Microsites and structures used by fishers (Pekania pennanti) in the southern Sierra Nevada: A comparison of forest elements used for daily resting relative to reproduction

Many wildlife species rely on unique features of trees during daily activities and fundamental parts of their life cycle. The fisher (Pekania pennanti) is a forest-dwelling carnivore of conservation concern in western North America that uses unique habitat elements as refuges during resting bouts and for reproduction. Prior to this study, little was known about the fine-scale habitat used during reproduction at the southernmost extent of the fisher’s range. Between 2007 and 2015, we attached radio-collars to 55 male and 72 female fishers in the southern Sierra Nevada and documented resting locations of males on 216 occasions (196 structures) and females on 824 occasions (737 structures). Beginning in 2008, we also monitored females over 8 reproductive seasons, confirming 45 females at dens and documenting 95 natal dens (83 structures) and 206 maternal dens (192 structures). We established 3 comparisons to guide our assessment of fine-scale habitat: resting males versus resting females, natal dens versus maternal dens, and resting fishers (both sexes) versus denning fishers (all dens). We expected the need for physical security and thermal protection in combination with morphology, predation risk, and aspects of reproductive ecology would influence patterns of use. Both sexes used a variety of microsites for resting, but females selected tree cavities most frequently (47%) while males used branch platforms most often (39%). For resting structures, live conifers were used most often by both sexes (males 44%, females 34%), but live hardwoods (males 16%, females 28%) and conifer snags (males 16%, females 22%) were also important. Comparing natal and maternal dens, we found that cavity microsites used early in the den season tended to be higher than those used later, and large live hardwoods comprised roughly half of all natal (46%) and maternal (51%) den structures. For resting versus denning, we found that large diameter hardwoods were an important source of cavities for both activities, live conifers used for denning were larger than those used for resting, and den structures tended to be on steeper slopes than rest structures. White fir (Abies concolor), California black oak (Quercus kelloggii), and ponderosa pine (Pinus ponderosa) were selected most often by both sexes for resting. In contrast, denning females relied on California black oak (55%), but also used white fir (24%) and incense cedar (Calocedrus decurrens; 12%). As noted in studies further north, our findings highlight the value of large trees with decay to support fisher reproduction and daily refugia.