Cholecalciferol for mouse (Mus musculus) control: limited effectiveness and the challenges of tracking tunnels for density monitoring

In New Zealand, predation on endemic biota by house mice (Mus musculus) is a pressing conservation issue. While rodenticides are commonly used, efficacy and bioaccumulation are a concern. Cholecalciferol (vitamin D) offers a reduced risk of bioaccumulation. Our 14-month field study assessed the effectiveness of cholecalciferol-laced bait, which reduced mouse density by up to 59% from peak levels, yet was insufficient for sustained suppression of mice to the low levels required for conservation management. Spatially explicit capture-recapture and footprint tracking revealed that while the block treated with cholecalciferol showed density patterns consistent with suppression, a footprint tracking rate < 5% was only achievable intermittently. Mouse density peaked in March and was highest in grassland habitat in the block without poison (106 mice ha⁻¹, 95% CI: 87–130), while densities in woody vegetated areas in the same block were lower (57 mice ha⁻¹, 95% CI: 41–80). During peak abundance, mouse densities in the non-poison block were 2.4 × higher than in the poison block. The seasonal variability in both mouse density and footprint tracking rates suggests that seasonality and food availability were the drivers of mouse population dynamics. The relationship between footprint tracking rate and density oscillated through time, with an up to 25-fold variation in mouse density for a given tracking rate. Consequently, we caution wildlife managers that footprint tracking rates may not always serve as a reliable proxy for density and require, context-specific interpretation. Our findings indicate that the success of future mouse toxicant interventions will hinge on both toxicant efficacy and its sensitivity to seasonal climatic and productivity shifts, highlighting the need to better understand the phenological links between mouse density and impacted species.