Pitfall Trapping of Microtus townsendii

-A field population of Microtus townsendii was trapped concurrently with live-traps and pitfall traps throughout the spring and summer of 1975. The population was at peak densities, with a maximum of 519 voles per acre. The number enumerated by pitfalls was up to two times larger than the number enumerated by live-traps. Capture of a large number of young in pitfalls indicated that mortality was higher among post-weanlings than among any other age group. Recruitment to live-traps was largely restricted to animals over 40 g, whereas in pitfalls animals were generally lighter. Over half of the 1,100 animals captured first by pitfalls were never caught by live-traps. Among those subsequently entering live-traps, many stayed away for 6 to 10 weeks, and some of the smaller ones stayed away even longer. Few animals dispersed to other nearby trapping grids. Pitfall trapping and live-trapping sample different subpopulations, young animals and transient adults being a large fraction of the individuals caught in pitfalls. Population estimation techniques for small rodents either assume equal catchability of all members in the population (Jolly, 1965) or that virtually all the animals can be caught and enumerated (Krebs, 1966; Hilborn et al., 1976). The assumption of equal catchability can be tested by trapping with two or more different methods and comparing the populations captured by each type of trap. This was the first purpose for the present study. The second purpose was to measure survival and dispersal rates of small juveniles between weaning and puberty. Juveniles are particularly difficult to capture by standard live-trapping techniques. However, changes in the rates of juvenile survival are of great importance in determining the rate of population growth in a number of Microtus species (Krebs and Myers, 1974). The technique used to investigate juvenile survival and the efficiency of total enumeration trapping involved the use of pitfall traps concurrently with Longworth livetraps. Other workers (Kott, 1965; Andrzejewski and Rajska, 1972) have had success in trapping the younger segment of the population by the use of pitfalls that were continuously open. We modified the pitfall trap so that it could be prebaited when not in