Bat homes in the Big Sky: habitat characteristics of northern Myotis (Myotis septentrionalis) in northeastern Montana

Abstract

The northern myotis (Myotis septentrionalis) is one of 9 endangered bat species in the United States but the only one to inhabit Montana and the only resident bat species newly discovered in the state in the last 10 years. The need for conservation efforts is hampered by the large gaps in our understanding about northern myotis in Montana. We started to build our knowledge base by characterising habitat, specifically summer daytime roosts. Summer roosts provide refuge for bats while they give birth, raise young, and prepare for hibernation or migration. We mist-netted for bats from May-August in 2022 and 2023 and attached transmitters to 36 individual northern myotis. We located a total of 76 roosts from 33 tagged individuals. We quantified characteristics of the roost trees, as well as the surrounding patch, and compared these values with the same features at random locations. Bats roosted primarily in eastern cottonwood trees (Populus deltoides) (97% of roosts), which was the most available tree species on the landscape. Bats showed a strong preference for tall trees in patches with a relatively high basal area, containing multiple trees similar to the roost tree. Bats also selected for cottonwoods that were in an early to mid-stage of decay with somewhat less canopy cover. Most consecutive roosts were within a few hundred metres of previous roosts (range = 2-881 m) and each roost was used for 1.7 days on average. Our work supports the notion that northern myotis display some flexibility in roost selection in different parts of their range, with the common theme of selecting areas that provide multiple possible roosts and facilitate their social interactions. This is especially important as studied population is one on the edge of the species known range, and edge populations are shown to be more plastic in behaviour and resilient in the face of change. Improving our understanding of habitat preferences of northern myotis in Montana will provide us with a more developed toolkit to aid in their habitat management and conservation. Managers can then design strategies that protect critical roosts as well as surrounding habitat.