Does Trap Odour Influence Estimation of Population Size of the Short-Tailed Vole, Microtus agrestis?

SUMMARY (1) A live-trapping and marking programme on a population of Microtus agrestis L. continued until the number of unmarked individuals in each trap clearance was very low. This was achieved after 13 nights when seventy-seven individuals had been caught in 1274 trap nights on a grid covering 0.36 ha. (2) After thorough cleansing and resetting an immediate surge of unmarked individuals occurred. Continued use of clean traps revealed the presence of further unmarked individuals. (3) Population estimates after using clean traps were up to 62% higher than those calculated by Hayne's trap-out and by a modified Lincoln's Index for the 13-night control period. (4) There was no evidence of immigration from outside the grid, peripheral traps obtaining smaller catches than central traps, nor was there evidence of serious competition for traps between M. agrestis and other species of rodents. (5) Application of tiger urine to the trap tunnels during one phase of the experiment resulted in substantially lowering the estimated population size. Clean traps used immediately after restored the estimate to its previous level. (6) Individuals deterred by tiger urine were those with a poorer previous trapping