Demography of the spring decline in populations of the vole, Microtus townsendii

(1) Population densities of small rodents typically decline in the spring at the start of the breeding season. Eighteen spring declines of Microtus townsendii were monitored by mark-recapture methods on undisturbed grasslands near Vancouver, British Columbia. (2) The duration and size of spring declines is highly variable: sometimes there is virtually no decline, and at other times more than 90% of the population may be lost in a spring decline that may last for 12 weeks after breeding begins. (3) Males usually disappear more rapidly than females. (4) The size of the spring decline is closely related to the rate of loss. (5) In light-to-moderate spring declines most of this loss is due to emigration; in severe declines most loss is due to death rather than movement. (6) In males, the amount of rump wounding is highly correlated with the size of the spring decline. (7) We suggest that spring declines in both sexes of M. townsendii are produced by spacing behaviour, but we do not know if differences in spacing behaviour are sufficient to explain all variations in spring declines