ABSTRACT
Townsend’s vole, Microtus townsendii, ranges from northern California to southern British Columbia. The present example is based upon a study of M. townsendii in the northern extremity of its range. M. townsendii can be enumerated with snap traps or live traps. However, in a live-trap enumeration, not all members of a vole population are equally trapped.1–3 Pitfalls are more likely to catch smaller voles, and live traps are more likely to catch the larger ones. Live traps caught only 55% of M. townsendii known to have been in the populations under study. About 50% of adult males first captured in pitfalls failed to enter live traps, as did about 40% of adult females. Live traps underestimate population size by at least 50% during the breeding season, regardless of whether the population is at low or high density.
