The Herbivore-Based Trophic System 363 



trappable lemmings was below about 12 ha"'. At higher lemming densi- 

 ties the density of breeding jaegers increased and approached an asymp- 

 tote of about 7.3 nesting pairs km"^ at lemming densities above 100 ha"'. 

 The lemming density required for breeding is higher for snowy owls than 

 it is for jaegers. Snowy owls have not bred on the Barrow peninsula in 

 the absence of breeding jaegers, but jaegers have bred during periods of 

 low to moderate lemming density when snowy owls did not. The density 

 of owls during breeding is much lower than that of jaegers, and the owls' 

 numerical response to the number of lemmings is less consistent. Pitelka 

 et al. (1955) estimated the density of snowy owls on the Barrow peninsula 

 to be 0.2 pair km"^ during the 1953 lemming high, and Pitelka (1973) esti- 

 mated a density of about 1 pair km"^ during the 1960 high. 



The difference in territory size in these two species may result from a 

 difference in timing of nesting. Snowy owls arrive, establish territories, 

 and begin their clutches well before snowmelt. The only exposed ground 

 at this time is on ridges, bluffs and high-centered polygons, where the 

 combination of wind and sublimation of snow produces small snow-free 

 areas. Owls watch for lemmings while sitting on these vantage points. 

 Thus, the extent of exposed ground in the spring may help to determine 

 breeding density, along with amount of lemming movement over the 

 snow. 



Pomarine jaegers generally arrive later and establish territories 

 around the time of snowmelt, when the tundra is rapidly becoming ex- 

 posed and lemmings are maximally exposed. Jaegers search for lemmings 

 while patrolling or hovering about 10 m above the ground. The strong 

 numerical response in jaeger populations suggests that territory size is set 

 by food supply. The territories are larger than necessary in June, but by 

 July the demand for food is higher, since the young must be fed, while 

 the lemming population has usually declined. 



For the first few weeks of summer, there are often many more pom- 

 arine jaegers and snowy owls present than eventually establish territories 

 and breed. At low to intermediate lemming densities, jaegers engage 

 sporadically in territorial behavior. Unless mating and nesting ensue, the 

 birds quickly abandon the territory and leave the area. In contrast, non- 

 breeding snowy owls may remain localized in an area for long periods of 

 time. Even when lemming densities are high, some jaegers do not breed. 

 Pitelka et al. (1955) estimated jaeger densities at 15 to 25 km"^ during ear- 

 ly summer 1953; the breeding population later that summer was 6.9 pairs 

 km"^ Maher (1974) estimated the nonbreeding population as 25 to 50% 

 of the breeding population in 1956 and less than 25% in 1960, two other 

 peak years. Thus, in a pre-high season, the density of snowy owls may in- 

 crease during the summer while jaeger density always declines after a 

 spring peak (Figure 10-11). 



The numerical response of other avian predators is less clear. Short- 



