Shrew populations were measured by recapture techniques at monthly intervals between May and October from 1952 to 1957 on two of three plots in southeastern Manitoba, and from 1952 to 1958 on the third. Annual populations per acre varied from 0.4 to 9.3 for Sorex cinereus, 0 to 4.1 for Sorex arcticus, and 0 to 1.8 for Blarina brevicauda. Populations of S. cinereus varied inversely with those of S. arcticus where the two species occurred together. The population turnover appeared to be about two generations a year for the two species of Sorex and one per year for B. brevicauda. An exceptional population peak of S. cinereus is described. The breeding season was from May to August, except in a particularly high population of S. cinereus when breeding extended into October. Breeding began at about 4 months of age or earlier in Sorex and at about 10 months in Blarina. The average number of embryos was 6.5 for all species except at the highest population of S. cinereus when the average for this species dropped to 3.0. Several litters may be produced per season by all these species. Mortality was greatest in the first 2 months after birth, after which it declined gradually. One S. cinereus lived for at least 23 months, and one S. arcticus 18 months. Population levels of S. arcticus were correlated with the depth of the water table below ground surface-the drier the site, the higher the population. Nesting S. cinereus appeared to be advsersely affected by rainfall. The rate of mortality was markedly lower in winter than in summer. Observations failed to implicate parasites or predators as important factors in the population dynamics of shrews. Immigration and emigration may be the chief causes of apparent fluctuations in populations in spring.
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