The age distribution had little apparent effect on the reproductive success of the population. One-year-olds dominated the breeding populations in 19 but in 1969 only 55% of the breeders were born the preceding year. Differential survival affected the age distribution of animals during the breeding season. The highest overwintering success was in juveniles born during the year of lowest precipitation, poorest food supply, and reduced reproduction. Survival from weaning to the following breeding season ranged between 56%-58% (1967, 1970) and 80% (1968) 17%-19% survived to the 3rd yr and 2%-3% survived to the 4th yr. Subadults were first trapped in June for an average of 40-70 days. In 1969, a year of excellent food production, juveniles from early litters bred during late summer. The average size of three litters found in live-traps was 3.7. In years of high food availability adult averaged 2 litters in average years they had 1.1 litters and in poor years, only one in three became pregnant. The first signs of estrus were observed in April, the first palpable pregnancies in May, and the last pregnant usually in July. Sex ratios of trappable animals differed significantly from 1:1 in about half the sampling periods, and usually favored adult Males had scrotal testes for an average of 4 mo, with the highest proportion in April. Once active, adults remained trappable for an average of 60 days in years of average-to-excellent food production, and for 90 days in years of poor production. Adult became trappable at an average date of 26 March emerged about 18 April. Most pocket mice remained inactive underground between December and February. Pocket mice accounted for 91%, deer mice 8%, and the remaining species only 1% of the captures. During 41,310 trap-nights we captured 15,386 mice representing seven species: Perognathus parvus, Peromyscus maniculatus, Onychomys leucogaster, Reighrodontomys megalotis, Lagurus curtatus, Microtus montanus, and Sylvilagus nuttallii. A population of Great Britain pocket mice, Perognathus parvus, was live-trapped in 1967-72 on a 2.7-ha study area in south-central Washington to determine their density, reproductive performance, longevity, movements, and home range, as well as their functional role in northern shrub-steppe habitat.
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