BREEDING BIOLOGY OF AN INSULAR POPULATION OF THE MASKED SHREW (SOREX-CINEREUS KERR) IN NOVA-SCOTIA

The breeding biology of a high-density population of the masked shrew, Sorex cinereus Kerr, from Bon Portage Island, Nova Scotia, was examin ed over 2 years. Data were collected on reproductive condition, litter size, age structure, and sex ratio from shrews pitfall-trapped during June-November 1986 and May-November 1987. Breeding activity among ove rwintered adults declined from spring to fall. Most, but not all, over wintered adults bred. A small proportion of young of the year bred in their year of birth. Among overwintered adults, reproductively active shrews were significantly longer and heavier than nonbreeders. Among y oung of the year, reproductively active individuals were significantly heavier but not longer than nonbreeders in their age-class. In both y ears some individuals were still reproductively active in November. Th e age distribution of the population suggested that recruitment extend ed beyond the spring and summer pulses typical of shrew populations at this latitude. Based on their age distribution and the presence of la te breeders in both years, it is likely that limited reproduction occu rs throughout the year in this population. In contrast to some island populations of rodents, there was no indication of reduced reproductiv e activity in this insular population of insectivores.