368 G. O. Batzli et al. 



of prey. Maher (1970) analyzed the remains of lemmings in pomarine 

 jaeger pellets collected in the high lemming years of 1956 and 1960. The 

 proportion of prey in the smallest size class, corresponding to nestlings, 

 was greater in 1956, when lemming density declined rapidly during the 

 summer, than in 1960, when lemmings remained abundant throughout 

 the summer. Osborn (1975) compared the 1960 distribution of prey in 

 jaeger pellets with estimates of relative abundance of different age classes 

 and sexes from snap-trapping data. Little or no difference was evident. 

 These observations do not rule out selectivity because snap trapping 

 probably overestimated the frequency of males in the population. 

 Greater movement by males than females exposes males to more traps, 

 just as it exposes them to more predation. Although more male lemmings 

 were found in jaeger pellets in 1960, Maher's 1956 sample contained a 

 greater proportion of females (54%). Thus, there seems to be no consis- 

 tent selection of males by jaegers. 



Pitelka et al. (1955) determined the sex of 76 lemmings accumulated 

 at a single snowy owl nest in June 1953; 25 were females and 51 were 

 males. A similar preponderance of males was found by Thompson 

 (1955c) in snowy owl pellets. The greater vulnerabiHty of males to owl 

 predation seems clear. 



During the winter weasels prey heavily upon lemmings and live in 

 the nests of their victims. MacLean et al. (1974) suggested that the large 

 nests that are subject to the greatest predation are built by breeding fe- 

 males and that weasel predation may be concentrated upon reproductive 

 females. This hypothesis may be tested by comparing the sex ratio of 

 lemming populations after winters with heavy predation with the sex 

 ratio after winters with little predation (Table 10-7). The sex ratio signifi- 

 cantly favored males after the winters of 1959-60 and 1964-65; a sex ratio 

 favoring males after the 1962-63 winter is marginally significant. During 

 all three of these winters weasel predation was heavy (MacLean et al. 

 1974). Breeding also occurred during these winters, so the stress of breed- 

 ing might have contributed to the skewed sex ratios. 



The quantity of food removed per predator can be examined by cal- 

 culating the number of prey needed to meet the energy requirement or by 

 field observation of prey capture rate. The latter approach allows for the 

 interaction of behavior and energy requirement in a natural setting, but 

 such data are not easy to obtain. 



Gessaman (1972) studied the metabolic rate of snowy owls by mea- 

 suring both the oxygen consumption and the food intake of birds con- 

 fined in outdoor pens. He concluded that the daily consumption of an 

 average adult owl was 6.6 60-g lemmings during the coldest period of the 

 winter (T = -29 °C) and 4 lemmings in summer (Ty -5°C). Pitelka et al. 

 (1955) observed food consumption by a captive immature snowy owl 

 kept in an outdoor cage during summer. During the late phase of growth. 



