 longspurs recorded between 1968 and 1972, then, should reflect changes 

 in the size of the potential breeding population, as influenced, in part, by 

 breeding success in preceding years (Figure 11-10). Since the study area 

 used by Custer and Pitelka was placed in optimal longspur habitat, a 

 slightly more conservative estimate of 0.30 nest ha"' is used for the area 

 as a whole (Table 11-11). 



Lapland longspurs are indeterminate layers, and clutches of four, 

 five, and six eggs are common. The modal clutch size, five eggs, occurred 

 in 45% of the nests examined. Mean clutch size was 5.06 eggs, and varied 

 between only 4.76 and 5.50 between years (Custer and Pitelka 1977). 

 Over this period as a whole, 64''7o of the eggs hatched and 68^/0 of the 

 chicks survived to fledging, about eight days after hatching. Thus, the 

 average female produced about 2.2 fledged young. 



The occurrence of starved nestlings in longspur nests after some of 

 the young have fledged indicates that food supply can influence repro- 

 ductive success; however, only 3.1% of all nestlings observed over the 

 seven-year period died of starvation. Loss to starvation was greatest 

 (7.1%) in the very cold summer of 1969. 



By far the major source of reproductive failure was predation; 

 22.3% of all eggs were taken by predators prior to hatching, and 22.7% 



