The Detritus-Based Trophic System 447 



studied over seven seasons. Seastedt and MacLean (1979) found that no 

 new male longspurs were successful in establishing a territory and obtain- 

 ing a mate after 12 June, only eight days after the first arrival on the 

 study area. 



Incubation lasts from 12 days in Lapland longspurs to 22 days in 

 dunlin (Table 11-11); thus, the peak of hatching occurs in late June in 

 longspurs and in the first half of July in the wader species. The altricial 

 longspur young remain in the nest for about eight days, during which 

 they are fed and brooded by both adults. Young waders are very preco- 

 cial. They usually abandon the nest within hours of hatching of the last 

 egg, and they gather all of their own food; however, they are metabolic- 

 ally unable to maintain their own body temperature (Norton 1974) and 

 require frequent brooding by adults (50 to 83% of the time during the 

 first week for semipalmated sandpipers; Ashkenazie and Safriel 1979a). 

 The young of both longspurs and waders appear on the tundra before the 

 warmest weather of the season. A large proportion of the birds have left 

 the tundra by the second week in August, when average temperatures are 

 only slightly below the mid-July peak. Thus, avian activities are strongly 

 skewed toward the early season and are not coincident with the warmest 

 weather. Neither temperature nor length of the snow-free season can be 

 considered major factors in the evolution of breeding phenology. It is 

 more likely that the timing of breeding is determined by the emergence of 

 adult Diptera. 



Prior to the mass emergence of adult flies, adult birds feed heavily 

 on dipteran larvae, which they capture by inserting the bill into the tun- 

 dra. Young waders have growing, incompletely ossified bills. They are 

 unable to probe into the tundra, and thus require surface-active prey for 

 the first three to four weeks of life. The appearance of wader young 

 closely follows the appearance of their prey. Longspur hatching precedes 

 the emergence of adult Diptera, and larvae and pupae are the major prey 

 fed to nestlings. The young leave the nest in early July, just as adult Dip- 

 tera become abundant. Similarly, the abrupt decline in emergence of 

 adult Diptera, which occurs sometime after mid-July, may limit the peri- 

 od in which newly hatched birds can forage successfully, and thus may be 

 responsible for the synchrony of reproduction. 



The phenological relationship of avian reproduction and insect 

 emergence may help to explain an apparent paradox in incubation peri- 

 ods (Norton 1974). In dunlin both adults incubate, providing almost con- 

 tinuous attention to the eggs; incubation lasts about 22 days. In pectoral 

 sandpipers, the eggs are slightly larger than those of dunlin; the female 

 incubates alone and is present about 86% of the time, yet the incubation 

 period is 19 to 20 days. Nest initiation is later in pectoral sandpipers than 

 in dunlin, presumably because of the later snowmelt of the lowland mea- 

 dows in which they feed. The shorter incubation period of pectoral sand- 



