Use of track stations to index abundance of sciurids

We developed rainproof track stations to index sciurid abundance. Track stations were effective at detecting presence of a species and provided a useful index of abundance in two validation efforts. In the more rigorous validation, we documented the correlation of two indices of abundance (our track index and catch-per-unit-effort, CPUE) with population size of Abert's squirrels (Sciurus aberti) as estimated by mark-recapture methods on eight 60-ha plots in April and August 1996. The track index (number of stations visited) was correlated highly with estimated population size in both April (r = 0.983) and August (r = 0.924). CPUE, as measured by the cumulative number of squirrels captured in the first day and first 2 days of the trapping effort, performed as well (r = 0.905 in April and r = 0.945 in August) as the track index. We also examined the correlation between these two indices for chipmunks (Tamias), golden-mantled ground squirrels (Spermophilus lateralis), rock squirrels (S. variegatus), and Abert's squirrels by double-sampling 30 plots (2.25 ha each) for diurnal sciurids in 1994 and 1995. The track index and CPUE were correlated highly for chipmunks (r = 0.815), golden-mantled ground squirrels (r = 0.881), rock squirrels (r = 0.868), and Abert's squirrels (r = 0.926). Similarity of regression slopes suggested that the track index was valid across seasons. Both the trap index and CPUE were less powerful statistically than mark-recapture estimates in detecting differences among plots and between seasons but did detect large differences without unacceptable Type I errors. Compared to live-trapping, track stations provided results in less time, were less expensive and easier to deploy in the field, caused no mortality to animals, and eliminated exposure of technicians to diseases transmitted by rodents.