High densities of the masked shrew, Sorex cinereus, in jack pine plantations in northern Ontario.

-The masked shrew Sorex cinereus was abundant in jack pine (Pinus banksiana) plantations in northern Ontario in a year which coincided with high numbers of jack pine budworm (Choristoneura pinus). Snap-trapping indicated that greater numbers of this shrew occurred in medium-age (40 yr) stands than very young (2-10 yr) stands. Continuous pitfalltrapping yielded many more S. cinereus than snap-trapping and also showed that shrew densities were greater in medium-age stands than in young (20 yr) stands. The difference in numbers was a result of much better recruitment of juveniles in the medium-age forest than in the young forest. Of all sex/age groups, adult females were the least likely to be captured, presumably because they did not disperse as readily as the other groups. Females in the medium-age stands produced slightly larger litters, which gave rise to significantly heavier juveniles compared to females from the young stands. The following characteristics were not significantly different between the two forests: number of adults, adult weight and length, juvenile length and litter size. A combination of greater cover (and thus a more suitable microclimate) and more of a preferred food (insect larvae) may explain why juvenile recruitment was much greater in the medium-age stands than in the young stands.