Small mammal capture data from summer and winter in northern Wisconsin, USA

Small mammal live trapping data from northern Wisconsin, USA. Data was collected from 10 trapping sessions across 5 years. Five trapping sessions occurred in the summer and 5 ocurred in the winter. Data was used to explore the season variation in small mammal abundance in northern Wisconsin. 

Methods
We established small mammal trapping sites at 9 study sites, 3 each in northern hardwood forest, eastern hemlock forest, and spruce-larch bog. We sampled small mammals in summer (August/September) and winter (January/February) from summer 2018 until summer 2022. Sites in spruce-larch bogs were discontinued after winter 2020 and replaced with 3 sites in white cedar swamp beginning in winter 2021 (Scott et al. 2022). Sites were >300 m apart to maintain independence.

Small mammal trapping. —To sample small mammals in the subnivium, we used chimney-like structures (“snow culverts”) comprised of sections of corrugated black culvert (Jolly et al. 2022; Scott et al. 2022). Snow culverts allowed continual observer access to the subnivium and partial protection from extreme cold for small mammals inside traps (Jolly et al. 2022). We placed traps (HB Sherman Traps, Inc. LFA) directly on the ground inside the snow culverts and baited each with sunflower seeds. At each site, we placed 25 structures in a 5 × 5 grid, 10 meters apart (Scott et al. 2022). Prior to the trapping season, we baited culverts weekly with sunflower seeds to ensure that small mammals located and entered the structures. For additional protection from the cold, we wrapped the traps in either foam pouches (30.4 × 30.4 cm, Duck Brand Foam Pouches 284867) or polyester quilt batting. Additionally, we hung plastic bags with extra bait and nesting material at the back of the traps. We left traps open continuously for 9 consecutive days and checked traps daily beginning at sunrise (Scott et al. 2022). We sampled 3 sites at a time, one of each forest type.

We identified each animal to species and tagged each rodent with two unique ear tags (National Band and Tag Co.). We marked short-tailed shrews with non-toxic paint pens for within-season recapture identification, as ear-tagging was not feasible (Pauli et al. 2006). We recorded mass (g), right hind foot length (mm), sex, age, and reproductive status for each individual. Additionally, we measured right ear and tail length of Peromyscus mice for identification to species (Stephens et al. 2014). In cases of extreme cold (< -20° C), we forewent body measurements and only applied ear tags before releasing the animals. We released animals on top of the snow adjacent to culverts where they were captured and ensured that each animal successfully burrowed back into the snow before moving on. Upon initial site set up, we established a single path through the snow to access all traps efficiently and reduce compaction of the subnivium.

During the summer trapping session (August/September), we placed 25 traps at each site directly on the ground outside of the culvert structures. We selected locations near coarse woody debris, out of the sun, and covered traps with bark, leaves, and other debris to keep them dry and shaded. We baited traps with sunflower seeds and continued to place bags of extra seed and nesting material in all traps but did not wrap the traps in foam. We followed the same protocols as winter for handling and processing all animals. We limited our analysis to the three most abundant subnivium-obligate species that we captured across the 9 seasons: white-footed mice, southern red-backed voles, and northern short-tailed shrews (Scott et al. 2022). These species also rely on different resources and dietary components. Deer mice (P. manicualtus) were excluded from analysis because of low winter capture numbers, likely due to both their regional decline and changes in their behavior in the winter, such as daily torpor (Pierce and Vogt 1993) and increased arboreal activity (Barry et al. 1984). Live capture of small mammals followed ASM guidelines (Sikes et al. 2016) and was approved by the University of Wisconsin – Madison Institutional Animal Care and Use Committee under protocol number A006063-R01 with a state scientific collection permit.